CORNELL
UNIVERSITY
LIBRARY
FROM
The Estate of S. Simpson
Cornell University Library QP 34.P371 1910
Practical physiology.
3 1924 003 128 729
The original of tiiis book is in tine Cornell University Library.
There are no known copyright restrictions in the United States on the use of the text.
http://www.archive.org/details/cu31924003128729
PKACTICA.L PHYSIOLOGY
PRACTICAL PHYSIOLOGY
EDITED BY
M. S. PEMBEEY
CONTRIBUTORS A. P. BEDDARD, M.A., M.D. LEONARD HILL, M.B., F-KS.
Assistant Physician, lato Demon- strator of Physiology, Guy's Hospital
J. S. EDKINS, M.A., M.B.
Lecturer on Physiology, St. Bartholomew's Hospital
Lecturer on Physiology, The London Hospital
J. J. E. MACLEOD, M.B.
Professor of Physiology, Western Reserve University, Cleveland, U.S.A.
MARTIN FLACK, M.A., M.D. M. S. PEMBREY, M.A., M.D.
Demonstrator of Physiology, London Hospital
Lecturer on Physiology, Guy's Hospital
ILLUSTRATED Br NUMEROUS BIAORAMS AND TRACINOS
THIRD EDITION
NEW YORK
LONGMANS, GREEN, & CO.
LONDON: EDWARD ARNOLD
1910
PREFACE TO THE FIEST EDITION.
Physiology is the Ijasis of medicine, and the further advance of these sciences depends mainly upon the "experimental method." The medical student, the future physician, should undergo a training in practical physiology, for thereby he learns the most important of all lessons ; he learns to observe, to draw conclusions from his observations, and to unravel the causes of his failures.
The importance of practical physiology is undoubted, but as to the nature and scope of the experimental work, which is most suitable for the medical student, there is considerable difference of opinion among teachers of physiology. In this country, perhaps, too much stress has been laid upon the physiology of muscle and nerve ; for the hope that a study of the properties of these tissues will unfold the enigma of life is likely ever to remain without consummation.
An advance in the knowledge of the living organism as a whole, one organ reacting upon another, has been gained by experiments upon the living animal, treated as a unit and not as a collection of separate organs and tissues. Such practical physiology needs extension in the courses of instruction given to students. It should, as far as possible, have a direct relation to medicine.
The methods which are used in the investigation of the respiratory system, the circulation, the body heat, the nervous system and special senses ; the chemistry of the blood, of digestion, and of urine — these are the subjects which are especially required by the clinician. These sub- jects, moreover, afford as excellent a mental training as the study of muscle and nerve.
In the present work the authors have attempted to give some exten- sion to practical physiology along the lines just indicated.
The book has been divided into an elementary and an advanced por- tion. Part I. treats of elementary experimental physiology (the physiology of muscle and nerve, circulation, respiration, animal heat,
vi PEEFACE
the central nervous system, and the special senses) ; Part II. of elementary physiological chemistry ; Part III. of advanced experimental physiology ; and Part IV. of advanced physiological chemistry. . The experiments upon the physiology of muscle and nerve are based upon the course given at Guy's Hospital — a course modelled on a reduced scale upon the excellent practical courses given at Oxford by Professor Burdon Sanderson and Professor Gotch. The experiments in this section have been limited as far as possible to those which can be conveniently performed with simple apparatus by a large class of students. For this reason the experiments with the galvanometer and capillary electrometer have been restricted to demonstrations, and very few details of such experiments are given.
There are some important experiments upon the circulation and respiration, which for various reasons cannot be properly performed by the student ; these have been collected together as demonstrations in Parts I. and III.
The subject of vision is so important from a medical as well as a physiological and psychological point of view, that it has here received more extensive treatment than is usually the case in works on practical physiology.
In those portions of the book which treat of physiological chemistry, an attempt has been made to demonstrate, step by step, the chemical relationships which exist between the various substances, and to illus- trate, by suitable experiments, the different properties of those bodies. The drawings of crystals were executed by Mr. W. E. M. Turtle, to whom the authors are deeply indebted.
Figures have been borrowed from The Physiological Action of Drugs, by M. S. Pembrey and C. D. F. Phillips. For the loan of numerous blocks illustrating physiological apparatus the authors are indebted to Messrs. Baird & Tatlock, of Hatton Garden, E.G. The sources of other diagrams and tracings, which have been borrowed, are indicated in the description of the figures. The initials of the author, who took the record of the original tracings, are appended to the respective curves.
Sept., 1902.
PKEFACE TO THE SECOND EDITION.
In the present edition considerable changes have been made in those portions of the work which deal with Physiological Chemistry. The new exercises have involved a slight increase iu the total number of pages, and several new figures have been added.
July, 1905.
PREFACE TO THE THIRD EDITION.
The present edition is in many respects a new book, for many parts: have been rewritten and the arrangement of the whole has been altered. The book now consists of two parts, Part I., which deals, ■with Experimental Physiology, and Part II., which is devoted entirely to Physiological Chemistry.
The experiments upon muscle and nerve have been reduced in number and the observations which can be made upon man have been increased.
The Authors wish to thank Drs. H. D. Haskins and J. H. Eyffel for their able assistance in the revision of the chapters on Physiological Chemistry and Dr. Kennaway for many criticisms and suggestions upon the whole of the work.
Dr. Hertz has contributed a special chapter upon the " Investigation of the Motor Functions of the Alimentary Canal by means of the X-Rays," and Dr. Eyffel one upon "Lactic Acid, its Estimation and Significance." For this valuable assistance hearty thanks are given.
Sept., 1910.
CONTENTS. PART I.
MUSCLE AND NERVE. CIRCULATION. RESPIRATION.
ANIMAL HEAT. CENTRAL NERVOUS SYSTEM
AND SPECIAL SENSES.
By A. P. Beddard, J. S. Edkins, L. Hill, and M. S. Phmbret.
CHAP. PAGE
Introduction, - 1
I. Electrical Apparatus for Physiological Experiments. By
A. P. B., 2
II. The Graphic Method. Maximal and Minimal Stimuli. Uni- polar Excitation. By A. P. B., ] 4
III. A Single Contraction of a Gastrocnemius Muscle. By
A. P. B., 22
IV. The Conditions which affect Single Muscular Contractions.
By A. P. B., 29
V. The Conditions which aifect Single Muscular Contractions
(continued). By A. P. B., 32
YI. The Conditions which aflfect Single Muscular Contractions
(continiied). By A. P. B., 35
VII. Two Successive Stimuli. Genesis of Tetanus. Tetanus. By
A. P. B., 40
VIII. The Properties of Nerve, Minimal and Maximal Stimuli.
By M. S. P., - 44
IX. The Relation between Muscle and Nerve. By M. S. P., 48
X. The Effect of a Constant Current upon Muscle and Nerve.
By M. S. P., 50
XI. The Electromotive Properties of Muscle and Nerve. By
M. S. P., 51
XII. Extensibility and Elasticity of Muscle when at Best and Contracted. Comparison with Eubber (Advanced). By A. P. B., 53
70
CONTENTS
CHAP. J"*"^
XIII. Load and After-load. Work done with Increasing
Loads (Advanced). By A. P. B., 57
XIV. Summation of Stimuli {Advanced). By A. P. B., 62
XV. Effect of Distilled Water and of Various Salts on Muscle
{Advanced). A. P. B., §7
X VI. Fatigue of a Voluntary Movement and of a Muscle-Nerve Preparation with its Circulation intact {Advaiiced). By A. P. B., XVII. The Bate of Transmission of a Nervous Impulse
{Advanced). By M. S. P., 76
XVIII. The Polarisation of Electrodes and Unpolarisable Elec- trodes {Advanced). By M. S. P., 78
XIX. Transmission of a Nervous Impulse in both Directions
{Advanced). By M. S. P., 79
XX. The Relation between Muscle and Nerve. The Inde- pendent Excitability of Muscle {Advanced). 'By M. S. P., 80
XXI. The Effect of a Constant Electrical Current upon the Excitability and Conductivity of Nerve {Advanced) By M. S. P., 81
XXII. The Absence of Fatigue in a Stimulated Nerve
{Advanced). By M. S. P., - 87
XXIII. The Electromotive Properties of Muscle and Nerve
{Advanced). By M. S. P., 88
XXIV. The Electromotive Properties of Muscle and Nerve {continued). The Galvanometer and the Capillary Electrometer {Advanced). By M. S. P., 90
XXV. The Anatomy of the Frog's Heart and its Contraction.
By L. H., 92
XXVI. Methods of Recording the Heart. By L. H., 97
XXVII. The Stannius Heart. By L. H., 100
XXVIII. The Cardiac Nerves and Ganglia. By L. H., 103
XXIX. The Sino-auricular Junction. The Action of Drugs.
By L. H., 109
XXX. The Effect of Nicotine, Chloroform, and Ether upon the
Heart. By L. H., 112
XXXI. Dissection of the Heart. The Cardiac Impulse. ByL. H., 115
XXXII. The Pulse. Human Blood Pressure. By L. H., 121
XXXIII. Blood. The Haemoglobinometer and the Haemacyto-
meter. By L. H., 128
CONTENTS xiii
CHAP. PAGE
XXXIV. Circulation of the Blood {Elementary Demonstrations).
By L. H., - 131
XXXV. The Heart {Advanced). By L. H., 149
XXXVI. The Heart {continued). The Action of Drugs. By L. H., 151
XXXVII. Gaskell's Clamp and the Effect of Local Warmth on the
Heart {Advanced). By L. H., 154
XXXVIII. Action of the Cardiac Nerves {Advanced). By L. H., 156
XXXIX. The Pulse {Advanced). By L. H., 161
XL. Vaso-motor System {Advanced). By L. H., 164
XLI. Intracardiac Pressure. Blood Flow {Advanced Demonstra- tions). By L. H., 166 XLII. Effect of Haemorrhage and Saline Transfusion {Advanced
Demonstrations). By L. H., 171
XLIII. Respiration. By M. S. P., 174
XLIV. Intrathoracic Pressure. By L. H., 176
XLV. Ventilation of the Lungs. The Spirometer and the
Stethograph. By M. S. P., 177
XLVI. Chemistry of Eespiration. By M. S. P., 180 XL VII. Determination of the Respiratory Exchange in Man.
By M. S. P., 184
XL VIII. Eespiration Apparatus. By L. H., 186 XLIX. TJie Chemistry of Respiration. The Gases of the Blood.
By L. H., 187
L. The Oxygen Capacity of Blood. By M. S. P., 190 LI. The Effects of Changes in Atmospheric Pressure. By
L. H., - 192
LII. The Influence of Carbon Monoxide. By M. S. P., - 194
LIII. The Regulation of Respiration. By M. S. P., 195
LIV. Cheyne-Stokes Respiration. By M. S. P., 197
LV. The Influence of the Vagus upon Respiration. By
M. S. P., 199
LVI. Animal Heat. By M. S. P., 200 LVII. Investigation of the Motor Functions of the Alimentary
Canal by means of the X-Rays. By A. F. Hertz, 203
LVIII. Salivary Secretion. By L. H., 210 LIX. The Functions of the Central Nervous System. By
M. S. P., 214
LX. Reaction Time. By M. S. P., 216 LXI. The Rate of Discharge of Nervous Impulses from the
Central Nervous System. By M. S. P., 217
V CONTENTS
CHAF. PAGE
LXII. The Functions of the Anterior and Posterior Roots of the Spinal Cord. The Bell-Majendie Law. By M. S. P., - 220
LXIII. MUUer's Law of the Specific Energy of Nerves. By
M. S. P., 222 LXIV. Cutaneous Sensations. Sensations derived from Move- ments (Advancd). By J. S. E., 223 LXV. The Dissection of the Eye. By J. S. E., 226 LXVI. The Eye as an Optical Instrument (Advanced). By
J. S. E., - - - - 228
LXVII. The Refracting Media of the Eye. By J. S. E., 232
LXVIII. The Retina. By J. S. E., 240
LXIX. Sensations of Light and Colour. By J. S. E., 245
LXX. Binocular Vision. By J. S. E., 254
LXXI. The Optical Defects of the Eye. By J. S. E., 258
LXXIL The Optical Defects of the Eye (Advanced). By J. S. E., 261
LXXIII. The Instruments used in the Clinical Investigation of
the Eye. By J. S. E., - 263
LXXIV. Dissection of the Ear in the Sisate. Auditory Sensations.
By J. S. E., - 267
PART II.
PHYSIOLOGICAL CHEMISTEY.
By J. J. R. MACLEOD and M. Flack.
Introduction, 270
I. Carbohydrates. By J. J. R. M., 272
II. Carbohydrates {continv^d). By J. J. R. M., - 279
III. Carbohydrates (continued). By J. J. R. M., - 288
IV. Proteins. By M. F., 297 V. Proteins (continued). By M. F., 304
VI. Fats, Fatty Acids, Phosphorised Fats and Cholesterol. By
J. J. R. M., 313
VII. Milk. By M. F., - 324
VIII. Blood. By M. F., 331 IX. The Spectroscopic Examination of Haemoglobin and its
Derivatives. By J. S. Edkins, 342
CONTENTS XV
CHAP. PAGE
X. Muscle. By M. F., 350
XI. Dietetics, Food, Metabolism. By M. F., 35&
XII. Digestion in the Mouth. By J. J. R. M., 371
XIII. Digestion in the Stomach. ByJ. J. E. M., 375
XIV. Digestion in the Intestine. By J. J. R. M., - 387 XV. Bile. Bacterial Digestion. By J. J. R. M., 397
XVI. The Chemistry of Urine. By J. J. R. M., 407
XVII. Urea. By J. J. R. M., - 415
XVIII. Uric Acid and other Purine Bodies. By J. J. R. M., 422 XIX. The Inorganic Acid Radicles of Urine. Urinary Deposits.
By J. J. R. M., 434
XX. Pathological Urine. By J. J. R. M., 442 XXI. Lactic Acid, its Estimation and Significance. By J. H.
Ryffel, 454
XXII. Haemolysis and Precipitins. By J. J. R. M., 460
XXIII. The Pigments of Urine. By J. S. Edkins, 467
Appendix— Analytical Tables. By J. J. R. M., 469
Index — Part I., Experimental Physiology, 474
Index — Part II., Physiological Chemistry, 478
WEIGHTS ABB MEASUEES.
Length.
Metric or Decimal. English.
I Metre (M.) - =39-3701 inches.
1 Decimetre (dm.) . . . = 3-9370
1 Centimetre (cm.) - = 0-3937
1 Millimetre (mm.) - = 0-0393
1 Micromillimetre (mkni) - =0-000039 „ The nnit of the Metric System is the Metre, which represents one ten-milliontb
part of a quarter of the meridian of the earth. The multiples and subdivisions are obtained by the use of decimals ; the former being designated by Grreek prefixes, the latter by Latin prefixes.
1 Myriametre (Mm.) = 6-2137 miles.
1 Kilometre (Km.) - - = 0-6214 „
1 Hectometre (Hm.) =109-361 yards.
IDekaraetre (Dm.) - = 32-8084 feet.
1 Metre (M.) • - - = 39-3701 inches.
Weight. Metric or Decimal. 1 Kilogramme (Kgm.) 1 Gramme (Gm.) 1 Decigramme (dgm.) 1 Centigramme (cgm.) 1 Milligramme (mgm.) The unit is the Gramme which represents the weight of water at 4° C.
English. = 2,-2046 pounds. = 15-4323 grains. = 1-5432 „ = 0-1543 „ = 0-0154 „
cubic centimetre of
Apothecaries Weight. 437 "5 grains (gr. ) =1 ounce. 16 ounces {§) =1 pound (lb.).
I 60 grains ! 20 grains
= 1 drachm (3). = 1 scruple O).
1 grain =0-0648 gramme.
*Not official.
Avoirdupois Weight. 16 drachms = 1 ounce (oz. ). 16 oz. =1 pound (lb.).
28 lbs. =1 quarter (qr.).
4 quarters = 1 hundredweight (cwt. ) 20cwt. =lton.
1 pound =453-592 grammes. 1 ounce = 28-35 grammes.
WEIGHTS AND MEASURES Capacity.
Metric or Decimal. 1 Dekalitre (Dl.) 1 litre (L.) 1 Decilitre (dl.) - 1 Cubic centimetre (c.c.)
or 1 MUlilitre (ml.)
}■
English. = 2-1998 Imperial gallons. = 35-196 Imperial fluid ounoee. = 3-5196 „
= 0-0352
60 minims (Tn.) 8 fluid drachms -
20 fluid ounces 8 pints
1 cubic centimetre 1 fluid ounce 1 pint - 1 gallon
= 1 fluid drachm (3). = 1 fluid ounce (5). = lpint(0). = lgaUon{C).
= 16 '9 minims. = 28*42 cubic centimetres. =568-34 cubic centimetres. = 4-54 litres.
THERMOMETERS.
Fahrenheit and Cbktigeade Scales.
To convert degrees F. into degrees C, deduct 32, multiply by 5, and divide by 9. To convert degrees C. into degrees F., multiply by 9, divide by 5, and add 32.
r.
212°
112
106
104
102
101
100
99
98
97
C.
100°-0 44-4 41-1 400 38-9 38-3 37-8 37-2 36-7 36-1
|
F. |
C. |
|
80- |
26-7 |
|
70 |
21-1 |
|
60 |
15-6 |
|
50 |
10-0 |
|
41 |
5-0 |
|
32 |
0-0 |
|
23 |
- 5-0 |
|
14 |
-10-0 |
|
5 |
-150 |
AVERAGE WEIGHTS AND HEIGHTS.
Average weight of a healthy male child at birth • - . = 6-8 lbs. It .. ,. six months' old ■ =12*4 ,,
J. » ,, twelve ,, - =18-8 „
An adult man (dressed) 5 feet 8 inches in height, should weigh 11 st. 1 lb. and should have a chest circumference of 38J inches.
PAET I.
MUSCLE AND NERVE. CIRCULATION. RESPIRATION.
ANIMAL HEAT. CENTRAL NERVOUS SYSTEM
AND SPECIAL SENSES.
THE PHYSIOLOGY OF MUSCLE AND NERVE.
Introduction. — Physiology, the study of the properties of living organisms, can be properly appreciated and learned only when it is approached from the practical and experimental side. The study of the simplest forms of life, the unicellular organisms, is as yet only in its infancy, and at the present moment experimental physiology deals almost entirely with the functions of the various tissues and organs which together make up a vertebrate animal.
The cold-blooded vertebrate, the frog, is the most suitable animal for elementary experiments upon muscle and nerve; it is readily obtained, and its tissues under suitable conditions retain their vitality for many hours after they have been excised and cut off from their supply of blood.
The muscular, nervous, and vascular systems of the frog are the most important in an experimental course of physiology, for although muscle and nerve are highly differentiated forms of protoplasm with correspondingly characteristic functions, yet they show only in an exaggerated way properties which are common to all living matter. Thus in muscle the power of contraction or movement is highly developed; in nerve the property of excitability or irritability, the
response to a stimulus.
A
CHAPTER I.
ELECTRICAL APPARATUS FOR PHYSIOLOGICAL EXPERIMENTS.
In experimental physiology the stimulus most frequently used is an electrical one, for it is convenient, easily graduated, and less injurious to tissues than efficient thermal, chemical, or mechanical stimuli would be.
The Daniell Cell, which has an electromotive force (e.m.f.) of 1"1 volts, is the best source of electricity, for it yields an almost constant strength of current. It consists (Fig. 1) of (i) a plate of copper dipping
into a solution of copper sulphate which is kept saturated by crystals of the salt, and (ii) a rod of amal- gamated zinc placed in a porous pot filled with a 10 per cent, solution of sulphuric acid; the porous pot is surrounded by the solution of copper sulphate. The whole is generally placed for convenience in a glazed earthenware pot with a handle.
When the copper and zinc elements are connected by a wire the zinc dissolves in the sulphuric acid, forming ZnSO^ -i- H2, The H ions thus liberated become charged with the electricity originally stored in the zinc; they migrate through the porous cell into the copper sulphate and split it up into HgSO^ + Cu, and their charge of electricity is transferred to the Cu ions. These in turn deliver up their charge of electricity to the copper plate and, as they discharge, become deposited on the plate as metallic copper.
Thus inside the cell electricity passes from the zinc, or positive
' ^BoriAofiMioj'e Pot Flo, 1. — Diagram of a Daniell cell seen in section.
ELEMENTARY EXPERIMENTAL PHYSIOLOGY 3
element, to the copper or negative element; outside the cell the current passes from the copper binding-screw, the positive pole or anode of the battery, to the zinc binding-screw, the negative pole or kathode.
If plates of copper and zinc were simply immersed in 10 per cent, sulphuric acid, the chemical action set up would soon cause the copper plate to be covered with bubbles of hydrogen gas. This would cause a resistance to the ilow of current inside the cell, and further, hydrogen being electro-positive to zinc, a polarisation current in the opposite direction to the original battery current would be set up in the cell and rapidly reduce its E.M.F. Daniell, by placing the copper plate in a solution of copper sulphate, which the hydrogen splits up, prevented polarisation from taking place within the battery.' Therefore as long as there is free sulphuric acid present and the copper sulphate is saturated, the current produced by the cell remains constant. Pro- vided too that the porous pot, which is to prevent the deposition of copper on the zinc rod, remains permeable to the H ions.
The zinc rod has to be amalgamated because commercial zinc con- tains iron and other metallic impurities ; these in the presence of the sulphuric acid would, with the zinc, constitute a number of minute batteries. By covering the impurities with zinc amalgam their dis- turbing action is removed, and as the zinc is dissolved away, the mercury combines with fresh zinc so that the electromotive properties of the zinc rod remain constant.
' A more accurate description of the chemistry of a Daniell cell is as follows : The cell consists of two metals, zinc and copper, dipping into an electrolyte containing various ions in solution ; these are H, SO4, OH, Cu and SO4, of which Cu and H, being positive ions, will work their way towards the negative element, the copper plate and the OH and SO4 being negative ions towards the zinc. When in use chemical changes take place around both metallic plates. The ziuo is attacked by the SO4 ions discharging, forming ZnS04, and energy is liberated, which is conducted across the electrolyte by the ions in solution. Around the copper plate the copper sulphate is being split up into SO4 and Cu ions, in which process energy is stored up. But the energy liberated at the zinc plate is greater than that stored in the neighbourhood of the copper plate, therefore the cell, when working, is always liberating a balance of energy which appears as an electric current. The SO4 ions, constantly being liberated in the copper sulphate solution and charged with electricity, migrate through the porous pot towards the zinc, discharge forming ZnS04 and a liberation of energy as explained. Towards the copper plate both H and Cu ions charged with electricity are constantly streaming. That it is the Cu ions and not the H ions which discharge and become precipitated on the plate depends simply upon the fact that it requires a less energy and a lower e.m.f. to separate Cu than H ions. Therefore as long as there are sufficient Cu ions present to conduct the current, Cu ions and not H ions will discharge and be precipitated on the copper plate.
PEACTICAL PHYSIOLOGY
Keys are instruments for making or breaking electrical circuits and for short-circuiting currents.
The Mercury Key consists of a small cup hollowed out of a piece of vulcanite (Fig. 2). From the cup, which is nearly filled with clean mercury, pass in opposite directions two stout copper wires with
Fio. 2.— The mercury key.
Fio. 3. — The spring key.
binding-screws ; one wire and binding-screw are fixed to the vulcanite base, the other wire can be raised out of or lowered into the mercury by an insulated handle. In some forms of mercury key the wires connecting the binding-screws to the mercury cup run through the vulcanite ; the ends of these wires are liable to become oxidised and
dirty, and in consequence they make
J bad contact with the mercury. In
I order to avoid this it is only necessary
L to fix the insulated wires from the
^ ^fy '^ ^^ii^Tcy to the binding-screws and to
r' * iiPiTO_jBBiBM turn the naked ends of these wires
<(i I lOflHRRH over into the mercury.
The Spring Key is made of a block of lacquered wood, to one end of which is attached a broad brass spring with a binding-screw, and co the other end a plate of brass with a binding-screw (Fig. 3). When the spring is depressed by the finger its free end touches the brass plate and connects together the two binding-screws. The brass plate carries a clip which can clamp the spring in contact with the plate. The Du Bois Key consists of two metal blocks each carrying two binding-screws and attached to a vulcanite base (Fig. 4). The
Fig. 4.— The Du Bois key.
ELEMENTARY EXPERIMENTAL PHYSIOLOGY. 5
metal blocks can be connected by a thick brass bar attached to an insulated movable handle. This key, like the mercury and spring key, may be used as a simple make and break key (Fig. 5) ; but its
FlQ. 5. — Plan of the use of a Du Boia key, as a simple make and break key.
Flo. G. — Arranged as a short-circuiting key : key shut.
Fw. 7. — Arranged as a short circuiting key: key open.
proper use is as a short-circuiting key (Figs. 6 and 7) ; and when a Du Bois key is directed to be used, it must be inserted into the circuit as a short-circuiting and not as a simple key.
A +
Fio. 8A.^The Pohl'a reverser. A and B the two side cups ; C, Bf B and P the four corner cups ; S the handle made of glass or vulcanite.
Fig. 8b.— Universal key (Gotch). The key is used by rotating the arm containing the screws connected with the wires A aud B, which come from the battery. In the position shown the current flows from the wire of C to that of D ; if rotated through 45° there is a complete double break of the battery-circuit ; if rotated through 90' then the current is remade and the current flows from the wire of B to that of C.
6 PRACTICAL PHYSIOLOGY
The PoM's Reverser consists of six mercury cups hollowed out in a block of vulcanite, each cup being connected to a binding-screw (Fig. 8a). The four corner cups are connected diagonally by stout copper wires which do not touch each other. The two side cups are
joined by stout copper wires to a non-conducting cross-piece, which acts as a handle Each end of the handle also carries a semicircle of copper wire which is connected to the wire going into the side cup, and is of such a length that it will dip into the cup at either end by turning the handle over towards that end. If the handle is in such a position that a current, entering the reverser by one of the side cups, emerges by an end cup of the same side, then, by turning the handle over, the cross-wires come into use, and the current will now emerge by the end cup of the opposite side.
The instrument may also be used to send a current into either of two circuits. The cross-wires are removed, the wires from the battery are connected to the two side binding-screws, and to each pair of end cups the wires of the two alternative circuits (Fig. 9). Then by turning the handle over the current may be sent into either of these two circuits. A much more efficient instrument is the universal key (Fig. 8b), which has recently been introduced by Gotch. It can be used as a double break-key, a reverser and a shunt.
The term Electrodes is applied to the free ends of the two wires which conduct the current to the tissue to be stimulated. They consist
Fio. 9. — Plau of the arrangement of the two alternative circuits.
^*jm!lMSlU-
Pih
Flos. 10 AND 11. — Two forms of electrodes.
of two insulated wires, the ends of which are clean and free from insulating material, carried in some form of holder ; this is generally made by running the wires through a piece of vulcanite, cork, or model-
ELEMENTARY EXPERIMENTAL PHYSIOLOGY
ling wax (Figs. 10 and 11). A form of electrode sometimes very useful is made by soldering the free end of each wire to the head of a needle. The Bheochord is used to alter the strength of a constant current to be sent through a muscle or nerve. In its simplest form it consists of
vmm
;^V HS —
■«"Br
^^ ' J
Fio. 12, — Simple form of monochord.
a single straight or zig-zagged wire with a binding-screw at either end and a movable contact between them (Fig. 12). If a Daniell cell be
4 +
Fig. 13. — To illustrate the principle of the monochord,
connected to the two ends of the monochord A and B (Fig. 13), there will be a fall of potential in it from A to B. If from A and the
Fio, 14, —The rheochord arranged to vary the strength of a current passing through a nerve. It consists of two pai-allel wires connected by a movable metal slider S. By moving the slider S to the right the resistance of the rheochord in circuit and therefore the amount of battery current passing through the nerve would be in- creased.
movable contact S two electrodes pass to a nerve, the current from the battery has two circuits open to it and can pass either through the nerve or along the monochord back to the battery. The
8 PRACTICAL PHYSIOLOGY
amount of current which will pass through the nerve will be directly proportional to the difference in potential between A and S, i.e. if the fall in potential in the monochord is uniform, proportional to the distance between A and S, being greater as S is moved away from A ; it is also inversely proportional to the resistance of the circuit through the nerve. But the resistance of this circuit may be considered con- stant for all positions of S, since the resistance in the nerve itself is enormously greater thau that caused by any change in the length of the monochord wire in the circuit.
Although the Daniell cell is the most convenient source of current, and its strength can be regulated by a rheochord, and although the
Fig. 15.— The induction-coil.
make and break of a constant current do act as a stimulus to muscle and nerve, it is often more convenient to use induced currents. These are obtained by connecting a Daniell cell to an induction coil, and their advantages are : (1) That being of extremely brief duration as com- pared with the make of a constant current, they set up practically no polarisation in the tissues (see page 78). (2) Having a comparatively large b.m.f. and rapid development, as compared with the galvanic current, they constitute a much more effective stimulus. For, the law of excitation states that the effectiveness of a current as a stimulus depends not only upon the total variation in its intensity, but also upon the amount of such variation in the unit of time, i.e. the greater the rapidity of the total variation, the more effective is the current as a stimulus.
The Induction-coil (Fig. 15) consists of two coils, of which the primary is made up of a few turns of insulated thick copper wire with only a small resistance. This is wound round a core of iron wire to increase the number of lines of magnetic induction which pass through it. The ends of the wire forming the primary coil are con- nected with the top binding-screws 1 and 2 (Fig. 16).
The secondary coil is made up of a large number of turns of insulated
ELEMENTARY EXPERIMENTAL PHYSIOLOGY 9
fine copper wire. The large number of turns of wire in the secondary as compared with the primary coil, transforms the low E.MF. of the current in the primary circuit into a high E.M.F. in the secondary circuit; for each turn in the primary coil induces an effect in every turn of the secondary coil, so that the sum of all these effects is a single one of greatly increased intensity.
The long fine wire of the secondary coil gives it a great resistance, but when the induced currents are passed through the relatively enor- mous resistances of animal tissues this is unimportant.^
The ends of the wire of the secondary coil are connected to the binding-screws 3 and 4 (Fig. 16).
The E.M.F. of the induced current varies with the following factors : (1) It varies directly with the intensity of the change of current in the primary circuit, so that if no current or a current of constant strength be running through the primary coil no induction occurs ; but if the strength of the current in the primary circuit does change, whether it be an increase or decrease, the greater the change the stronger will be the induction. (2) It varies directly as the rate of change in the
Flo. 16. — DiagraiD of an induction-coil and its connections.
strength of the inducing current, so that, if the constant current be increased or decreased greatly in strength, but sufficiendy gradually, no induction takes place ; on the other hand, for a given change in the constant current the more rapid the change the greater the induction. (3) It varies with the angle between the primary and secondary coils in such a way that when the two coils are accurately at right angles there is no induced current ; but the strength of the induction increases as the angle between the coils is altered until the maximum is reached, when the wires are parallel to each other. If the secondary coil be
^The resistance of a piece of a frog's sciatic nerve 1 cm. long is about 100,000 ohms.
10 PRACTICAL PHYSIOLOGY
movable horizontally on a central point, the strength of the induced current can be graduated by altering the angle between the two coils. (4) It varies inversely as the distance between the two coils, being greatest when the secondary is completely over the primary coil, and becomes less and less as the coils are separated. The strength of the induced current is usually regulated by varying the linear distance between the coils, and most induction-coils are graduated by a milli- metre scale fastened to the side of the carrier, so that the pointer in the secondary coil is at the zero of the scale when the one coil is exactly covered by the other. This graduation, however, is purely arbitrary, for the absolute decrease in the strength of the induced current becomes less and less for every centimetre that the coils are separated. An exact graduation can be obtained by a scale corresponding to equal galvanometric deflections.
The direction of the induced current in the secondary coil is, at make of the battery-circuit, in the opposite direction, and at break of the battery-circuit, in the same direction as the battery-circuit in the primary coil. Most coils are so wound that when at make the battery current enters the primary coil by one top binding-screw, the induced current leaves the secondary coil by the binding-screw of the opposite side (Fig. 16).
The Use of Make- and Break-Induction Shocks as Stimuli. — Two wires are connected with the poles of a Daniell cell ; the free end of one wire is fastened to one binding-screw of a spring-key, and to the other screw of the key is fixed a third wire. The clean free ends of the wires are placed on the tongue, and the key is opened and closed ; no shock is produced, but only a sensation of taste ; the intensity of the current is insufficient to produce a marked excitation.
The free ends of the wires are now connected with the screws, or terminals, 1 and 2 of the induction-coil and a Du Bois key is placed in the secondary circuit (Fig. 16). The secondary coil is pushed far apart from the primary, and the Du Bois key is opened; make and break of the primary circuit produces no excitation, for the induction- currents are too weak. The secondary coil is gradually moved towards the primary, and the spring-key is opened and closed from time to time, until a point is reached at which a shock is felt at break, but not at make of the constant current. The position of the secondary coil on the scale is noted. As the secondary coil is moved up further, the break-shock becomes greater, and a slight shock is also felt at make ; in a similar way the two shocks can be further increased, but the break-shock remains greater than the make-shock.
It is especially to be noted that there is no induction-shock if the
ELEMENTARY EXPERIMENTAL PHYSIOLOGY 11
primary circuit remains closed by the spring-key. An induction shock is produced only at the make or the break of the constant current.
Closure of the Du Bois key short-circuits the electrodes, and no shock will be felt on make or break of the constant current. By means of this key the make- or break-induction shock, or both, can be shut off from the electrodes.
The secondary coil is now removed from the grooves of the carrier, and is placed close to, but at right angles to, the primary coil : no shock is produced when the primary circuit is closed or broken. The secondary coil is gradually turned on its vertical axis, and the spring- key is opened and closed from time to time. A shock will be felt first at break, then at make, and these will increase until the maxima are reached when the secondary coil is parallel to the primary.
These simple experiments show that the make and break of a galvanic current can act as weak stimuli; that on connecting the Daniell cell with the induction-coil induced currents are produced in the secondary coil only at make and break of the battery-current and not when it is running with constant strength through the primary coil ; that the induced currents are very effective stimuli, can be easily graduated in strength and short-circuited by a key. It has further been shown that the break induction-shock is stronger than the make. The cause of this difference lies in the primary coil, and needs explanation.
When the battery-current enters the primary coil, it induces a current in it as well as in the secondary coil. This " self-induced " or make extra current, like that induced in the secondary coil, is a momentary current in the opposite direction to the battery -current ; hence it delays the rapidity with which the battery-current reaches its maximal intensity in the primary coil and weakens the effect which change in current in the primary coil will induce in the secondary coil. On the other hand, when the battery-current is broken, the current in the primary coil suddenly runs down to nothing; and although a break extra current, running momentarily in the same direction as the battery-current, is induced in the primary coil, it cannot delay the rapidity of the fall of the battery- current, because a primary circuit no longer exists in which the extra current could run.
Demonstration of the Break Extra Current. — Connect a cell with Hnding-screws 1 and 2 of the induction-coil, placing a spring-key in the circuit. Fasten to the same binding-screws of the primary coil two wires, the free ends of which are placed on the tongue. On closing the spring-key no shock is felt, but, on opening it, the shock of the break extra current.
12 PRACTICAL PHYSIOLOGY
A purely ph3'sical proof of the break extra current can be obtained by connecting one pole of a battery to the primary coil, and by touching with the other wire from the battery the milled head of the other binding-screw of the primary coil. Every time that the battery circuit is broken, the break extra current will pass across from the screw to the wire as a minute spark ; no spark, or a very feeble one, is seen on touching the first terminal, for in this case there is no current in the primary coil.
EcLualisation of Make and Break Induced Currents. — From what has been said it is clear that, if the break extra current were provided with a circuit to run in, the strength of the current induced in the secondary coil at break would be reduced to that of the current induced at make; and so they would be equalised. In order to effect this the battery-circuit is not broken, but is nearly completely short-circuited out of the primary coil by a Du Bois key (Fig. 17). Now again test the relative strengths of the make and break induced currents.
S.C.
f xo. 17. — Arransfemeut of apparatus for equalising the make and break iuduced currents.
They may be approximately equal, but the original difference is not infrequently overcorrected, and now the break-shock is the weaker. This is caused by the make and break extra currents running in circuits of different resistance. At make the extra current runs not only through the primary coil but also through the resistance of the Daniell cell ; but at break the extra current has to run only through the resistance of the primary coil, hence it is the more effective current of the two, and reduces the effect induced in the secondary coil at break more than the make extra current does on closing the primary circuit.
Faradic or Tetanising Shocks. — Induction-coils are provided with an automatic arrangement for rapidly making and breaking the primary circuit by means of Wagner's hammer. Connect up the battery to screws 5 and 6 of the coil, interposing a spring-key, and follow out the primary circuit (Fig. 18). The current passes up the pillar A along the spring H to the screw Sj, through the primary
ELEMENTAEY EXPERIMENTAL PHYSIOLOGY 13
coil to the electro-magnet E, and so to the pillar B. "When the circuit is thus made, E becomes an electro-magnet, pulls down the spring H from its contact with Sj and breaks the circuit ; consequently E ceases to be a magnet, the spring flies up into contact with S,, and again
Fig. 18. — Diagram to show the actlou of Wagner's hammer.
completes the circuit. The number of times the circuit will be thus made and broken per second depends upon the length of the spring H ; in most coils it is of such a length as to give 50 complete vibrations per second. At each make and break of the circuit a current is induced in the secondary coil, just as when the circuit was broken by hand ;
Via. 19. — Diagram to show the action of the Helmholtz side-wire.
further, the break-shock is stronger than the make-shock, and for the sime reason as before.
Determine the distance necessary between the two coils for the shocks just to be felt on the tongue.
14 PRACTICAL PHYSIOLOGY
Helmholtz showed that it is possible to equalise these Faradic shocks by short-circuiting, instead of completely breaking, the battery-current, and for the reason already explaiited. For this purpose (Fig. 19) a stout wire, W, connects the binding-screws 7 and 1, Sj is screwed up out of reach of the spring, and Sj is screwed up. Follow the circuit of the current which passes from binding-screws 7 to 1 by the side-wire, and so to the primary coil, back to the electro-magnet E, to binding-screw 6 and to the battery. When, however, the current reaches £, it becomes a magnet, and pulls down the spring into contact with Sj. This short-circuits the battery-current out of the coil, for the current will now pass from the pillar A, by way of H, to the pillar B, and so back to the battery. There is still left the circuit 7 W, 1, PC, E, H, A, 7, in which the break extra current can run and reduce the strength of the current induced in the secondary coil at break.
Determine the distance between the coils at which the shocks are now just felt on the tongue ; it will be found to be reduced, showing that the break-shock which was alone felt before has been reduced down to or even below the strength of the make-shock.
CHAPTER II.
THE GRAPHIC METHOD. MAXIMAL AND MINIMAL STIMULI. UNIPOLAR EXCITATION.
The graphic method is applied to muscle in order to obtain a permanent magnified record of the change in form of a muscle during contraction, and further, to investigate the time-relations of the con- traction. For this purpose it is necessary to describe the method of preparing the muscle and then three special pieces of apparatus: (1) a magnifying lever, the muscle lever, or myograph, which can write on (2) a surface either stationary or moving at a uniform rate, the drum, and (3) an instrument for recording time on the drum, the chronograph, which will be described in Chapter III.
The Muscle- and Nerve-Preparation. — The quickest way to kill a frog is to "pith" it. The articulation between the skull and the vertebral column can be felt with the tip of the finger ; it is severed by a transverse cut with a pair of scissors, and then a probe or blanket- pin is inserted into the skull to destroy the brain. The spinal cord is destroyed in a similar way, and this final stimulation of the nerve-cells
ELEMENTARY EXPERIiVIENTAL PHYSIOLOGY
15
causes a discharge of motor impulses to the muscles of the body, which give a series of convulsive twitches or contractions. These twitches quickly cease, the body and limbs are in a toneless, relaxed condition, and all reflexes have been abolished.
The frog is then placed belly downwards on a frog-board, and the skin at the ankle is divided by a circular incision ; the tendo-Achillis is exposed and a thread passed under the tendon and tied just above the sesamoid bone. In this way a ligature is attached to the muscle with-
Flo. 20. FlO. 21.
Muscles of the frog's leg. (After Ecker.)
Fio. 20. — Dorsal aspect.
1. Trineeps femorls.
2. Biceps femoris.
3. Rectus internus.
4. Semi-membranosus.
5. C^strocneiDius. & Tendo Achillis.
Fig. 21. — Ventral aspect.
1. Rectus Internus.
2. Gracilis.
3. Adductor longus.
4. Vastus internus.
5. Sartorius.
6. Adductor brevis.
7. Adductor magnus.
8. Gastrocnemiiis.
9. Tendo Achillis.
out damage to or irritation of its fibres. The tendon is divided below the sesamoid bone, and a pull upwards towards the knee frees the gastrocnemius muscle and the skin from the remaining structures of the leg, which are cut away just below the knee. The gastrocnemius muscle is protected from drying and from contact with foreign sub- stances b}' drawing down the "trouser" of skin. The sciatic nerve is now dissected in the following way. The skin over the posterior surface of the thigh is divided by a longitudinal incision in the middle line, the biceps and semi-membranosus muscles are separated, and the sciatic nerve is exposed. The nerve must not be pinched with forceps, for it is easily damaged. The muscles on each side of the urostyle and then the urostyle itself are cut away; the three constituent ends of the sciatic nerve are now exposed. The spinal column is divided transversely between the 6 th and 7th vertebrae and the 9 th, 8th, and 7th vertebrae are bisected. The piece of bone, from which the nerve to be prepared issues, can be grasped with the forceps without damage
16
PRACTICAL PHYSIOLOGY
to the nerve, and the sciatic nerve is freed from the surrounding tissues as far as the knee. The thigh is then severed from the body by a
Fig. 25. Fig. 23.
Diagrams of a muscle- and nerve-preparation. (Pembrey and Phillips.) Fig. 22. — The first stage of dissection.
Fig. 23. — The second stage of dissection. The sciatic nerve exposed and the gastroc- nemius muscle covered by skin.
transverse cut close to the articulation of the head of the femur (Figs.
22 and 23). In order that the best results may be obtained the muscle- and
nerve-preparaiion should be as fresh and irritable as possible, and in order to obtain this the following precautions should be observed, (a) All apparatus for the experiment should be in working order before the dissection is commenced. (6) The muscle must be prevented from drying by keeping the "trouser" of skin pulled down over it, and since the nerve is even more easily killed by drying, it should, when not required for immediate stimu- lation, be allowed to lie among the muscles of the thigh, the lymph of which will keep it The nerve must not be placed upon the frog's
Fig. 24. — The crank-lever, muscle-board and stand,
moist and irritable.
ELEMENTARY EXPERIMENTAL PHYSIOLOGY
17
skin, the secretions of which quickly injure it. (c) When the nerve is on the electrodes it must be kept moist by normal tap-water saline solution (-70 per cent, sodium chloride in tap-water) upon a piece of filter-paper, but care must be taken that the current from the electrodes is not short-circuited thereby, (d) The nerve itself should not be picked up by forceps, but should be lifted by the pieces of the vertebral column. Consequently the whole length of the nerve should always be dissected out; as a rule it should not be cut across in the thigh nor simply exposed in the thigh and two electrodes pushed under it.
The Muscle-lever takes one of two chief forms :
(a) The crank-lever (Fig. 24) consists of an L-shaped piece of metal, the horizontal arm of which is long and carries the writing
«==-
Fio. 26. —The simple lever with after-loading acrew. F, clamp ; L, lever j M, muscle.
point, whilst the vertical arm is short and to this the thread round the tendo-Achillis is firmly tied. The muscle rests, in the same straight line as the lever, on the muscle-board, a horizontal piece of wood covered with cork. The whole is carried on a vertical stand (Fig. 24), the arm of which is movable on the base, so that the writing point of the myograph can be swung towards and away from the drum without altering the position of the base of the stand. When the thread has been tied to the lever, a pin is pushed through the lower end of the femur into the cork; this gives the muscle a fixed point from which to pull. It is necessary to see that, when the muscle is at rest, the thread attached to the lever is taut, and that there is no "slack" to be taken in when contraction begins; further, the writing arm should be horizontal.
In this form of lever the movement of the writing point is at right
18
PRACTICAL PHYSIOLOGY
angles to the movement recorded. The magnification of the movement of the muscle recorded by the lever is calculated by dividing the dis- tance of the writing point from the axis by the distance from the axis of the point of attachment of the thread from the tendon. The nearer to the axis the muscle is attached the greater will be the magnification. It is quite sufficient to magnify the movement of the muscle 5 times. (6) The simple lever (Fig. 25) consists of two parts : a rigid femur- clamp to hold the piece of femur, and a horizontal writing lever below it to which the thread on the tendo-Achillis is tied. Care must be taken that the femur-clamp and lever lie in the same plane, and that
the muscle is tied to a point on the lever vertically below the clamp. In this case the movement of the writing point is in the same plane as that of the movement recorded. The magnification, as before, is calculated by dividing the distance of the writing point from the axis by the distance of the point of attachment of the muscle from the axis.
The writing lever must be as light as possible (see page 27, Chap. III.), but it must be sufficiently rigid to prevent vibra- tions being set up in it. For this purpose writing levers are generally made of light metal, glass, Japanese cane or straw.
The actual writing point is made of thin metal foil or moderately sti£f paper bent at its free end slightly over towards the drum. The writing point must lie as nearly as possible parallel to the recording surface, or, in other words, at right angles to a radius of the drum. Further, the bend near its end is necessary; it acts as a weak spring and keeps the writing point up against the recording surface in difierent positions of the lever. For the end of the lever describes a curved line, and the more it leaves the horizontal position the greater will be the distance of the end of the straw from the recording surface.
The Kymograph or recording drum (Fig. 26) consists essentially of a stout brass cylinder which is made to revolve round a vertical axis by either clockwork or string belting from a motor. It is necessary to have some arrangement by which the speed of revolution can be altered within wide limits ; this is obtained by various mechanical devices in different patterns of drum, one of which is shown in Fig. 26.
"imiaTAiioeK. Fio. 26. — Kymugraph.
ELEMENTARY EXPERIMENTAL PHYSIOLOGY 19
The drum is covered with white glazed paper, the surface of which is then blackened by a thin layer of soot, obtained by revolving the drum through either the luminous part of a broad gas flame or the smoke of burning turpentine or camphor. The writing point of the lever, as the drum revolves, rubs ofi' the layer of soot and leaves a white magnified image of the movement of the muscle or heart or whatever change is being recorded.
The white paper is of the same width and longer than the surface of the drum, and the under-surface of the overlap is gummed. The paper must be laid evenly and without wrinkles round the drum, the gum is then moistened and the paper fastened. The layer of soot obtained from the gas flame should be dark brown in colour, and care must be taken to revolve the drum suflSciently rapidly through the flame to prevent scorching or burning of the paper. The film of soot from camphor is less firmly attached to the paper, and must not be made too thick, otherwise the writing point does not, without undue friction, rub off' enough of it to leave a distinct tracing. In recording it must be so arranged that the tracing does not come at the overlap, for the joint in the paper is liable to make the point of the lever jump. Further, it is very important that the drum should be made to revolve away from and not towards the writing point, in other words, the tracing as it is taken should pass from the writing point, not towards but away from the lever. When the tracing is finished, the paper is cut through at the overlap and the details of the experiment written on it. The tracing is preserved by drawing it once through a varnishing solution ^ and pinning it up to dry.
This graphic method, as we shall see, introduces several errors, but such accuracy as it has must depend upon the drum remaining a true cylinder ; it is therefore very important that a drum should never be dropped or in any way dented.
Minimal and Maximal Stimuli. — If the strength of the stimulus applied to a muscle be varied within certain limits, it is found that the muscular response also varies, so that the greater the excita- tion the greater is the shortening of the muscle.
In order to demonstrate this, connect up a Daniell cell to an induc- tion coil so as to give single induction shocks, placing a mercury key in the primary circuit and a Du Bois key in the secondary circuit ; cover and smoke a drum. Dissect out a gastrocnemius preparation and attach it to the myograph lever, arrange the electrodes to stimulate the muscle directly; one needle-electrode is used which passes through
' A rapidly drying varnish is made by dissolving 250 c.c. of the best white hard varnish in a litre of methylated spirits and then adding 10 c.c. of castor oil.
20 PRACTICAL PHYSIOLOGY
and fixes the lower end of the femur; the other wire from the Du Bois key is joined to a piece of capillary copper wire which has been threaded by means of a needle through the tendo-Achillis. In this way the current can be passed through the length of the muscle, and the very fine wire will not cause any obstruction to the free movement of the muscle when it contracts. Bring the writing point on to the surface of the stationary drum.
With the secondary coil at 20 cm. and the Du Bois key open, make and break the primary circuit, no contraction will take place. Gradu- ally move up the secondary coil towards the primary, opening and closing the key in the primary circuit at each new position. With the secondary coil at about 16 cm. the muscle will contract at break but not at make, showing that the break induction shock is stronger than the make-shock. The contraction is recorded on the drum by a nearly vertical line, and shows a minimal contraction in response to a minimal stimulus; the make-induction shock is still a sub-minimal stimulus and no contraction results. Sotate the drum on a short distance by hand, move the secondary coil up 1 cm. and stimulate again. Repeat this process, moving the drum on after each contraction and increasing the strength of the stimulus after each make and break of the primary circuit (Fig. 27). As the strength of the stimulus is increased the contraction at break increases in height rapidly at first and then more
Fig. 27. — Heights of conttaction of a muscle with different strengths of stimuli. M marks the make and B the break of the primary circuit. The numbers refer to the distances in cms. of the secondary from uie primary colL (A.P.B.)
slowly until, with the secondary coil at about 7 cm., a point is reached beyond which the height does not increase. At 7 cm., therefore, the break-shock and the contraction which it causes are maximal. All stimuli intermediate in strength between minimal and maximal are called sub-maximal. At a certain point the make-shock will be found to become an effective stimulus and cause a minimal contraction. As the make-shock is increased in strength, the contraction rapidly increases in height until, with the secondary coil at about 7 cm.,
ELEMENTAEY EXPEKIMENTAL PHYSIOLOGY 21
it becomes maximal and of about the same height as the break contraction.
The higher the contractions become the more obvious is it that the writing point describes on the stationary drum, not a straight line, but an arc of a circle. The shortening of the muscle, after allowing for the magnification hj the lever, is measured not by the length of this arc but by a perpendicular line dropped from its highest point on to the base line.
It is necessary to point out here that, when the primary circuit is made — and the same is true if it be broken — a momentary induced current is both made and broken through the nerve, and yet there is only one contraction of the muscle. It has been found that in a current of such short duration the break stimulus is ineffective because it falls within the refractory period of the make stimulus (see Chap. VII., p. 40). In both cases, whether the primary circuit is made or broken, the effective stimulus to the nerve is only the make stimulus of the induced current.
Unipolar Excitation. — Connect a battery to a coil so as to give tetanising shocks ; connect a wire to one pole of the secondary coil and place its free end on the tongue. If the secondary coil be moved completely over the primary, faint shocks will be felt. The explana- tion of this phenomenon is that the making and breaking of the primary circuit causes free electricity to collect at the end of the wire connected with the secondary coil ; when the e.m.f. of this charge is sufficient to overcome the resistance of the tissues of the body, the circuit is completed through the body, the floor and desk, and so back to the other pole of the secondary coil. With the wire still on the tongue, touch the other pole of the secondary coil with a moistened finger; much more powerful shocks are felt because a more direct circuit from one pole to the other of the secondary coil has been provided.
Eepeat the experiment on a sciatic-gastrocnemius preparation in the following way, with either tetanising or single-induction shocks. Lay the preparation on a perfectly clean and dry glass-plate and place a wire connected with one pole of the secondary coil under the nerve; no contraction of the muscle takes place because the dry plate insulates the preparation and the secondary circuit cannot be completed. Now touch the muscle with a wire, the other end of which rests on a gas or water pipe; the muscle contracts because the circuit is completed through the earth. It is not even necessary that the conductor should touch the preparation, for, if a moistened finger is brought as near the muscle as possible without touching it, the muscle contracts, especially
22 PRACTICAL PHYSIOLOGY
if a moistened finger of the other hand touches the other pole of the secondary coil. In this case the human body acts like a condenser charged with electricity, which by its approach can stimulate muscle or nerve. Further, if the nerve be ligatured between the electrode and the muscle, or cut across and the two cut ends laid over eacjb other, which will prevent the passage of a nervous impulse along it, contrac- tion of the muscle is still produced, because the discharge takes place along the whole length of nerve and muscle between the electrode and the point by which the muscle is connected to the earth, so that any irritable tissue in the course taken by the charge is stimulated.
If, however, the muscle and nerve preparation is laid on an ordinary moistened muscle-board, the insulation is so slight that one electrode, connecting the nerve and the secondary coil, will by itself cause the muscle to contract.
It is in order to guard against accidental stimulation of muscle and nerve by unipolar action that a Du Bois key must always be placed in the secondary circuit, and must always be kept closed except when the tissue is being intentionally stimulated. The brass bridge of the key, which has many thousands of times less resistance than the tissue between the electrodes, affords a perfect closure of the secondary circuit and prevents static electrification of the electrodes.
Errors from unipolar action are liable to take place, especially in the study of the electromotive phenomena of muscle and nerve by the electrometer and galvanometer (see Chap. XVIII.).
CHAPTER III. A SINGLE CONTRACTION OF A GASTROCNEMIUS MUSCLE.
In order to study the contraction given by a muscle in response to a single stimulus, it is not sufficient to inspect the curved line traced by the myograph-lever on a revolving drum. It is also necessary to study the length of time occupied by the whole twitch and the time- relations of different parts of it. For this purpose a time-tracing must be simultaneously recorded by a special apparatus, which generally takes one of two forms.
(1) The Tuning Fork; to one prong of this a writing point, similar to that on the myograph-lever, is attached. With the writing point lightly touching the blackened surface of the drum, a sharp tap is given to the fork, and the drum set in motion ; care must be taken that the drum does not make more than one revolution, otherwise the time-
ELEMENTARY EXPERIMENTAL PHYSIOLOGY
23
tracing will run over itself. The number of complete vibrations per second and the time value of each will depend upon the note of the fork. The most useful fork is one that gives 100 complete vibrations per sec. When more rapid vibrations are required the above method is not suitable, because the vibrations of a fork of a higher note cease so soon after a single tap.
In order to obtain a time-tracing in ^^ths or less of a second, it is necessary to use —
(2) A Chronograph or time-marker, which records on a drum the number of times per second a current through it is made and broken by
Fio. 28.— A time-marker.
another special piece of apparatus. The chronograph (Fig. 28) consists essentially of an electro-magnet, which, when the current through it is made, attracts and pulls down a metal lever carrying a writing point. When the current through the electro-magnet is broken, a spring at the other end of the lever raises the writing point.
The apparatus used to make and break a current through the chrono- graph at any definite known rate is a tuning-fork of the corresponding note. To one prong of the fork is attached a platinum wire which.
J r'^T'^
Fio. 29. — A tuning-fork with electro-magnet.
with each complete vibration of the fork, makes and breaks the chrono- graph circuit by touching and receding from a brass contact or mercury cup (Fig. 29). The tuning-fork, when once started vibrating by a tap, is kept vibrating automatically by an electro-magnet in the same circuit (Fig. 30). Thus, when the platinum wire touches the mercury cup the battery current is made through the chronograph and the writing point is pulled down; at the same time the current is made through the other electro-magnet, which attracts the tuning-fork and pulls the platinum point away from the mercury. Both electro-magnets now cease to act, the writing point of the chronograph is pulled up by the
24
PRACTICAL PHYSIOLOGY
spring, and the platinum wire of the tuning-fork again touches the mercury, thereby making the circuit again.
To record the contraction of a muscle in response to a single maximal induction-shock, the apparatus is set up in the following way (Fig. 31).
Fia. 30. — Diagram of the chronograph circuit.
Connect one pole of a Daniell cell to one top binding-screw of the primary coil, and the other binding-screw of the coil to a binding-screw on the base of the stand of the drum. The current passes through the metal work of the stand to a metal striker carried beneath the drum on its axle. As the drum revolves this striker touches a strip of naked wire attached to, but insulated from, the rest of the stand. The
binding-screw in connec- tion with this naked wire is connected to the other pole of the battery. It is only when the striker and naked wire are in contact that the primary circuit is completed.
A sciatic and gastroc- nemius preparation is made and attached to the myograph-lever, which is grams, and should then be
Fio. 31. — Diagram of the apparatus for recording a single muscular contraction.
weighted near its axis with 10 or 20
horizontal. The nerve is laid across the electrodes coming from the Du Bois key, and the secondary coil is arranged to give maximal induction-shocks. A tuning-fork giving 100 complete vibrations per second is arranged to write just beneath the myograph lever. Before the two writing points are brought into contact with the smoked surface, the drum should be made to revolve in order to see that it will rotate away from the writing points and at a suffi-
ELEMENTAEY EXPERIMENTAL PHYSIOLOGy 25
ciently rapid rate ; the rate of rotation should not be less than 20 cm. per sec. Adjust the writing points to touch the smoked paper lightly, and with the Du Bois key open, and the fork vibrating, let the drum make one revolution and no more. The curve of the muscular contraction and the time below it in y^^ths of sec. will be recorded (Fig. 32). Close the Du Bois key, remove the tuning-fork, but do not alter the position of the base of the stand carrying the myograph. With the writing point of the lever accurately on the abscissa line of the muscle curve let the drum revolve so as to complete a base line beneath the actual curve corresponding to the muscular contraction. With the writing point still on the base line, rotate the drum by hand until the striker just touches the naked wire. At this position of the
Fio. 32.— Single contraction of gastrocnemius in response to a maximal make shock. Muscle loaded with lever and 30 grms. at axis of lever ; actual load on muscle, 6 grms. Magnification, 5. Temp., 15° C. Time marker, 100 per sec. (A.P.B.)
drum a maximal make induction-shock was sent into the nerve ; with the finger on the lever make the writing point describe a vertical arc, which cuts the time-tracing below and the abscissa line above. In the same way, by rotating the drum by hand, vertical arcs are drawn through the muscle-curve and time-tracing at the three following points : (1) the point at which the curve leaves the base line, (2) the highest point of the curve, and (3) the point at which the curve regains the base line.
It will be noted that, during the single revolution of the drum, the primary circuit has not only been made but also been broken again by the striker leaving the naked wire. The nerve has consequently received a maximal make and then a maximal break shock, but has only responded by a contraction to the first ; for, owing to the rapid rotation of the drum, the second stimulus has reached the muscle too soon after the first for the muscle to be able to respond (see Ee- fractory period of muscle, p. 42). If, however, the drum is revolving but slowly, the second stimulus may follow the first after a sufficient interval of time for the muscle to partly respond to it. This leads
26 PRACTICAL PHYSIOLOGY
to a deformation of the curve (Fig. 32), in which the hump near the top of the up stroke of the lever is caused by the muscle re- sponding to the second stimulus (see Effect of two successive stimuli. Chap. VII , p. 40). If with a slowly revolving drum it is desired to send into the nerve a single stimulus, it is only necessary to place the secondary coil at such a distance from the primary that the break but not the make shock is effective.
The curve (Fig. 32) occupies about ^nr^^^ ^^c- ^^^ can be divided into three parts.
(1) The first part extends from the point at which the stimulus reached the nerve to that at which the contracting muscle began to raise the lever. This is the latent period, and is seen to last about
•j^th of a sec. During this period several distinct processes take place ; (a) a nervous impulse has to pass down the strip of nerve between the point stimu- lated and the muscle, this will occupy about i^nny*^® °^ * second (see Velocity of nervous impulse). Of the remaining , A ^ths (6) the
Fig. 33.— Contraction of the same preparation d iuuu \ /
as in Fig. S2, recorded on a drum revolving at paSSage of the UerVOUS impulse
a slower rate. The hump near the top of the i i «
upstroke of the lever represents a second con- along the fine motor nerVO-
traction in response to the breai: shock. Time ^. .
marker, 100 per sec. (A.P.B.) eudlUgS OCCUpieS about looftths
sec, and (c) the latent period of the muscle itself about xjnsTr*^^ *'f * sec. This in turn is due to several factors, of which two must be mentioned. When muscle fibres begin to contract a certain time must elapse before the muscle is able to exert a sufficient pull to move the recording lever; in other words, there is instrumental inertia to be overcome. Again, when muscle, which is highly extensible, begins to contract, every part of every fibre does not simultaneously begin to shorten; but the contracted part of a fibre stretches at first the uncontracted part, and is therefore not united to the lever by a rigid connection. It is only when the tension in the stretched part has sufficiently increased, or the fibre as a whole has passed into a state of contraction, that the lever begins to be pulled upon.
(2) The second period extends from the point at which the lever begins to rise to the point highest above the base-line. This is the period of active contraction or shortening of the muscle, and occupies about x^ths of a sec.
(3) The third portion extends from the highest point of the curve to the point at which the curve rejoins the base-line. This is the period
ELEMENTARY EXPERIMENTAL PHYSIOLOGY 27
of relaxation, and lasts about xrirths of a sec. Eelaxation is a passive process brought about by the falling lever and weight doing the same amount of work on the muscle as the muscle during its period of shortening has done in raising the lever and weight to a certain height.
The muscle-curve, although roughly a magnified record of the change in length of the muscle, is deformed by certain errors of instrumental origin, which it is necessary to mention in order to avoid, so far as they are preventible. The most important are the mass and length of the lever and the disposition of the weight along it. They affect all parts of the curve. The weight of the lever tends to prevent the muscle from beginning to raise it (inertia of position) and so lengthens the latent period ; therefore the lever should be as light as possible. During the stage of shortening the lever, when once in motion, tends to be carried on by its own momentum after the muscle has ceased to
Fio. 34. — Single coiitiaction of gastrocnemius. Muscle loaded only with a I'atLer heavy lever. Magnification, 5. Temp., 16° C. Time marker, 100 per sec. (A.F.B.)
pull on it (inertia of motion), and so makes the muscle appear to have shortened more than it really has. For the same reason, during the period of shortening, the tension on the muscle is not uniform, but becomes less as the lever undergoes acceleration ; during the relaxation exactly the opposite happens, a heavy lever as it falls again undergoes acceleration and increases the tension on the muscle throughout the relaxation and may even stretch it beyond its original resting length (Fig. 34). In order to reduce these errors the lever again should be as light as possible.
On the other hand, to attach to the muscle no other weight than that of a very light lever would introduce fallacies. For, unless the muscle is sufiiciently weighted to keep it taut, there may be, when the muscle begins to contract, a certain amount of 'slack' to be taken in which would cause an apparent lengthening of the latent period.
28 PRACTICAL PHYSIOLOGY
Again, when the muscle does begin to pull on the lever, it will do so with a sudden jerk, which may cause a light lever to fly up out of control of the contracting muscle ; this, again, makes the muscle appear to have undergone greater shortening than it really has (see, however. Chapter XIII.). Further, the relaxation of a muscle being purely passive, the period of relaxation of an insufficiently weighted muscle is much prolonged, and the writing may fail to reach the base- line again. 1
In order to get over these instramental difficulties, the muscle-lever must be as light as is consistent with rigidity, and the muscle must be suitably loaded, the weight being attached near the axis of the lever for the following reasons : the nearer it is to the axis, the less move- ment will it undergo, and therefore the less will be its inertia of move- ment and the more uniform the tension on the muscle throughout the curve. This disposition of the weight also helps to reduce the after- vibrations or ' shatter '-curves which are frequently seen following the relaxation (Fig. 34). Compare with this Fig. 32 taken from the same muscle ; by hanging a weight of 30 grams near the axis of the lever the shatter curves have been nearly eliminated, and are represented by the slight oscillation between the two vertical lines at the end of the curve.
It may be pointed out that in the living body the muscles are always weighted when they contract, and even when relaxed they are under considerable tension ; for they are really shorter than the distance between their points of origin and insertion, and their antagonists are always exerting a certain pull on them, and some muscles, such as the deltoid, are considerably stretched by the weight of a limb.
The length of the lever is of some importance ; for, besides the fact that length reduces the rigidity of a light lever, a further deformation of the curve is introduced by increasing the magnification. As the writing point is raised, it tends to leave the drum, and in the course of a much magnified curve is only kept on the drum by the lengthening out of the spring formed by the writing point. Therefore the more the writing point is raised above the horizontal, the more the magnifi- cation is constantly increasing. For this reason the muscular move- ment should not be magnified more than is sufficient to make the record of it clear.
Although muscle curves, as accurate records of the muscular move-
' Muscles during the cold of winter, even when properly weighted, frequently show this ' contraction-remainder.' If cold be the cause, turn back the ' trouser ' of skin and pour over the muscle some normal tap-water saline heated in a test tube to 25° C. Cf . footnote on p. 33.
ELEMENTARY EXPERIMENTAL PHYSIOLOGY 29
ment, have fallacies inseparable from the method of recording them, it is possible to make two rough deductions from them :
(1) The amount of actual shortening a muscle undergoes during con- traction can be calculated by measuring the vertical height of the top of the curve above the base line and dividing it by the magnification ; in rig. 32 the height is 20 mm., and the magnification 5, therefore the muscle became shorter by 4 mm. The length of the resting muscle when loaded by lever and weight was 25 mm., consequently the muscle during contraction became shorter by 4xt^, i.e. nearly a sixth of its original length.
(2) The amount of work done by the muscle during its contraction is the product of the load and the height to which it was raised, W = L x H. In Fig. 32 the actual load which the muscle raised was not the whole of the 30 grams, hung near the axis of the lever, but a proportion of it, calculated by multiplying by the distance from the axis of the point of the suspension of the weight, and dividing by the distance from the axis of the point of attachment of the muscle ; this fraction was \, and the actual load lifted 6 grams. The height to which it was raised was 4 mm. ; consequently the work performed was 24 gramme millimetres.
CHAPTEE IV.
THE CONDITIONS WHICH AFFECT SINGLE MUSCULAR CONTRACTIONS.
(a) Different Muscles, (b) Veratrine. — The curve produced by the contraction of a muscle may be altered not only by such influences as temperature, load, fatigue, and drugs, but also by the difierences in structure of various muscles. The muscular fibres of the frog are found to present two varieties, clear and granular, which differ both in structure and in physiological properties. The gastrocnemius may be taken as an example of a muscle whose fibres consist largely of the clear variety, and the hyoglossus of the granular variety, i.e. a muscle in which the majority of muscle-fibres contain more nuclei and are rela- tively richer in undifferentiated living material, the sarcoplasm. The chief physiological difference between granular and clear muscles are, that granular muscles have a slower and more prolonged contraction, are less excitable, more easily tetanised, and less readily fatigued.
In mammals the same differences between red and white muscles can be shown to exist. Red muscles, such as the masseter or soleus of the rabbit, differ structurally in having more sarcoplasm and nuclei in their fibres, and are redder in colour owing to a much richer capillary net-
30 PRACTICAL PHYSIOLOGY
work between their fibres and to the presence of myohaematin in the fibres themselves ; physiologically they are far less readily fatigued and show a contraction four or more times as long as that of the white gastrocnemius (Fig. 35).
For comparison with the single twitch of the gastrocnemius, that given by the hyoglossus may now be studied. This muscle, arising from the anterior edge of the body of the hyoid cartilage, runs forwards into the substance of the tongue.
A Hyoglossus Preparation is made by cutting ofiF the whole of the lower jaw, including the tongue and hyoid cartilage. Place it on the myograph board, mucous surface upwards, turn the tongue
Fio. S5. — Gompaiison of contractions of red and white muscle of rabbit, stimulated indirectly. Upper curve is response of the red soleus and lower curve that of the white gastrocnemius. Time marker, 50 per sec. The tracing to be read from right to left." (M.S.P.)
forwards, and connect its tip to the lever by a thread. Firmly fix the hyoid cartilage by running a pin through it into the cork. Two needle electrodes transfix the base of the muscle just in front of the hyoid.
All the other connections are the same as when studying the single contraction of the gastrocnemius; a weight of 5 or 10 grams is placed near the axis of the lever.
Compared with the single twitch of the gastrocnemius, that given by the hyoglossus (Fig. 36) shows the following differences : the whole contraction lasts more than twice as long, the latent period is slightly longer, but it is the period of shortening and still more that of relaxa- tion which is more gradual and prolonged.
Action of Veratrine. — A brainless frog is poisoned by injecting into the dorsal lymph sac 5 minims of a saturated (1 in 1000) solution of veratrine in normal tap-water saline. In order that the drug may
ELEMENTARY EXPERIMENTAL PHYSIOLOGY 31
be rapidly absorbed it is important not to 'pith' the frog, but to destroy its cerebrum with a pair of Spencer- Wells pressure forceps. In about ten minutes it will be observed that the hind legs are very slowly and imperfectly flexed after a jump, and a few minutes latei the frog will be seized by a spasm when it jumps. As soon as these
Fia. 36. — Contraction of the hyoglossus muRcle. Time marker, 100 per second. (A.P.B.)
symptoms appear the remaining portions of the central nervous system are destroyed, and a sciatic and gastrocnemius preparation made.
In the meantime the action of veratrine may be studied on the hyoglossus preparation used in the previous experiment. Five minims of the veratrine solution are injected into the lymph sac in which the muscle lies. The drum is arranged to revolve at a slow rate of about 2 cm. in 10 sees., and a simple key instead of the " striker " of the drum is placed in the primary circuit. After waiting ia few minutes the muscle is stimulated by a single maximal induction-
FlQ. 37. — ContiactioD of the gastrocnemius muscle of a frog. The etfect of vera- trine. The first two contractions show the characteristic effect of the drug ; further stimulation produced twitches without the prolonged contraction. Tiie curve has f been reduced to one-half the actual size. The time is marked in seconds. (Fembrey and Phillips.)
shock, and its contraction recorded. The curve shows that the response is a single slow contraction with an enormously prolonged relaxation^ Eeplace the hyoglossus by the gastrocnemius and sciatic preparation and stimulate it in the same way. As soon as the iirst contraction is over, the muscle is stimulated again, and so on for half a dozen contractions. It will be seen that the first contraction (Fig. 36) con- sists of a smart initial twitch followed by a much longer contraction, and an even more prolonged relaxation. The second contraction shows the same characters to a less extent, and the subsequent con- tractions become of shorter and shorter duration until they reach the
32 PRACTICAL PHYSIOLOGY
normal. If the muscle be allowed to rest, the veratrine eflfect returns again. The absence, in the case of the hyoglossus, of the sharp initial twitch seen in the gastrocnemius contraction, is probably due to more complete poisoning of all the muscle-fibres. The gastroc- nemius is more bulky, some of its fibres remain unpoisoned and respond with a normally rapid contraction, followed by the slower and more prolonged contraction of the poisoned fibres.
CHAPTER V.
THE CONDITIONS WHICH AFFECT SINGLE MUSCULAR CONTRACTIONS— Continued.
(c) Temperature. — Since the shortening of muscle during its contrac- tion is but the outward and visible sign of chemical changes taking place in the muscle, it is not surprising that changes in temperature should greatly affect the single muscle-twitch.
In warm-blooded animals whose bodily temperature does not undergo a greater variation than about 2° C, the effect of different temperatures on muscular activity is unimportant. But it is quite otherwise in cold- blooded animals whose range of bodily temperature is that of their external medium. In them, the muscular activity of which they are capable at any moment is determined largely by the temperature of their muscles. Again, the subject becomes important for warm-blooded animals when, from any cause, their bodily temperature is materially altered, as it may be by disease. These abnormal variations in their temperature may be sufficiently great to affect the muscular activity of which the animal is capable. More frequently, however, they are important because of the effect which an abnormally high bodily tem- perature, especially when long continued, may have upon the actual chemical constituents of muscle, and especially upon its proteids.
In order to study these effects, the apparatus is arranged to stimulate a muscle with single maximal induction shocks, using the "striker" of the drum, in the primary circuit. Either a hyoglossus or gastroc- nemius preparation may be used ; if the latter, it must be prepared without a covering of skin, in order that its temperature may be more readily altered. Also, the muscle must be stimulated directly and not through its nerve, since changes of temperature affect nerve.
It is important to use maximal stimuli, for cold increases the excitability of muscle, and a stimulus which is minimal at 5° C. vrill be sub-minimal at 25°. The lever should be weighted near its axis and the drum should revolve at a rate of about 20 cm. per sec.
ELEMENTARY EXPBEIMENTAL PHYSIOLOGY 33
Cold tap-water saline solution, which has been cooled by ice to nearly 0° C, is slowly poured upon the muscle ; the temperature of the solution is noted, the muscle is stimulated, its contraction recorded and the point along the tracing at which the stimulus was sent into the muscle is marked. Swing the writing point off the drum, but do not move the base of the stand carrying the myograph. Take a series of super- imposed curves at temperatures of about 3°, 13°, 23°, and 33° C. (Fig. 38). Sufficient time must be given and fluid used to allow the bulk of the thick gastrocnemius to attain approximately the temperature of the saline solution. In order to get exact results, it would be necessary to
KiG. jj8. — The effect of temperatu''e upon the contraction of the gastrocnemius muscle. The time is marked in y^^ second. The tracing should be read from right to left. Figures on curve are the temperatures of the salt solution. (Fembrey and Phillips.)
suspehd the muscle in the solution at a given temperature until its substance had attained that temperature.
It will be seen that cold lengthens the whole curve, especially the latent period and the phase of active contraction ; the period of relaxation is relatively less affected, but a tendency to incomplete relaxation is often seen.^ As the muscle is warmed, the liberation of energy becomes more and more rapid, consequently the time occupied by the whole twitch decreases progressively, and especially the latent period and period of shortening ; the passive stage of relaxation is
' Cooled excised muscles, even when weighted, are liable to show a ' contrac- tion-remainder,' or incomplete return to their former length after contraction. It is also seen after strong direct stimulation, in poisoning with veratrine, and as fatigae or death come on.
C
M PRACTICAL PHYSIOLOGY
relatively less shortened, although muscle does become more extensible as its temperature rises from 0° to 30° C. (Fig. 38).
The relation between temperature and the height of the contraction is not quite so simple. Between 0° and about 15° C. the actual height of the contraction may fall slightly, and for two reasons : as the tem- perature increases, the irritability of the muscle decreases; further, other things being equal, the more slowly a muscle contracts, the more time it has to shorten up as much as it will in response to a given stimulus. From 15° to 25° the height of the curve rapidly increases; this is largely, if not entirely, instrumental in origin, and is
Fio. 39.— Curve of the shortening of the gastroenemius muscle during heat-ri|;ar. (Pembi'ey and Fhillips.)
due to the fact that, as the liberation of energy becomes more rapid, the lever." receives a considerable jerk from the rapidly contracting muscle. In other words, the increased height of the contraction is due, not to a greater liberation of energy, but to the greater rate at which the, same quantity of energy is liberated. From 25° to 35° C. the irritability of muscle and its height of contraction rapidly fall.
Now poar on some solution warmed to 50° C. When the muscle- fibres reach a temperature of about 40° C. they undergo a rapid shortening (Fig. 39), which, as the temperature of the muscle rises, passes into the permanent shortening of ' heat-rigor.' This condition is due to coagulation of some of the muscle proteids, and in consequence the muscle, becomes hard, opaquo, inelastic, and has permanently lost its irritabilitj-.
ELEMENTARY EXPERIMENTAL PHYSIOLOGY 35
CHAPTER VI.
THE CONDITIONS WHICH AFFECT SINGLE MUSCULAR CONTRACTIONS— Continued.
(d) Load. — In order to study the effect of variations in load upon a single muscular contraction, the apparatus is arranged for stimu- lating the muscle by a single maximal induction-shock, the drum being placed as a key in the primary circuit and arranged to rotate at a fast rate. Make a gastrocnemius-sciatic or hyoglossus preparation.
Fio. 40. — The effect of load upon the contraction of the gastrocnemius muscle (A F.B.)
Fio. n is the continuation of the experiment In Fig. 40. Single contractions of the gastrocnemius with different loads. The figures on the curres represent the weights in grms. hung at the axis of the lever ; actual load on muscle was in each case one- fifth. Magnification, 5. Temp., 12' C. (A.F.B.)
Eecord a single contraction of the muscle weighted only by the lever, mark the latent period and draw a base-line. Then hang on to the lever near its axis weights increasing by 20 grams at a time, and for each addition of weight record a contraction. The base of the stand carrying the myograph should not be moved during the experi- ments, but the curves should be superimposed as in Figs. 40 and 4L Each increase of weight stretches the muscle, consequently it is
36
PRACTICAL PHYSIOLOGY
necessary to bring back the writing point accurately on to the base-line before each contraction is recorded.
The general effects to be noticed are — that, as the load is increased, the latent period becomes slightly longer, the height of the contraction generally becomes less, the rise of the lever during the period of active contraction becomes more gradual, and the period of relaxation, which may be at first much decreased, gradually lengthens out again.
If the muscle be fresh and in really good condition, the early effect of increasing the load may be to increase the height of the first few contractions (Fig. 43). This stimulatory effect of initial tension on the power of a muscle to liberate energy during a subsequent contraction, is seen, within certain limits, in all kinds of muscular tissue; and it is of importance. For, in the body, as has been already pointed out, the skeletal muscles are, even when relaxed, under a certain tension produced by the pull of their antagonists and their being really shorter than the distance between their points of origin and insertion.
But when we study the work done by the muscle during a series of contractions with increasing loads, and not merely the height of the individual contractions, the stimulating effect of increased load is much more obvious. After the tracing has been varnished and dried, measure off the vertical heights of the curves corresponding to the different loads, and calculate the work done during each contraction (see p. 29). In the following table are given the details of the work done during the contractions recorded in Figs. 40, 41.
|
Number on Contraction. |
Actual Load in grms. |
Actual Height of Contraction in mm. |
Work done in grm. mm. |
|
20 |
4 |
4 |
16 |
|
50 |
10 |
3 8 |
38 |
|
80 |
16 |
3-6 |
57-6 |
|
lOO |
20 |
3-4 |
68 |
|
150 |
30 |
3-3 |
99 |
|
200 |
40 |
3-2 |
128 |
|
300 |
60 |
2-8 |
168 |
|
500 |
100 |
2-6 |
260 |
It will be seen that, although the height of the contraction decreases as the load increases, the work performed increnses throughout. This process of course has limits, which will be dealt with on p. 60. The important deduction to be made from these results is that muscle as
ELEMENTARY EXPERIMENTAL PHYSIOLOGY
37
(» o
as
aft
9 9
S - ° a » o
« g
11
3 a
I-
38 PEACTICAL PHYSIOLOGY
a machine for doing work is found to have its output of energy regulated, not merely by the strength of the stimulus reaching it, but also to a large extent by the amount of work it is called upon to do (see p. 60).
Effect of Fatigue. — When discussing the fatigue of muscle it is necessary to draw a distinction between the fatigue of a movement produced by the voluntary contractions of the muscle concerned in it (see p. 70), and the fatigue of a muscle caused by the artificial stimula- tion of the muscle itself or of the nerve supplying it (see p. 72). Further, in the second case there is a marked difference in the effect of continued stimulations on a muscle whose circulation is still intact (see p. 74), and on one which has been excised from the body. Here we shall deal only with the simplest case of a muscle excised from the body and stimulated directly and not through its nerve, in order to exclude any possibility of fatigue of nerve or of nerve endings.
Prepare either a hyoglossus preparation to be stimulated by two needle electrodes, or a gastrocnemius-sciatic preparation to be stimulated by one needle-electrode and by fine capillary copper wire threaded through the tendo-Achillis, as the other electrode. The drum is placed in the primary circuit, so that each time it revolves the muscle receives a maximal make induction-shock ; it should revolve at such a speed that the muscle will be stimulated once or twice a second. Weight the muscle near the axis of the lever, using 20 grms. for a hyoglossus and 50 grms. for a gastrocnemius preparation. With the Du Bois key closed, describe a base line and mark on it the point at which the stimulus will enter the muscle. Now open the Du Bois key, allow the drum to revolve, and record the first contraction and every tenth or twentieth subsequent contraction. For this purpose, directly the first contraction is over, the writing point is swung away from the drum, which goes on revolving and causing the muscle to contract. The base of the stand carrying the myograph must not be moved so that for each con- traction the point of entrance of the stimulus will be the same. The writing point should be a fine one, otherwise the number of super- imposed curves will to some extent obliterate each other.
When a series of curves taken in this way is examined (Fig. 43) it is seen that they show the following changes as fatigue progresses, — the latent period becomes slightly longer, the shortening of the muscle takes place more slowly and reaches its maximum more gradually, but the actual height of the curves does not begin to decrease much until the other features of fatigue are well marked ; the lengthening out of the period of relaxation is the most marked feature, it is evident from the first, and, as it progresses, a ' contraction remainder ' also appears.
S i3
SB ^
s3
il
S'g
G S O CI
40 PRACTICAL PHYSIOLOGY
The rate at which fatigue comes on in a muscle under the above conditions is increased by raising the temperature and the load.
Another method of studying the effects of fatigue on a hyoglossus or gastrocnemius muscle is as follows. In this case the primary circuit is made and broken by hand, and the contractions are recorded as nearly straight lines on a drum revolving at the slowest possible speed. The secondary coil is moved up to the primary until both make and break shocks are maximal, and the muscle receives a stimulus once every 5 sees. In this way Fig. 45 was produced. It will be seen that the height of the contractions, after remaining fairly constant at the beginning, gradually decreases until, at the end of 15 minutes, the muscle was incapable of lifting the load. Further, it is seen that in the last two-thirds of the tracing the basal points of the twitches gradually fail to reach the base line, thus showing a 'contraction remainder.' If the muscle had been stimulated at shorter intervals, this appearance would have come on earlier ; for, as soon as the period of relaxation began to increase, the next stimulus would have reached the muscle before there had been time for relaxation to be completed.
If the muscle be allowed to rest for a few minutes and then the stimulation is continued, it will be found that even excised muscle is capable of slight recovery from fatigue (Fig. 45).
One other point shown by Fig. 45 must be referred to ; the 'height of the first twenty twitches increases, showing a 'stair-case' effect. This short and tetn|»orary improvement in the condition of muscle, brought about by the repetition of a stimulus of constant strength, was at one time thought to be peculiar to cardiac muscle (see Heart) ; but although shown best perhaps by the heart, it is also shown by all forms of muscular tissue.
CHAPTER VII.
TWO SUCCESSIVE STIMULI. GENESIS OF TETANUS. TETANUS.
When a second stimulus reaches a muscle after the contraction caused by the first is over, the muscle responds with a second contraction similar to or perhaps slightly higher than the first (see Fig. 45). When, however, the second stimulus reaches the muscle before the contraction caused by the first is completed, the response given by the muscle to the second stimulus depends upon the exact phase of its twitch, in which it happens to be when the second stimulus reaches it. In order to investigate this point, arrange the drum and circuits as
ELEMENTAEY EXPERIMENTAL PHYSIOLOGY
41
Fm. 46.— Effect of two auccegslTO maximal stimuli, with gradually dlminisliing Intervals, upon the gastrocnemius. To be read from below upwards.
S. Time tracing, 60 per sec. In the two upper curves are shown both the con- traction in response to the first stimulus alone and the combined contractions caused by the two successive stimuli. (M.S.P.)
A. Time tracing, 100 per sec. Recorded on a drum revolving at a much faster rate. The second stimulus was sent in well within the latent period of the first. (A.P.B.)
42 PRACTICAL PHYSIOLOGY
in experiments for recording a single maximal contraction on a rapidly revolving drum (p. 24); it is only necessary in addition to place a second ' striker ' in the primary circuit through the drum. If the rate of revolution of the drum remains constant, then, by simply altering the angular distance between the two ' strikers,' a second stimulus can be sent in at varying intervals after the first. Make a gastrocnemius preparation and stimulate it either directly or through its nerve. Set the drum in motion and, with the Du Bois key open, approximate the 'strikers' until the muscle clearly to the eye just responds with a complete contraction to each stimulus. Close the Du Bois key, bring the writing point on to the bottom of the drum, describe a base line and mark on it the point at which each stimulus enters the preparation ; then open the key, record both contractions, and close the key again. Now raise the myograph until the writing point will just clear the top of the curves, approximate the strikers a little, and again record the contractions, after marking a base line and the points of entrance of the two stijnuli. This process is repeated until the ' strikers ' are finally at such a distance apart that the second stimulus falls within the latent period of the first.
In this way Figs. 46 A and B were obtained. It shows that when a second maximal stimulus reaches a muscle during any part of its period of relaxation or of shortening, the rest of the contraction due to the first stimulus is omitted and the muscle starts off on a fresh contraction in response to the new stimulus. Since the second contraction may be as high as the first and starts with the writing point above the base line, it follows that the height of the second twitch above the abscissa is greater than and may be nearly double that of a single contraction ; in other words, a summation of contraction has taken place. . If, how- ever, the second stimulus falls within the latent period of the first, then the muscle responds by a contraction only to the first stimulus (Fig. 46 A) ; that is, the muscle is refractory to the second stimulus so far as its being able to respond by a second contraction is concerned ; therefore in skeletal muscle the ' refractory ' period corresponds in time to the latent period (cf. the 'refractory' period of cardiac muscle, p. 64).
(Jenesis of Tetanus. — In order to study the response of a muscle to a series of stimuli, it is necessary to have an apparatus which will auto- matically make and break the primary circuit of an induction coil at any desired rate.
The vibrating reed is a convenient form and consists of a ilat steel spring which can be clamped in various positions along its length ; by altering the length of spring which is allowed to vibrate, the number of
ELEMENTARY EXPERIMENTAL PHYSIOLOGY 43
vibrations per second can be changed. The spring has numbers stamped on its upper surface, corresponding to the position at which it must be •clamped to give that number of complete vibrations per second. The free end of the spring carries a platinum point which makes and breaks contact with a mercury cup in connection with the primary circuit {Fig. 47). In order to maintain the vibrations of the spring it is usual to place above it, and in the same circuit, an electro-magnet, so that, when the spring makes contact with the mercury, it is attracted out of the cup again by the magnet. In performing a complete vibration, the spring will both make and break the primary circuit and, in order that the two stimuli may not cause contractions of unequal height, the secondary coil must be so placed that either the make shock is just
p.c
Fig. 47. — Diagram of the vibrating reed in circuit.
ineffective, in which case the number of effective stimuli per sec. will be the same as the number of complete vibrations of the spring, or the make and break shocks are made equal and maximal, in which case the number of contractions per sec. will be double that of the complete vibrations of the spring.
Place the vibrating reed in the primary circuit so as to give 10 effective stimuli per sec. Make a gastrocnemius and sciatic preparation, with the Du Bois key closed, set the spring vibrating and bring the writing point of the myograph on to the surface of the drum, rotating at a slow rate, about 3 to 4 cm. per sec. ; open the Du Bois key and record the contractions for about 1 sec. Stop the drum, adjust the spring to give 20 effective stimuli per sec, and record the contractions as before. Repeat again with 30 stimuli per sec. Then remove the vibrating reed from the primary circuit, connect the battery with the coil so as to set the "Wagner's hammer vibrating, and record the contraction of the muscle for a few seconds.
Since each twitch of a gastrocnemius at 20° C. lasts about j^g^th sec, a muscle at that temperature could just respond without any summation to 10 stimuli per sec. If, however, the muscle is colder or fatigued, and each contraction therefore lasts longer, with 10 stimuli per sec, some slight summation may be seen, i.e. relaxation is not complete before the next
44 PRACTICAL PHYSIOLOGY
contraction begins, and the line joining the apices and basis of the successive contraction ascends slightly. With 20 stimuli per sec. the summation and fusion of each individual contraction is more complete > but the apex of each individual contraction will probably still be seen : the curve is therefore one of incomplete tetanus (Fig. 48). With 30 stimuli per sec. fusion may be complete from the first, i.e. complete tetanus, or if not complete at first, it gradually becomes so. This gradually increasing fusion (Fig. 48) is really due to fatigue : for the period of relaxation of the individual contraction tends to become longer and longer, and therefore the next stimulus reaches the muscle progressively earlier in each individual twitch, until a point is reached in which there is no time for the muscle to begin to relax between the stimuli, and fusion becomes complete. With the Wagner's hammer, which causes the muscle to receive 50 or more stimuli per sec, fusion is com- plete from the first. One other point is to be noted in nearly all these curves : at first the rise in the lever is very rapid, then it suddenly becomes more gradual, but, even when fusion has been complete from the first, the lever may still rise slowly for a short time until the muscle has reached the utmost shortening of which it is capable. If the stimulation is still continued, this height may be maintained for a short time, but sooner or later the lever will begin to drop, showing the onset of marked fatigue. In all cases when the stimulation ceases, the relaxation is at first extremely rapid, then becomes more gradual and a ' contraction-remainder ' varying in extent according to the degree of fatigue is generally seen.
The same experiments ma>y be performed with a hyoglossus prepara- tion. This muscle, however, being of the 'granular' variety and having a contraction which lasts twice as long as that of the ' clear gastrocnemius (see p. 30), is sent into complete tetanus with half the number of stimuli, i.e. about 15 per sec.
CHAPTER VIII.
THE PROPERTIES OF NERVE, MINIMAL AND MAXIMAL STIMULL
A NERVE is not a unit ; it is that branch of a nerve-cell which conducts an impulse to, or from, the periphery. A nerve-cell with its dendrites and axis-cylinder process or axon forms a unit, the neuron. It is con- venient, however, to examine the characteristics of a nerve apart from its nerve-cell. The chief of these are excitability and amductimty. Excitability, or, as it is sometimes called, irritability, is the response to a.
ELEMENTARY EXPERIMENTAL PHYSIOLOGY
45
■a
0, □Q
a
S
.9
to
46 PRACTICAL PHYSIOLOGY
stimulus; a nervous impulse, the real nature of which is unknown,, is started at the point stimulated, and is transmitted or conducted along the nerve.
Nerves can be stimulated by electrical, mechanical, chemical or thermal agents; of these the most important in experimental physiology is the electrical, for it can be finely graduated, is of extremely short duration, and can be applied repeatedly without damage to the nerve. The first experiments will therefore be the electrical stimulation of nerve.
The Electrical Stimulation of Nerve. — An induction-apparatus is. arranged for single induction-shocks, and a simple pair of electrodes- is connected with the secondary coil by means of a Du Bois key. A preparation of the sciatic nerve in its entire length and of the gastroc- nemius muscle of a pithed frog is made, and near the origin of the- nerve is applied the pair of electrodes.
On the passage of an induction-current through the electrodes the nerve is stimulated, and an impulse is sent down the nerve, reaches the muscle, and causes it to contract. This is indirect stimulation of the muscle, and is, if a weak current be used, not due to an escape of the electric current along the nerve towards the muscle. This is proved by the following experiment. A moistened thread is tightly tied round the nerve at a point between the electrodes and the muscle. The passage of a weak induction-current of the same strength as that previously used will stimulate the upper portion of the nerve, but the nervous impulse will not pass through the block produced by the thread. A breach in the physiological continuity has been produced, and the nervous impulse is not conducted through the ligatured nerve. The moistened thread would not prevent the passage of a purely electric current.
The response of the nerve to a stimulus bears within certain limits a relation to the strength of the stimulus. This can be shown by the following experiment.
Maximal and Minimal Stimuli. — The muscle of the preparation is attached to a myograph, the lever of which is arranged to write upon a drum covered with smoked paper. The electrodes are placed between the muscle and the ligatured portion of the nerve which was used in the previous experiment. The induc- tion shock is made so weak that no response is obtained, and is then gradually increased until a contraction is observed with the break-she ck Contraction does not follow each break-shock; the stimulus is sui ■minimal. The contraction is recorded as a vertical line upon the stationary drum, and before each stimulation the drum
ELEMENTARY EXPERIMENTAL PHYSIOLOGY 47
is turned by hand about half an inch. The strength of the current is slowly increased until a small contraction follows each break-shock; this is the minimal stimulus. The distance in centimetres of the secondary from the primary coil is noted upon the drum. The make- shock is weaker than the break, so that it is necessary to use only the one or the other in this experiment.
The intensity of the current is still further increased until the most powerful contraction of the muscle, as indicated by the height of the nearly vertical lines upon the drum, is obtained ; the stimulus is now maximal. Any further increase in the strength of the stimulus is not accompanied by a bigger contraction ; a supra-maximal stimulus only produces a maximal contraction, and is liable to damage the nerve.
It may be, as Gotch has suggested, that the diflference between maximal and minimal stimulation depends upon the number of the constituent fibres of the nerve stimulated. A weak electric current may afiect only a few fibres, and therefore the result will be only a slight contraction, due to the excitation of those muscle-fibres alone which are supplied by the nerve-fibres.
It will be found that the excitability of the nerve changes, so that with the same strength of stimulus there will not be the same minimal point. A loss of excitability readily occurs if the nerve be allowed to dry, but during this process there may be irregular fluctuations in the excitability of the nerve above and below the normal.
Mechanical Stimulation of the nerve can be shown by pinching the nerve with a pair of forceps ; the muscle contracts, showing that a nervous impulse was produced. Such a method of stimulation injures the nerve, but by means of simple arrangements a nerve can be stimulated mechanically without damage. A light hammer worked by an electro-magnet may be used to tap the nerve, or small drops of mercury from a funnel may be allowed to fall upon the nerve. Such methods are useful in experiments in which an electrical stimulus might introduce a source of fallacy, but for ordinary experiments they are undesirable, since there is a difficulty in maintaining a constant strength of stimulus, and there is a danger of damage to the nerve.
Thermal Stimulation is next shown by the application of a hot wire to the nerve. The muscle contracts. The damaged portion of the nerve is cut away, and to the end of the living nerve is applied a crystal of common salt ; the muscle soon shows irregular twitches due to the chemical stimulation of its nerve. The last form of stimulus is obviously limited to special experiments, for the stimulus is not easily graduated and damages the nerve.
48 PRACTICAL PHYSIOLOGY
CHAPTER IX. THE RELATION BETWEEN MUSCLE AND NERVE.
The motor nerves by means of their end-plates are so intimately con- nected with the muscle-fibres that it is impossible to stimulate the muscle-substance alone by the direct application of a pair of electrodes to the intact muscle. The question, therefore, arises whether muscle possesses indejaendent excitability, whether it can respond to a stimulus without the intervention of its nerve. The development of muscle from protoplasm, which is contractile and excitable although possessing no nerves, would suggest that muscle itself is excitable and can respond to a stimulus. This can be shown, for the fully developed muscle, after its nerve has been paralysed by the action of a drug.
Curare^ is an alkaloid used as an arrow-poison by some natives of South America. The following experiments show that it paralyses the terminations of the motor nerves, but that the muscle still responds to direct stimulation :
(i) Two watch-glasses are almost filled with a 1 per cent, solution of curare in normal tap-water saline. Two muscle and nerve-preparations are made, care being taken to bisect the lower portion of the vertebral column and thus obtain the entire length of the sciatic nerve. The excitability of the nerve and of the muscle in the case of each preparation is tested by the determination of the minimal stimuli. Then the nerve of preparation A is placed in one watch-glass full of the poison, but its muscle is left outside upon a piece of filter-paper moistened with normal tap- water saline. The gastrocnemius muscle of the preparation B is placed in the solution of the drug and its nerve upon the damp filter- paper (Fig. 50). Stimulation of the nerve B will soon produce no contraction, even if the strongest induction-shocks be used ; on the other hand, an examination of the nerve A will show that its excitability has practically undergone no decrease. Stimulation of the muscle B which has been exposed to the action of the drug readily produces a contraction. The poison, therefore, must act upon some portion of the terminations of the nerves, probably upon the end- plates, for both muscle-substance and nerve-trunk retain their excita- bility even after long exposure to the drug.
Muscle will contract on direct stimulation even after its nerves have degenerated. This experiment, however, is not suitable for a class, for it would be necessary to keep the animal alive for two or three weeks in order that the nerve-fibres might completely degenerate.
' It is prepared from various plants of the genus Stryohnos.
ELEMENTARY EXPERIMENTAL PHYSIOLOGY
49
A further experiment with curare can be made, (ii) The cerebral hemispheres of a frog are destroyed, and then the sciatic nerves are carefully exposed in each thigh ; a strong ligature is passed under the sciatic nerve of one side, A, and is tied tightly around all the structures of the thigh except the nerve. The circulation of the blood is thus com- pletely stopped in the structures below the ligature. Stimulation of either sciatic nerve produces a contraction of the muscles of the corre- sponding leg. Under the skin of the back of the frog are injected two or three drops of a 1 per cent, solution of curare. The poison is
Fio. 50.— Biaf^^m of the ezperimeiit on the action of curare.
absorbed by the blood-vessels and is circulated in all parts of the body except those below the ligature. Paralysis is produced, and the frog lies in a toneless condition and does not move if its toes be pinched. Stimulation of the sciatic nerve produces in the case of the ligatured leg, A, a contraction of the muscles, but in the case of the other leg, B, no contraction occurs. The muscles, however, of the leg, B, contract on direct stimulation.
Both nerves in their upper portions have been exposed to the poison, the muscles of both legs respond to direct excitation, but the ligatured leg alone to indirect stimulation. The ligature has prevented the poison from reaching the terminations of the nerves inside the muscles. It is upon these terminations that the curare acts.
The independent excitability of muscle can also be shown in the case of cardiac muscle. The apex of the ventricle of the frog's heart con- tains no ganglia, but it responds to a stimulus, and under appropriate conditions will even contract rhythmically.
Further experiments upon the independent excitability of muscle are given in Chapter XX.
■■ThiB operation should be performed with a pair of Spencer-Wells pressure- forceps in order that no blood may be lost.
D
AO PEACTICAL PHYSIOLOGY
CHAPTER X.
THE EFFECT OF A CONSTANT CURRENT UPON MUSCLE AND
NERVE.
Muscle and nerve consist of complex chemical substances, and con- tain about 70 per cent, of water in which various salts are dissolved. Moreover they are bathed in lymph.
The passage of a constant current through a liquid produces electro- lysis; thus, in the case of water, oxygen is given oflF at one plate, hydrogen at the other. Animal tissues, containing, in addition to a large percentage of water, salts and proteins, are also the seat of electro- lysis during the passage of a constant current ; the ions are probably of a complex nature. These changes in nerve and muscle are shown by alterationaJn excitability and conductivity.
These it is necessary to consider in relation to the changes which occur at the anode and kathode during the make and break of the
Fio. 51. — Diagram of the frog's heart to show the effects of the make and break of a constant current upon muscle. In A the ventricle is represented as pale and contracted, with a small shaded area to represent the flushed and uncontracted poi-tion of the ventricle ; that is, a local diastole during general systole. This condition can he produced by the make of the anude or the break of the kathode of a constant current. In B the veutiide is dilated and flushed, with a small pale area of contracted muscle ; that is, a local systole during general diastole. This condition can he produced by the make of the kathode or the break of the anode.
constant current. The simplest experiment can be made upon the frog's heart.
The Effects of Anode and Kathode upon the Frog's Heart. — The brain and spinal cord of a frog are pithed and then the heart is exposed. Care should be taken to avoid the severance of large blood-vessels in order that the vascular system may be well filled with blood. The pericardium is opened and the heart is observed ; the ventricle during systole is pale owing to the contraction of its muscle fibres forcing out the blood from its spongy walls ; during diastole, when the muscle is relaxed the ventricle is flushed owing to its distension with blood. There are no blood-vessels in a frog's cardiac muscle.
ELEMENTARY EXPERIMENTAL PHYSIOLOGY 51
The ends of two pieces of ordinary insulated wire are well cleaned and are connected with a Daniell battery ; the clean free ends of the wires are bent back so that there will be smooth surfaces to apply to the heart. The wire connected with the copper of the battery is the amade, that with the zinc is the kathode.
In the frog's mouth is placed the kathode, for there good contact is obtained with a moist conductor; the anode is placed upon the ventricle. Now it will be found that during the systole of the ventricle that portion of the muscle which is around the anode will be flushed, uncontracted, and bulging outwards — the anode at the make of the circuit produces a heal diastole during general systole (Fig. 51 A). The rhythmic power of the cardiac muscle around the anode is diminished, so that it remains uncontracted.
If now the wire be suddenly removed from the heart, the break of the anode causes an increased excitability of the muscle to which it had been applied, there is a local pallor ; the cardiac muscle is here contracted during the general diastole of the heart. The break of the anode produces a local systole during a general diastole.
The kathode is now applied to the heart and the anode is placed in the frog's mouth. There is produced a local systole during the general diastole of the heart. The kathode increases the excitability of the cardiac muscle, and thus the fibres affected remain contracted. The end of the wire is kept in contact with the ventricle for about a minute and is then suddenly removed ; a flushed and bulging spot will indicate the region to which the wire had been applied. The break of the kathode produces a local diastole during general systole, for the dis- appearance of the condition of katelectrotonus is accompanied by a fall in excitability.
This simple experiment shows that the make of the kathode and the break of the anode excite, that the make of the anode and the break of the kathode depress. This is also true in the case of nerve.
CHAPTER XI.
THE ELECTROMOTIVE PROPERTIES OF MUSCLE AND NERVE.
In uninjured and resting muscle and nerve there is no electric current, but during activity a current, the '■current of action,' is produced. Injury causes local activity around the damaged tissue, and is there- fore accompanied by an electric current, the so-called ' demarcation or
52
PRACTICAL PHYSIOLOGY
vnjwry-cwrreni.' This electrical current produced by injury is, as Gotch pointed out, to be considered as a current of action. These facts can be demonstrated by the following experiments.
The Bheoscopic Frog. Oalvani's Experiment, Contraction without Metals. — A long length of the sciatic nerve is dissected in a pithed frog and the muscles of the thigh are exposed and cut across. The trunk of the sciatic nerve is laid along the longitudinal surface of the muscles of the thigh, and then by raising the end of the nerve by a small glass rod the transverse section of the nerve is allowed to fall upon the cut surface of the muscles (Fig. 52). At this moment a twitch of the muscles of the leg moves the foot or toes. The circuit of the electric current in the muscle has ' been completed through the nerve. The section of the muscle-fibres has produced a local contrac- tion of the fibres, and this is accompanied by an electrical change which is sufficient to produce excitation when it is passed through an excitable nerve.
Secondary Contraction or Secondary Twitch. — Two muscle- and nerve-preparations are made ; the nerve of A is so placed upon the muscle B that the cut surface of the nerve lies upon the tendon and its longitudinal surface upon the muscle-fibres Pio. 62.— Diagram of Gal- (Fig. 53). The nerve of preparation B is stimu- toaction^witiiout metals, lated by a Weak induction-shock, and thus its muscle is excited and made to contract ; the muscle A will also contract. The contraction of the muscle B is accompanied by an electrical current, the 'cwrent of action,' which passes through the nerve A and thus produces a contraction in the muscle A. This is not due to an escape of electrical current from the electrodes, for a secon- dary twitch can be obtained if mechanical or thermal stimuli be used to excite the nerve of pre- paration B. Further, ligature of the nerve B with a moist thread will show that there is no escape with a weak induction-shock; the ligature destroys the physiologi- cal continuity and prevents the passage of the excitatory state but not that of an electrical current.
Fia. ss.-
-Diagram of the experiment on secondary twitch.
ELEMENTARY EXPERIMENTAL PHYSIOLOGY 63
Secondary Tetanus. — If the nerve be stimulated with a rapid series of induction-shocks the muscle B goes into tetanus and its ' cvrrertts of action ' stimulate the nerve A, with the result that the tetanus is also observed in the muscle A. This ' secondary tetanus ' can be produced by rapid mechanical stimuli.
Further experiments upon the electromotive properties of muscle and nerve are given in Chapter XXIII.
CHAPTEE XII. (Advanced).
EXTENSIBILITY AND ELASTICITY OF MUSCLE WHEN AT REST AND CONTRACTED. COMPARISON WITH RUBBER.
Muscle is both extensible and elastic, that is, it can be stretched beyond and will return more or less to its original length when the extending force is removed. These are important properties ; for, unless muscle were readily extensible the sudden contraction of one set of muscles would in the body be liable to rupture their antagonists.
In the study of these properties a gastrocnemius preparation may be used, but a muscle whose fibres run more nearly parallel to each other is preferable, such as a sartorius preparation from a large frog or better still a gracilis-semimembranosus preparation.
A gracilis-semimembranosus preparation consists of the two large internal thigh muscles (Figs. 20, 21). The gracilis is a large muscle lying along the inner side of the sartorius ; it arises from the ischial sym- physis and is inserted into the head of the tibia. The semimembranosus is a bulky muscle behind the gracilis on the posterior aspect of the thigh ; it also arises from the ischial symphysis and is inserted into the back of the head of the tibia. To make the preparation, isolate these two muscles from those surrounding them near their points of insertion, cut through the tibia below this point and through the femur just above the knee joint. Holding this piece of bone, separate the two muscles up to the symphysis and remove with them the bone from which they arise. If a larger or longer muscle still is required, a double preparation may be made with the muscles of both thighs and the two hung side by side, or one below the other, united in the middle by the piece of the symphysis.
The following experiments should be performed. The bone at the
54 PEACTICAL PHYSIOLOGY
upper end of the preparation is rigidly fixed in a clamp and to the lower end is attached by a short thread or pin a brass mm. scale, having its zero at the bottom. The lower end of the scale has a small tray to carry weights or a hole by which weights can be hooked on. A jpointer carried by a separate stand is placed opposite the zero of the scale. A weight of 10 grms. is attached to the scale and the amount of extension read off; then another 10 grms. is added and so on until the load is 100 grms. or more. It will be found that the length to which the muscle is extended is not proportional to the weight used, but that, by each increase of weight the muscle is stretched rather less, the greater the previous extension. By removing the weights one by one the elasticity of the muscle is observed ; it is not complete ; for when all the weights have been removed the muscle does not at once return to its original length. An ' extension-remainder ' is present, and this is the more marked the more the muscle is fatigued by the degree and duration of the extension. Therefore the observations should be made as rapidly and on as fresh a muscle as possible. It is probable that muscle in the body with its circulation intact is completely elastic.
If the muscle is replaced by a suitable piece of rubber band and the same observations are repeated on it, it will be found that the series of elongations are more nearly proportional to the weights used, thus con- forming nearly to Hooke's Law, which states that the successive increments in length produced by equal increments of weight are, in a perfectly elastic body, equal. Also, as the weights are successively removed, it will be found that the elasticity of rubber is more nearly perfect. But, if the extension be great and of long duration, an ' exten- sion-remainder ' does appear and only gradually disappears.
Another method of demonstrating the same properties is to fix the upper end of a muscle-preparation in the clamp of a simple myograph and to attach its lower end to the lever by a bent pin. Attached to the lever vertically below the muscle is a scale-pan or hook to which weights can be suspended. The writing point of the lever is brought on to the surface of a stationary smoked drum and a zero line described by rotating the drum by hand. The drum is rotated back so that the point of the lever is 5 mm. from the beginning of the zero line, a weight of 10 grms. is attached to the lever, the muscle will be extended and the writing point will record a new vertical line on the drum. Turn the drum by hand so that the writing point will describe a horizontal line 5 mm. long.i attach
' By thrusting the points of a pair of fine forceps through a thin piece of cork a means of measuring off equal distances is obtained ; there is a mm. scale on the induction-coiL
ADVANCED EXPEEIMENTAL PHYSIOLOGY 55
another 10 grms. and repeat the process until 100 grms. or more are extending the muscle. In the same way reverse the process and remove the weights of 10 grms. one by one. If now the lower ends of the vertical lines drawn by the fall and rise of the lever are joined, a curved line will be formed, showing that the extension of the muscle becomes less and less for each additional weight. Further, when aH the
Fia. 54..— Curve of extensibility and elasticity of gastrocnemiuB. The figtireB on the curve are weights in grms. Temp., 15' C. (A.P.B.)
weights have been removed, the writing point will be below the original zero line, showing an ' extension-remainder ' (Fig. 54). It will also be seen that the line corresponding to the elasticity of the muscle is a flatter and more gradual curve than that corresponding to the extension ; this is caused by the long continued load impairing the elasticity of the muscle.
Fio. 56. — Elasticity curve of quiescent muscle. To be read from right to left. The figures on the curve are for weights in grms. (M.8.F.)
If the experiment be repeated on a piece of rubber band, the line join- ing the lower ends of the vertical lines will be nearly straight, and little or no 'extension-remainder' will be seen. Figs. 55, 56 show a com- parison of the lines thus described for a muscle and piece of rubber loaded from 0 to 500 grms. and then gradually unloaded again.
A contracted muscle is more extensible than a resting one. This is of importance in the body ; for, otherwise, a sudden and powerful con- traction of a muscle, trying to lift a heavy weight, would be liable to rupture either the muscle itself, or its tendon, or the bones to which it
56 PEACTICAL PHYSIOLOGY
is attached. As a matter of fact, of these three structures muscle, owing to its increased extensibility during contraction, is the least often ruptured. In order to demonstrate this properly the muscle-prepara- tion is attached to the clamp and lever, as in the last experiment. Arrange the apparatus, for stimulating the muscle directly with single
|
/ |
\ |
||
|
5C |
1 1 1 1 '0 "J-OC 300 200 100 0 |
1 ! 1 1 1 ) 100 200 300 400 500 |
Fio. 56. — Elasticity curve of rubber tubing. The figures represent weights in grms. (M.S.F.)
maximal induction-shocks, using a spring-key in the primary circuit. Bring the writing point on to a stationary drum and, with the muscle weighted only by the lever, describe an abscissa line corresponding to the resting muscle. With the writing point again at the beginning of this line, stimulate the muscle once and, from the top of the ordinate so marked,- draw another abscissa line corresponding to the muscle when contracted. Eotate the drum by hand, so that the writing point is now 5 mm. along the ' resting ' abscissa line ; hang 20 grms. on to the lever and stimulate, so as to record a second ordinate 5 mm. from the first. Eepeat this process, increasing the weight by an equal amount each time. In this way Fig. 57 was produced. It is clear that the distance of the lowest point of each ordinate below the ' resting ' abscissa line represents the extension of the resting muscle by a given weight, and that the distance of the top of the same ordinate below the ' contracted ' abscissa line represents the extension, by the same weight, of the muscle when contracted. If the lowest and then the highest points of the ordinate are joined, two curved lines are produced which represent respectively the curves of extension of resting and contracted muscle (Fig. 57). It will be seen that the extensibility of contracted muscle is absolutely greater, and increases more rapidly, than that of resting muscle. Hence, if the observations were carried
ADVANCED EXPERIMENTAL PHYSIOLOGY 57
far enough, the two curve lines would ultimately cross; this means that if a muscle were loaded by a weight greater than it could lift, it
Fia. 67. — Comparative extensibility of resting and contracted gastrocnemius. Temp. 12' C. Magnification, 5. Fi|;rures represent actual weights In grms. U is the ' resting' and G the * contracted' abscissa line, (A.P.B.)
would during its stimulation actually lengthen (Weber's paradox). If this were not so, we should, when trying to lift a load greater than the muscle could move, run a great risk of rupturing our muscles.
CHAPTER XIII. {Advanced). LOAD AND AFTER-LOAD. WORK DONE WITH INCREASING LOADS.
Muscles may be loaded in two ways ; the load may be applied before the muscle has begun to contract, or only after it has already begun to contract ; this latter method, in order to distinguish it from the former, is called ' after-loading.' Most of the muscles in the body are both loaded and after-loaded; that is, they are constantly loaded by the pull of their antagonists, and it is only after they have already begun to shorten that the main load — the weight of the limb, etc. — is applied to them. The deltoid, however, is an instance of a muscle constantly loaded by the weight of the arm ; the ventricle of the heart, on the other hand, is a muscle which is only after-loaded.
58
PRACTICAL PHYSIOLOGY
The effect of load, and of its method of application on a single muscular contraction, will be studied in the following ways : (a) the contraction given by a muscle loaded and after-loaded with the same
weight will be compared; (J) a con- stant load will be thrown on to a muscle as an after-load later and later in its period of shortening, and the eflfect on the contractions noted ; (c) the muscle being just completely after- loaded, the height of contraction, with increasing loads, will be mea- sured and the work done with each calculated.
Comparison of the Contractions of a Loaded and After-loaded Muscle. — Arrange the apparatus for stimulating a muscle with single maximal induc- tion shocks, using the drum as a key in the primary circuit. Fix a gastro- cnemius preparation to a myograph lever, provided with an after-loading screw ; by raising the screw the metal part of the lever can be supported at any level (Fig. 25). Hang a weight of 60 grms. near the axis and raise the screw until the whole of the weight is just after-loaded ; this point can be ascertained by supporting the weight with the finger, and when the muscle no longer tends to raise the lever off the after-loading screw, the muscle is unstretehed by any load. Arrange the apparatus so that with the screw in this position the lever is horizontal. Eecord a single contraction of the muscle on a rapidly revolving drum, mark the point of stimulation, and draw an abscissa. Then lower the after-loading screw until the muscle is loaded with the whole weight, and super-impose on the same abscissa and with the same point of stimulation a contraction of the loaded muscle (Fig. 58).
ADVANCED EXPERIMENTAL PHYSIOLOGY 59
The main differences between these two curves are — in the purely after-loaded muscle there is an appreciable lengthening of the latent period owing to the muscle in its unstretched condition having to take in ' slack ' ; a diminution in the height of the contraction, owing to the absence of tension on the muscle before the contraction began. In other words, moderate initial tension increases the power of a muscle to do work.
Progressive After-loading of a Muscle. — With the same arrangement of apparatus as in the preceding experiment, record a single con-
Fio. 59. — ^Effect of progresBiTe after-loading of a gastrocnemius. Actual load on musdO} 4 grms. Magnification, 6. Temp., 10* C (A.P.B.)
traction of the muscle when just after-loaded, draw a base line and mark the point of stimulation. Now raise the after-loading screw until the writing point is on a level with the highest point of the preceding curve; draw a fresh abscissa at this level and record a contraction; the point of stimulation will be the same as before. Eepeat this process until the muscle can no longer lift the lever off the after-loading screw (Fig. 59).
From this experiment we see that, in a series of contractions each more after-loaded than the last, a muscle is able to undergo a little further shortening each time until it reaches its maximal shortening. Also by measuring the heights of the contractions above their respective abscissae, we learn that the longer after stimulation it is before the muscle meets the resistance of a given weight, the less is the muscle then able to overcome that resistance and raise the weight. In other words, as a muscle contracts its extensibility progressively increases, and its absolute contractile force decreases, until at the height of its contraction its extensibility is greatest and its absolute contractile force
60 PEACTICAL PHYSIOLOGY
nil. Hence a muscle would contract under the most favourable circumstances, if the load, as it was raised, progressively decreased. Belation of Load to Work done during Contraction.— Tn order to record the height of contraction for a large range of weights, it is more convenient to record on a stationary drum simply the heights of a series of twitches than to super-impose a large number of curves. The apparatus is arranged for stimulating the muscle with a single maximal induction-shock, using a simple key in the primary circuit. A weight is hung near the axis of the lever of such a size that the actual load on the muscle is 50 grms. ; the method of calculating this weight has been already given on p. 29. The muscle is just completely after-loaded throughout the experiment in order to get rid of the effect of alterations in the initial tension. With the lever horizontal, the muscle is stimulated, and the height of its contrac- tion recorded on a stationary drum. The drum is rotated a short distance by hand; an additional load of 50 grms. is hung from the lever, and another contraction recorded. The process is repeated until the muscle is no longer able to raise the load off the after- loading screw. Fig. 60 gives the result of such an experiment; in it the magnification was 5, and the actual load on the muscle hali of the weight hung near the axis of the lever. The following table gives in grm. mm. the work done by the muscle with the various loads.
|
Actual load in grm. |
Actual lift in mm. |
Work in grm, mm. |
|
50 |
4-0 |
200 |
|
100 |
3-2 |
320 |
|
150 |
2-2 |
330 |
|
200 |
1-8 |
360 |
|
250 |
1-2 |
300 |
|
300 |
1-0 |
300 |
|
350 |
•8 |
280 |
|
400 |
•5 |
200 |
|
450 |
•4 |
180 |
|
500 |
•3 |
150 |
|
550 |
•2 |
110 |
|
600 |
•1 |
60 |
|
700 |
0 |
0 |
From the last column in this table we see that, although the height of the contractions diminishes continuously, the actual work done by the muscle increases at first rapidly and then more slowly, until it reaches its maximum with a load of 200 grms. After that point the work done begins to decrease slowly, and then more rapidly until at 700 grms. a load is reached which the muscle is unable to
ADVANCED EXPERIMENTAL PHYSIOLOGY 61
lift. This weight represents the 'absolute contractile force' of this muscle, that is, the load which, brought to bear on the muscle at the instant of contraction, is just able to prevent it from shortening. Although the muscle is unable to lift this load, and therefore, when stimulated, does no visible mechanical work, it nevertheless liberates energy chiefly as heat.
FiQ. 60.— Height of coutractiona of gastrocnemius with increasing load. The number above each contraction is its observed height in mm. Magnification, 5. The number below each contraction is the weight in grm. hung at the axis of the lever ; the actual load on the muscle was half of this number. (A.P.B.)
We are now in a position to recapitulate, so far as load is concerned, the conditions necessary to obtain an optimal contraction of a muscle and to see how far they exist in the living body. Initial tension, we have seen, decreases the latent period and increases the power of the muscle to do work. In the body the muscles are constantly loaded to a slight extent, and are thus kept stretched and free from 'slack' In this way movements with a short latent period, and with an absence' of jerkiness are obtained ; and the muscles by being stretched are kept irritable, awake and fit for sudden work. On the other hand we see that a muscle, when purely after-loaded, is at a disadvantage for doing work; yet in the body the main load is thrown on as an after-load. The advantage of this arrangement depends upon the increased extensibility of contracting muscle; for, in this way liability to rupture is reduced ; further, there is a saving of energy in pulling at a dead weight through an elastic spring, instead of through an inelastic cord, since some of the energy expended would be lost in a sudden jerk, but, in the case of the spring, is stored up in it and given out again as its elastic recoil. Thus smooth- ness is imparted to even the most sudden movements. We have
62 PRACTICAL PHYSIOLOGY
also seen that as a muscle shortens its absolute contractile force decreases ; therefore, it is clear that the after-load should be thrown on to the muscle at the instant of contraction, when the contractile force of the muscle is at its maximum, and not later; this is the arrangement in the body. Further, it would be an advantage if the load decreased as the contractile force of the muscle during its con- traction decreased; this is not usually the case in the body, but it does occur in certain movements, as, for instance, in jumping or when, with the upper arm horizontal, a weight in the hand is raised by flexing the forearm on the elbow.
CHAPTER XIV. {Advanced).
SUMMATION OF STIMULI.
In a previous chapter the subject of summation of contractions has been dealt with. This summation of 'effect' must be distinguished from the summation of stimuli, by which an inadequate stimulus, if repeated suflSciently often, becomes first adequate and then for a time increas- ingly effective. This is a summation of 'cause,' and probably plays an important part in the life of all living matter.
In order to demonstrate the summation of stimuli, arrange the apparatus for stimulating a gastrocnemius muscle directly with single induction-shocks, using a simple key in the primary circuit. Place the secondary coil at such a distance from the primary that the break- shocks are just subminimal. Eepeat the stimulus every 5 seconds.
ADVANCED EXPERIMENTAL PHYSIOLOGY 63
It will be found that sooner or later the summed excitations will cause a contraction, and, if the contractions are recorded on a slowly revolving drum, that a well-marked ' stair-case ' effect is produced (Fig. 61).
In dealing with the response of muscle to two successive stimuli, it has been seen that, when the second stimulus falls within the latent period of the first, the muscle is refractory, so far as being able to
Fio. 61. — Efifect of subminimal stimuli repeated every 5 seconds on gastrocne- mius stimulated directly. The dots mark the points at which stimuli were sent in before they became obviously effective, lime marking in secunds. (A.P.B.)
respond with a second contraction is concerned ; but it is not true that a muscle during its refractory period always entirely ignores a second stimulus.
In order to investigate this point, the apparatus is arranged as in demonstrating the effect of two successive stimuli (p. 42). The two 'strikers' are placed at such an angular distance apart that the second stimulus falls well within the latent period of the first ; the muscle is stimulated directly. The secondary coil is placed at such a distance from the primary that when, by rotating the drum by hand, one of the strikers is made to pass over the naked wire, a minimal or submaximal break, but no make contraction is obtained. A tuning fork is arranged to write under the myograph-lever, the drum is allowed to make one revolution at a rapid rate, a base line is drawn, and the points of stimulation corresponding to each 'striker' are marked. Swing the lever away from the drum, but do not alter the position of the base of the stand carrying the myograph. The single contraction so recorded is the response of the muscle to two break shocks. In order to determine whether the muscle has been in any way influenced by the second stimulus, raise the second 'striker,' so that it will no longer touch the naked wire, and record the contraction due to the first stimulus alone (Fig. 62). It will be found that the contraction in
64
PRACTICAL PHYSIOLOGY
response to the single stimulus is not so great as that due to the two
stimuli. In other words, there has been a summation of stimuli during the refractory period. In the same way subminimal stimuli can be summated, but two maximal stimuli are summated only when they follow each other after an interval of less than -g^th second.
As has been pointed out on p. 25, when a ' striker ' passes over the naked wire, there is both a make and break of the primary circuit; consequently in these experiments the muscle really receives four induction-shocks, of which, accord- ing to the position of the secondary coil, all four might be individually subminimal, or the two break-shocks might be alone effective, or all four might be eflFective. In order to deal with the summation of two break-shocks alone, it is usual to perform these experiments with the following special piece of apparatus.
The Spring or Trigger Myograph (Fig. 63). — It consists of a heavy metal base which is clamped to the bench. The essential part of the apparatus is an oblong metal frame carrying a smoked glass plate, the recording surface, which is shot on two horizontal wires past the writing points. In order to prepare the apparatus for use, the frame is pulled to one side by one of the arms attached to it ; this compresses a spring on the other arm, and the frame is held in position by a catch or trigger. When the catch is re- leased the spring gives the frame and
ADVANCED EXPEETMENTAL PHYSIOLOGY
65
glass plate a rapid and uniform horizontal motion, and the momentum carries the recording surface across until stopped by the buffers at the opposite side. The frame carries on its under surface two pins which knock over two vertical keys and so breaks two primary circuits (Fig. 64). Ki is fixed, but K^ is movable horizontally, and its position can
Fia, 63. —The spring myogrraph.
be adjusted so that it will be knocked over at any desired interval after Ky A pointer is attached to K^, and when this is opposite the zero of the scale this key will be knocked over at the same instant as K■^ ; there- fore, in order that K^ may be knocked over after K^ and that the second
FlQ. 64. —Diagram of the spring myograph in circuit.
stimulus may still fall within the latent period of the first, it is necessary to move K^ a short distance along the scale from K^ Place both keys in the primary circuit of the same coil and arrange the secondary coil at such a distance from the primary as to give sub- maximal break-shocks. With the spring compressed, the catch down and both keys vertical, the writing points of the lever and tuning fork are placed against the recording surface at its spring end in order that the whole contraction may be recorded. Release the catch. The frame is then pulled back to its original position, both keys are made vertical
ee
PRACTICAL PHYSIOLOGY
again, and the pins on the frame are slowly brought up against the two keys in turn and the points along the curve marked at which the two stimuli entered the muscle; the second stimulus should have fallen well within the latent period of the first. Reset the apparatus, leaving K^ horizontal, but placing K^ vertical, and record the contraction due to the first stimulus alone. This second contraction will be found to be smaller than that caused by the summation of the two sub- maximal stimuli.
Fig. 65 shows the contractions obtained by a Pendulum Myograph which is fundamentally the same as a spring myograph, and differs only in that the smoked plate, instead of being shot horizontally across by a spring, swings across at the end of a long and heavy pendulum and describes an arc of a circle.
The glass plate in either case is varnished in the ordinary way, and, when dry the curves are reproduced by exposing to daylight sensitive paper covered by the smoked plate.
ADVANCED EXPERIMENTAL PHYSIOLOGY 67
CHAPTER XV. {Advanced).
EFFECT OF DISTILLED WATER AND OF VARIOUS SALTS ON MUSCLE.
The various tissues of the body are all bathed in the same fluid, the lymph, which so far as the water and salts it contains are con- cerned, has a uniform composition. The tissues, although immersed in the same fluid, show different and characteristic properties owing to their difference in structure and chemical composition. If, however, the composition of the fluid, in which any given tissue is immersed, be altered, the composition and consequently the properties of its proto- plasm must also be altered. The first effect on living matter of such a change is to cause its stimulation, and then if the change be sufiioiently profound and long-continued to produce its death.
Only two changes in the tissue fluids will be considered here, namely — {a) Gross change in the osmotic pressure of the fluid, by using distilled water or a strong saline solution ; and (b) Change in the ions in solution without alteration in the osmotic pressure of the fluid, by using solutions of various salts isotonic with frog's blood-plasma.
Effect of Distilled Water. — Dissect out a gastrocnemius muscle and place it, without a 'trouser' of skin, in a watch-glass containing distilled water. For a few minutes the muscle may show irregular contractions, then it becomes opaque, swollen and incapable of re- sponding to a stimulus with a contraction. The muscle is said to have passed into a condition of 'water-rigor.' Test the muscle with induction shocks and demonstrate that it will no longer contract.
By placing the muscle into distilled water two effects are produced — the inorganic salts in the muscle diffuse out into the water, and water is attracted by osmosis into the muscle so that each fibre becomes greatly distended with fluid. The first effect of these changes is to produce stimulation, but, as the muscle fibres are distended with fluid, they become incapable of contracting, and finally there are not enough salts left in the muscle to keep the globulins in solution ; hence the muscle becomes gradually opaque and dies.
Effect of Strong Saline Solutions. — This effect will be exactly the opposite of that due to distilled water; for water will be abstracted from the tissue, and large quantities of the salt will diffuse into the muscle.
The effect on a tissue of mere abstraction of water from it is best seen by allowing a nerve to dry. Make a gastrocnemius and sciatic
68 PEACTICAL PHYSIOLOGY
preparation, keep the muscle and lower half of the nerve just moist with tap-water saline, but allow the upper half of the nerve to dry. As the nerve begins to dry, irregular contractions of the muscle come on which are stopped by moistening the nerve ; showing that loss of water acts as a stimulus to nerve. If the drying is allowed to continue, the dry portion loses its irritability and dies.
Now place upon the muscle a few crystals of NaCl; irregular con- tractions will soon appear. These are partly due to the abstraction of water, but also, as we shall see in the next experiments, to the stimulatory effect of NaCl.
The above experiments show that, in order to keep muscles and nerves irritable and in good condition, they must be moistened with a fluid which will neither give up nor abstract water from the tissue, i.e. which is isotonic with the animal's lymph. For this purpose a •7 per cent, solutioa of NaCl in distilled water has frequently been used. This solution, although isotonic with frog's blood, does not contain the calcium and potassium salts found in blood-plasma and lymph; and the question arises whether this alteration of the ions in solution affects in any way the properties of muscle.
In order to investigate this point, prepare two sartorius preparations with their, bony attachments and without injury to their muscular fibres. Place one muscle in Biedermann's solution ('5 grms. NaCl, •2 grms. NajHPO^, 2-04 grms. NajCOg in 100 c.c. distilled water), and the other in -7 per cent. NaCl in distilled water.
The muscle in Biedermann's solution, especially if the solution be cool (3° — 10° C), will after a shorter or longer interval begin to show fibrillary twitches and may even contract regularly and rhythmically as a whole. As soon as the result has been obtained, transfer the muscle to a solution made by adding to 100 c.c. of '7 per cent. NaCl solution in distilled water, 10 c.c. of a saturated solution of CaSO^, or of a 10 per cent, solution of CaClg in distilled water. The spontaneous contractions will soon cease.
The other muscle placed in the pure NaCl solution may remain quiescent; very often it will show fibrillary twitchings and irregular contractions, which are rapidly stopped by transferring the muscle to the solution containing a calcium salt as well as NaCl. Should the muscle, however, remain perfectly quiescent ^ it can still be shown that it is no longer in a perfectly normal condition. After it has remained in the solution for half an hour, remove it and connect it
'Frog's muscle differs somewhat in its behaviour in any given solution accord- ing to the time of year, there being a marked difference between muscle in the autumn and spring.
ADVANCED EXPERIMENTAL PHYSIOLOGY 69
to a myograph lever and stimulate it with a single maximal break shock. The contraction recorded on the drum will be no longer an ordinary single contraction, but a series of tetanic twitches of abnormal height and duration. Now remove the muscle, immerse it for ten minutes in the solution containing the added calcium salt, and again record its response to the same stimulus. A normal single contraction will be obtained. It is clear that sodium salts, when acting alone on skeletal muscle, have a powerful stimulatory effect, and that this can be neutralised by adding a certain proportion of calcium salt. For this reason ' normal ' saline solution is always made with tap-water instead of with distilled water. Some tap-waters, however, do not contain nearly enough calcium to bring about complete neutralisation of the sodium salt.
From the above experiments we learn certain facts of considerable practical importance. We see that tissues are greatly affected by changes in the osmotic pressure of the fluid surrounding them. Care must therefore be taken not to expose the tissues of an animal or man to fluids which are not isotonic with the blood-plasma. In man the solution of NaOl isotonic with the blood-plasma is only just under 1 per cent., and therefore differs widely in strength from the solution for a frog ; it is very necessary to bear this in mind when injecting fluid into veins or under the skin, and when irrigating the peritoneal cavity during operations. We further see that, when isotonic solutions of electrolytes are used, the tissues are by no means indifferent to the ions in solution. A really ' normal ' saline solution would, therefore, be one which contained the same salts in the same proportion as the animal's own blood-plasma. Einger's ^ fluid is an attempt to make such a solu- tion for the frog. Since in man it would often be difficult to obtain such a solution when wanted, it might be preferable, instead of using an imperfectly ' normal ' saline solution, to use an isotonic solution of a non-conductor, such as dextrose. A 5'8 per cent, solution of dextrose is isotonic with human blood-plasma.
In all the above experiments it has been found that skeletal muscle responds to the abnormal constant stimulus by an activity which is not constant, but intermittent or rhythmical. This raises the question whether the rhythmical contraction of the heart may not be the normal response of that particular kind of muscle to the constant chemical stimulus of the blood-plasma, and the same might be also partly true of the rhythmical activity of the respiratory and vasomotor centres.
'A modified Ringer's solution contains NaCl "7 per cent., CaCL, -0026 percent., and KCI '035 per cent.
70 PEACTICAL PHYSIOLOGY
CHAPTER XVI. (Advanced).
FATIGUE OF A VOLUNTARY MOVEMENT AND OF A MUSCLE- NERVE PREPARATION WITH ITS CIRCULATION INTACT.
When a voluntary movement is repeated sufficiently often fatigue is produced. The seat of this fatigue has to be investigated ; it might be in some part of a neurone in the central nervous system, or in some part of the peripheral nerve and muscle : in other words, the fatigue might be primarily central or peripheral. As the result of certain ergographic experiments it has been answered that this fatigue is of central origin. The experiments consisted in lifting a heavy weight suspended over a pulley by flexing a finger and registering the height of each successivo lift. When the movement had been repeated until the muscle was no longer able to lift the weight at all, it was found that electrical stimulation of either the nerve supplying the muscle or of the muscle itself caused the weight to be again lifted, but to a less height than before. When the electrical stimulation had in turn fatigued the movement it was found that a voluntary contraction of the muscle was again able to lift the weight, owing, it was supposed, to the resting of the cells in the central nervous system. From these experiments it was argued that the fatigue of a voluntary movement is purely central.
The methods used in the above experiments are open to grave objections, and it is necessary to touch upon some of these in order to avoid them. The use of a heavy weight is open to the objection that the muscle, when no longer able to lift that weight, is still capable of contracting, and could well lift a lighter weight ; therefore, it is better to make the muscle bend or pull on a spring, which will enable the feeblest as well as the strongest pull exerted by the muscle to be recorded. Again, electrical stimulation of a nerve or a muscle can be a much more powerful stimulus than that resulting from the maximal discharge of a motor nerve-cell ; consequently the fact that peripheral stimulation can make the muscle again lift the weight after voluntary impulses fail, is no proof that the fatigue was central. Further, when a nerve or muscle is stimulated by electrodes placed upon the skin, it is impossible to produce equal stimulation of all fibres ; some muscle-fibres will receive a maximal and others only a sub-maximal or minimal stimulus, and the pull of the muscle as a whole will be equivalent to that of a weaker muscle. When the muscle appears to be fatigued by peripheral stimulation, then a return to volitional stimulation, by pro-
ADVANCED EXPERIMEKTAL PHYSIOLOGY
VI
ducing equal stimulation of every fibre, leads to an apparent recovery of voluntary power. In this way is to be explained the apparent paradox, that a muscle fatigued by either voluntary or peripheral stimulation shows a recovery of power when stimulated in the opposite way.
In order to investigate this subject we shall compare the curve of voluntary fatigue taken with a spring ergograph from the human abductor indicis, with the curve obtained from the frog's gastrocnemius, with its circulation intact and stimulated through the sciatic nerve.
The Spring Ergograph. — A simple form of this instrument is shown in Fig. 66 to consist of a rigid upright iron bar which is clamped to
Fig. 66. —Spring ergograph. (Porter.)
the table. From the upper end of this projects a horizontal straight steel spring, the free end of which carries an ordinary writing point. The spring carries on its under side a short vertical steel arm, the lower end of which fits over the distal end of the second phalanx of the index finger. When the abductor indicis contracts the spring is pushed up ; by sliding the vertical arm along the spring the magnification of the movement and the strength of the spring can be altered. The hand is placed along the vertical side of the wooden support and the three outer fingers tied to it, leaving the thumb and index finger free. The forearm should be fixed to the bench in some form of support, but care must be taken not to tie down the arm sufficiently tightly to interfere with its circulation.
The subject of the experiment should sit comfortably and with his eyes shut, should not be spoken to nor in any way have his attention diverted, but should confine himself to giving a maximal contraction of his musclo every time he hears the beat of a metronome, which is set to give a beat eviery second. The observer takes the time of the experiment in minutes and so calculates the number of contrac- tions recorded, further he has to see that the vertical arm does not slip
72 PEACTICAL PHYSIOLOGY
out of position along the finger. In this way take 300 to 600 con- tractions on a drum revolving at an extremely low rate (Fig. 67).
At first sight the most striking feature of the curve is the more or less rhythmical waxing and waning in the height of the contractions ; this seems to he purely central in origin and to be due to variations in the strength of the voluntary impulse communicated to the muscle. Practice to a large extent does away with this rhythm. When the height of the contraction is measured it will be found that the average height decreases during the first 180 contractions and then attains a fairly constant level, which represents about 85 per cent, of the height of the original contractions. The initial decrease is better marked in Fig. 68, and here the fatigue-level was only about 45 per cent, of the original height. The characteristics of an ergographic fatigue-curve, therefore, are an initial fall which takes place during a variable number of contractions, and the attainment of a fairly constant level, which represents varying percentages of the height of the original con- tractions. This curve strongly suggests that during a series of con- tractions two processes are at work ; one by which available combustible material is being used up and the products of kataboljsm are accumu- lating, and the other by which both these defects are made good by the eirculation. During the early part of the curve the first process preponderates over the second and the height of the contraction decreases, but as soon as the two processes exactly balance each other a uniform level is maintained for hundreds of contractions. The probable seat of these processes will be referred to after the next experiment has been performed.
In order to obtain a record of the contractions of the gastrocnemius with its circulation intact, arrange the apparatus for stimulating the sciatic nerve with maximal induction shocks, using a simple key in the primary circuit. The cerebrum of the frog must be destroyed and the muscle-nerve preparation made without causing bleeding. The cerebral hemispheres are destroyed by compression, leaving the medulla and spinal cord intact, and the gastrocnemius is prepared in the usual way. A string ligature is placed beneath the gastrocnemius and tied tightly round the upper part of the tibio-fibula and the remaining muscles ; the leg is then cut through below the ligature. The whole frog is placed belly downwards on the myograph-board, a strong pin is pushed through the lower end of the femur and driven firmly into the cork. A piece of moistened flannel is then pinned down over the trunk to prevent the contractions of the muscles of the trunk from disturbing the lever connected with gastrocnemius. The skin over the middle of the thigh is divided longitudinally for a short distance, the muscles carefully
ADVANCED EXPERIMENTAL PHYSIOLOGY
73
■c
2&
St
Ma o o 3 o ■Oo CS rH %4 n
1
74
PRACTICAL PHYSIOLOGY
separated and the sciatic nerve exposed ind freed ; the nerve is gently raised by slipping a thread beneath it and the electrodes, insulated
c " — to
s s
|a
a
1 1
9 9
gg ■gS
m «
is
5 S
a a
ADVANCED EXPERIMENTAL PHYSIOLOGY 75
from the underlying muscles by a small piece of cork, are placed beneath the nerve. It is essential that the nerve should not be injured and
C 3
should be kept properly moistened throughout the experiment. The muscle is suitably weighted and just after-loaded. The nerve is stimulated by a maximal shock every 6 sees., and the contractions recorded on a drum revolving at the slowest possible rate (Figs. 69, 70).
76 PKACTICAL PHYSIOLOGY
It will be seen that the height of the contractions, although increasing at first, gradually falls off until at the end of about 200 contractions it reaches a uniform level, which represents about 85 per cent, of the original height and was then maintained with scarcely any alteration for three-quarters of an hour. This curve, therefore, is identical in general form with that obtained by the ergograph. Here again we see an initial fall and then a constant level of contraction, representing probably the equilibrium between two opposite processes, which must in this case be affecting some part of the peripheral nerve and muscle. The actual seat of this peripheral change is not absolutely certain (see further Expts. in Chapter XXII.).
Now cut through the leg in the middle of the thigh, so as to destroy the circulation through the gastrocnemius and continue the stimulation (Fig. 71). It will be seen that the height of the contractions rapidly and continuously decreases, and that at the end of about 320 contrac- tions the muscle is no longer able to lift the lever off the after-loading
CHAPTER XVII. (Advanced).
THE RATE OF TRANSMISSION OF A NERVOUS IMPULSE.
1'he rate at which an impulse is transmitted along a nerve is important because it throws some light upon the nature of the impulse. It travels much more slowly than an ordinary ekctric current, and, although it is accompanied by an electric change, it is something more complex. Its rate of propagation is 27 metres per second (88^ feet per sec.) in the frog's sciatic nerve, and 60 metres per second (196 feet per sec.) in the motor nerves of man.
(a) In the Motor Nerves of the Frog. — The following experiment should be performed for the determination of the velocity of the nervous impulse in the sciatic nerve of a frog :
A recording drum is arranged with a ' striker ' for completing the circuit of the primary current of the induction-coil. To the secondary coil are attached two Du Bois keys in the manner shown in the diagram (Fig. 73) ; from these pass two pairs of electrodes, one of which will be applied to the upper portion of the nerve, the other to the lower
ADVANCED EXPEEIMENTAL PHYSIOLOGY
77
portion. The entire length of the sciatic nerve is dissected out, and the gastrocnemius muscle is connected with the lever of a myograph ; the drum is arranged for rapid revolution, and a maximal shock is to be used for excitation. The latency of the muscular contraction (Chapter III, p. 22) is then determined, first for stimulation by the upper pair of electrodes, the lower pair being short-circuited by closure of its Du Bois
Fio, 73. — Diagram of the experiment on the rate of transmiBsion of a nervous impulse.
key ; then the experiment is made with the lower pair of electrodes for the exciting point. The time of this latency is determined by recording underneath the curves the vibrations of a tuning fork with 100 vibra- tions per second ; the difference in time between the moment of stimulation and the resulting contraction in the two cases represents the time taken for the nervous impulse to pass along the length of nerve between the two pairs of electrodes (Fig. 73). This piece of nerve is measured in millimetres,'^ and then the velocity of the transmission of the nervous impulse is calculated.
For the accurate determination of the rate of propagation of a nervous impulse a very rapid rate of movement of the recording surface is required ; for this reason the spring-myograph (Fig. 63, p. 65) or the pendulum-myograph may be used with advantage in the place of the drum.
(b) In the Motor Nerves of Man. — The velocity of the transmission of a nervous impulse in the motor nerves of man can be determined in the following way : A thick-walled india-rubber ball, similar to that used with a photographic ' shutter,' is connected with a recording tambour. Two clinical electrodes are moistened with strong saline solution in order to improve their conduction and contact with the skin; the large flat electrode is fastened to the leg of the subject, and the small electrode placed above the clavicle will be pressed over the brachial nerves. These electrodes are connected with the secondary
' There is a millimetre scale upon the slide of the induction-coil.
78 PRACTICAL PHYSIOLOGY
coil of an inductorium, and in the primary circuit is interposed the ' trigger ' key of the spring-myograph.
The india-rubber ball is held between the middle finger and the thumb, and the contraction of the flexor muscles will be recorded by the lever of the tambour, when the nerve is excited. The moment of stimulation is determined in the usual way (p. 25), and then the experi- ment is again performed, but with the small electrode pressed over the median nerve at the bend of the elbow. The moment of stimulation is again determined, in order to show that the resting position of the point of the lever has not been changed. The difference between the latency in the two contractions is measured by a t>ining-fork vibrating 100 times per second, and the length of nerve between the two points of stimulation is estimated ; from these data the rate of transmission of the nervous impulse can be calculated.
CHAPTEE XVIII. {Advanced).
THE POLARISATION OF ELECTRODES AND UNPOLARISARLE ELECTRODES.
Polarisation of Electrodes. — Ordinary metal electrodes in contact with a muscle or nerve will be surrounded by lymphj and in this fluid electrolysis will take place during the passage of an electric current. The ions resulting from this electrolysis will be positive and negative respectively ; if, therefore, the circuit of this seat of chemical and electrical change be suddenly made or broken, a shock will be produced, for the wires of the electrodes surrounded by the electrolysed fluid will form a minute battery. This can be demonstrated by the following experiment : A pair of electrodes, connected with a Du Bois key, is placed under the sciatic nerve, which has been exposed in the thigh of a pithed frog. Making or breaking the circuit causes no contraction. The two wires of a Daniell battery are connected with each side of the Du Bois key, and the current is allowed to pass through the nerve for several seconds. Then these two wires are rapidly disconnected from the battery and key ; the key is closed and opened, and each time a contraction of the muscles of the leg is caused. This make and break can be repeated several times with a similar result, until the polarisation has disappeared.
This experiment shows the necessity of unpolarisable electrodes in experiments upon the efiects produced in nerve and muscle by the
ADVANCED EXPERIMENTAL PHYSIOLOGY
79
passage of a constant electric current, and also the necessity of using a Du Bois key as a bridge to short-circuit the electrodes.
Unpolarisable Electrodes. — The preceding experiment has shown that the electrolysis occurring around the ordinary metal electrodes may easily act as an exciting electric current, and thus cause errors in experiments. In order to avoid this unpolarisable electrodes are used. The electric current from the battery is conducted through media which are not liable to polarisation.
The structure of Burdon-Sanderson's electrodes is shown in the following diagram (Fig. 74). A smooth amalgamated zinc rod dips into a saturated solution of zinc sulphate, which in turn conducts the current by means of a plug of kaolin or china clay, made into a thick paste with normal saline solu- tion ('75 per cent, sodium chloride). The plug rests upon a small glass tube with a flange; this delays the spread of the zinc sulphate into the kaolin. The nerve or muscle can be placed in contact with the plug of kaolin, or may be connected thereto by threads saturated with normal saline solution and kaolin. The plug must be kept moist with normal saline solution, for the electrodes have a high resistance.
The electrodes must be set up with clean hands and material, other- wise polarisation will occur. The solution of zinc sulphate must not be allowed to touch the tissue, for chemical excitation would occur. Kaolin and normal saline solution do not stimulate muscle and nerve.
The previous experiment on the polarisation of electrodes should be repeated with the unpolarisable electrodes. The result will be negative if the electrodes have been well and truly made.
Flo. 74. — Unpolarisable electrode. Burdon. Sanderson's pattern.
CHAPTER XIX. (Advanced). TRANSMISSION OF A NERVOUS IMPULSE IN BOTH DIRECTIONS.
The excitatory state produced by stimulation of a nerve can be transmitted in both directions. This can be shown by the following
experiments.
Sartorius Experiment.— The sartorius muscle is dissected out and its
80
PEACTICAL PHYSIOLOGY
iliac end is divided into two portions (Fig. 75). Stimulation with a weak induction shock at (a) or (a'), when there are no nerve-fibres, will produce a contraction of the one half of the muscle. Excitation, how- ever, at (b) or (&'), where there are nerves, will evoke a contraction of both halves
Gracilis Experiment. — The gracilis muscle of the frog is in two por- tions completely separated by a tendinous intersection (Figs. 21, 76). Both halves of the muscle are supplied by a single nerve, the individual
Fio. 75, — Diagram of the sartorius experiment to show the transmission of a nervous impulse in buth directions.
Fig. 76. — Diagram of the gracilis experiment to show the transmission of a nervous impulse in both directions.
fibres of which divide and supply both halves of the muscle. Stimula- tion of any kind at (a) or (a') where there are no nerve-fibres causes only the corresponding half of the muscle to contract ; but excitation at (b) or {¥), where the nerves lie, will cause both halves to contract.
CHAPTER XX. (Advanced).
THE RELATION BETWEEN MUSCLE AND NERVE. INDEPENDENT EXCITABILITY OF MUSCLE.
THE
In addition to the experiments which have been described in the elementary course (page 48), the following experiment upon the eartorius muscle should be performed.
The sartorius muscle lies on the ventral surface of the thigh (Fig. 21),
ADVANCED EXPEEIMENTAL PHYSIOLOGY
81
and its outlines can be made distinct by sponging it with the frog's heart full of blood. The muscle is carefully dissected out and will contract when its nerve, which passes into the muscle at the middle of its inner border, is cut across by the scissors. If the muscle be placed between two glass- slides and examined under a microscope, the distribu- tion of its nerve can be seen to resemble that shown in the diagram (Fig. 77). The finer branches of the nerves and even the end-plates can be more readily seen if the muscle be treated with acetic acid. There are no nerves in the terminal portions of this muscle, which consists of fibres running in a direction parallel with its length.
The sartorius muscle is dissected from the other thigh and the nerveless parts are stimulated by a pinch with a pair of forceps or by an electrical shock ; they contract, the muscle possesses independent excitability.
The absence of nerves from the terminal portions can also be shown in the following way. The muscle is suspended from its tibial end and is lowered until the cut iliac end touches some strong glycerine contained in a watch-glass ; it does not contract. A thin transverse slice is cut away and the muscle is again lowered into contact with the glycerine ; there is still no contraction. This pro- cedure is repeated until the nerves are cut across and on contact with the glycerine are stimulated and make the muscle pass into a contracted condition.
Fia. 77. — Bin- gram of the Bartorius musclo to show the distribution of its nerves.
CHAPTEE XXI. (Advanced).
THE EFFECT OF A CONSTANT ELECTRICAL CURRENT UPON THE EXCITABILITY AND CONDUCTIVITY OF NERVE.
The passage of a constant current produces changes in the excitability of nerve, at the anode there is a condition known as anelectrotonus, the excitability is diminished ; at the kathode there is an increase in excitability, a state of katelectrotonus. The conductivity is also affected, there is a fall in both the anodic and kathodic regions. These effects can be shown by the following experiment.
F
82
PRACTICAL PHYSIOLOGY
One Daniell battery is connected by two wires with a Pohl's reverser whereby the direction of the current can be changed; from the reverser the wires pass by means of a Du Bois key to a pair of unpolarisable electrodes. This is the polarising circuit. The stimu- lating ciicuit is set up separately for the production of single induction- shocks (Fig. 78). A preparation of the sciatic nerve and gastrocnemius muscle is carefully made from a recently pithed frog, and is placed in a moist chamber ; a pin is fixed through the lower extremity of the femur, and the tendo Achillis is connected by a thread with a lever.
Fig. 78.
-Diagram of the experiment on the effects of a constant electrical current upon the excitability and conductivity of nerve.
The sciatic nerve is placed across the kaolin plugs of the unpolarisable electrodes. The drum can be moved by hand. A minimal stimulus for the nerve is obtained, care being taken to use only the break or make-shock. The minimal contraction is recorded on the stationary drum.
The current from the polarising circuit is closed in an ascending direction, so that the current enters the nerve on the side near the muscle and immediately above the stimulating electrodes, which are connected with the inductorium. The nerve around the point of entry or anode of the polarising current is depressed in its excitability, and the application of a minimal, or even stronger, stimulus is no longer effective (Fig. 79). The polarising current is short-circuited by the Du Bois key, and by means of the reverser is changed in its direction, so that on opening the Du Bois key the current is descending, and the area of nerve near the stimulating electrodes passes into a condition of
ADVANCED EXPEEIMENTAL PHYSIOLOGY
83
84
PRACTICAL PHYSIOLOGY
SS3.2
BO -^ J
■S-Sg-g
111-
S.2 I "
i< s
1 o Of
£"■2" E § S S
O "0 3
.0 .fl*^
e" T5 o a>
i: « "^
•S ^ O m ASP'S
u a o> g «a g aoN^
■« » 3 ">
.M M o 0) 13 '^^ i-i . O
ADVANCED EXPERIMENTAL PHYSIOLOGY
85
t*-. 3 3 " fl
„ g ns CO S
ej O O O-fJ c]^
Sb P.-2 1=1 S Ego "■" ■O'?
OOP Qi-s fI J3 ill
PEACTICAL PHYSIOLOGY
S =« _ .2
■*3 rr
o ^ - 5 la
a^-g 53
si - a
■i I CO
O CS CD M
13 g » -S
•S y o 2
-? '-^ W) .
" rt (U 1" M
g I ^ H CO g <D i-i 03 bO T3 2 g eJ ^
§ § B g .s
E I & a «
o 2 " Ss
-O .rt ►*< cS jj C 03 ^ tM e« =S M u O _,
2 g m 2
2 -a S o t«
o •" 3 rQ 3
S tt 03 ^ 03 ■2 03 "^ 03 «
, 'S S 'a 03
° M _ Ij C
c o .2 ^ .2 f^ -g " ts
1 " ^ i i
o a _ * 3
^i« g| H ^ -g To §
03 C 03 r-S ^ d U O
S S i 5Po.Sfi
n-( m "^ U ^ p
fill's "^s
« I ill's ■g
'^ja ?s « 9 **
£ a*^ " g °|Si-| I
s-g3|§.gg
& B ^ fH o 'r;
g^ ID 2 a E'g
l«3SaS;§ ■s:|g.s.2's::;
■ill i til
'""Mil
e
sst-a
^S| Nil
ADVANCED EXPEEIMENTAL PHYSIOLOGY 87
The effect of the constant current upon the conductivity of the nerve is determined upon the same preparation. The stimulating electrodes are placed upon the central part of the nerve; a minimal stimulus is found, and its effect is recorded upon the stationary drum. The polarising circuit is now closed through the nerve in either the ascending or descending direction, and then the minimal stimulus is again applied. It is no longer effective owing to the decrease in the conductivity of the nerve. This change in the conductivity of nerve is also shown in the experiment upon the absence of fatigue in a stimulated nerve (Chapter XXII )
CHAPTER XXII. {Advanced).
THE ABSENCE OF FATIGUE IN A STIMULATED NERVE.
Nerves are not subject to fatigue, even if they be repeatedly stimulated for long periods of time. The following experiment not only demon- strates this fact, but at the same time shows that the passage of a constant electrical current through a portion of a nerve blocks the transmission of the excitatory state which is produced in the nerve by a stimulus applied above the polarising electrodes (page 86).
An induction coil is arranged for faradic shocks, and a pair of un- polarisable electrodes are connected by a Du Bois key with a Daniell cell. The two sciatic nerves of a pithed frog are dissected up to their points of exit from the vertebral column, which is then cut across above the nerves. The thighs are out away above the knee, and the two legs with their nerves are placed in a moist chamber, and are fixed by pins pushed through the lower extremities of the femora. The stimulating electrodes, which are connected with the secondary coil by means of a
88 PRACTICAL PHYSIOLOGY
Du Bois key, are placed under both sciatic nerves ; the unpolarisable electrodes are placed under one sciatic nerve midway between the muscle and the stimulating electrodes. The induction shocks are now allowed to pass through both nerves for a few seconds ; the muscles of both legs are thrown into tetanus. The stimulation is stopped and the polarising current is passed through the one sciatic nerve. The faradisation of both nerves is again commenced; the muscle in the one case will be sent into tetanus and quickly fatigued, but the other muscle shows no contraction, for the polarising current passing through its nerve blocks the passage of the nervous impulses evoked by the stimulating electrodes. When the first muscle is fatigued the polarising current should be broken ; the block is removed from the course of the sciatic nerve of the other muscle, which is at once tetanised by the stimulation of its nerve.
CHAPTER XXIII. {Advanced). THE ELECTROMOTIVE PROPERTIES OF MUSCLE AND NERVE.
Three simple experiments upon the electromotive properties of muscle have already been described (page 51). The following ex- periments require more care and very excitable tissues.
Secondary Twitch from the Heart.— If a freshly prepared and very excitable nerve be laid upon the heart of a frog,i so that the cut end of the nerve is on the base and the longitudinal surface upon the apex of the ventricle, a twitch of the muscle connected with the nerve is observed at each contraction of the ventricle. Each time the muscle-fibres of the ventricle contract, a "current of action" is pro- duced and stimulates the nerve
A fine glass rod should be placed under the middle portion of the
' For these preparations the frogs should have been kept cold for some time before the experiment.
ADVANCED EXPERIMENTAL PHYSIOLOGY 89
length of nerve, which lies on the ventricle, so that the current may not be short circuited.
Flo, 84. — Diagram of the experiment to show the stimulation of a muscle by the "current of action " of another muscle.
Stimulation of a Muscle by the "Current of Action" of another Muscle. — The sartorius muscle is very carefully dissected on each side, and then the one muscle is placed overlapping the other; the contact of the two muscles is secured by gentle pressure with two pieces of cork (Fig. 84). Stimulation of one muscle will produce a contraction in both; the "current of action" in the first stimulates the second muscle.
Fio. 85. — Diagram of the experiment to show the stimulatiou of a nerve by its own "current of injury,"
Stimulation of a Nerve by its own "Current of Injury." — Two plugs of kaolin moistened with normal saline solution are placed upon a piece of glass, and the tails of the plugs are made to hang over the edge (Fig. 85). The sciatic nerve of a pithed frog^ is carefully dissected down to the knee, the thigh is cut across, but the leg and foot are left intact. The nerve is so placed that its cut surface is upon one plug and its longitudinal surface upon the other plug. A watch-glass filled with strong saline solution, which is a good con- ductor of electricity, is suddenly brought in contact with the ends of the kaolin plugs ; thus the circuit is suddenly made and can be suddenly broken by the removal of the watch-glass. If the prepar- ation be very excitable, a twitch is observed at each make and
1 For these preparations the frogs should have been kept cold for some time before the experiment.
90 PEACTICAL PHYSIOLOGY
break of the circuit : the nerve is stimulated when the circuit of its
"current of injury" is completed or broken.
CHAPTER XXIV. {Advanced).
THE ELECTROMOTIVE PROPERTIES OF MUSCLE AND NERVE— Continued. THE GALVANOMETER AND THE CAPILLARY ELECTROMETER.
Demonstrations. — The galvanometer and the capillary electro- meter are delicate instruments which are easily damaged; they are employed to investigate the electromotive properties of muscle and nerve. The essential experiments upon that subject have already been performed by means of the so-called " rheoscopic frog." In this course.
Fig. 86.— Galvanometers.
therefore, the experiments with the galvanometer and the capillary electrometer will be demonstrated to the student and only brief details will here be given.
The Galvanometer employed in these experiments is Kelvin's reflecting galvanometer. It consists of a suspended system of magnets so arranged as to make the system nearly " astatic " ; the magnets are
ADVANCED EXPERIMENTAL PHYSIOLOGY 91
surrounded by coils of many turns of fine insulated wire. The resist- ance is high, from 5000 to 20,000 ohms. The movements of the mirror attached to the magnets are indicated by a spot of light upon the scale.
The amount of current sent through the galvanometer is regulated by means of a shunt, which is a resistance box whereby yV'^j tJi7*^> °^ TTTiTS*^ of the total current can be sent through the galvanometer
Fio. 87. — Scale nnd lamp for tlie reflecting galvanometer.
The electric current from the muscle or nerve is led off by means of unpolarisable electrodes, but before an experiment is performed the electrodes are tested, for in most cases they are not perfectly iso- electric. Any small deflection of the galvanometer due to this cause is compensated by a graduated current from a standard battery sent through the galvanometer in the opposite direction.
Perfectly uninjured muscle and nerve are iso-electric, but they are generally slightly damaged during the process of dissection and preparation. The deflection due to this current of injury or demarca- tion current (wrongly called the current of rest) is measured and is then increased by a more pronounced injury caused by touching one end of the muscle with a hot wire. The muscle is now stimulated by a tetanising current applied to its uninjured end ; the deflection of the galvanometer is in the reverse direction, due to the current of action (formerly called the negative variation) which is produced when the muscle contracts.
The current of injury is, as Gotch pointed out, to be considered as a local current of action ; around the injured portion the tissue is in a condition of excitation.
Similar experiments are demonstrated upon nerve.
Lippmann's Capillary Electrometer. — This instrument is a delicate electrical manometer, and is more suitable than the galvanometer for
92
PEACTICAL PHYSIOLOGY
the investigation of the electromotive properties of the fi-og's heart ; it responds to very rapid changes of electrical potential. It consists (Fig. 88) of a glass tube 0 drawn out at one end to a fine capillary tube; this is filled with mercury and is connected with a pressure apparatus by the rubber tubing BT. The capillary tube dips into a small trough filled with 10 per cent, sulphuric acid ; the bottom of this vessel is covered with mercury M in order to provide good electrical conduction with the platinum wire. The movements of the column of mercury in the capillary tube are observed by means of a microscope fitted with a micrometer scale.
The passage of an electrical current through the capillary tube alters the surface tension, and this alteration causes a movement of the mercury in the capillary tube. The movement of the column of mercury is from positive to negative, and the extent of the movement is roughly proportional to the difference in electrical potential. Based upon, these facts are the determination of the direction of, and the measurement of the electromotive force of, the current which is under investigation.
"With the capillary electrometer the electromotive properties of the frog's heart are demonstrated. The base and the apex of the ventricle are led off by unpolarisable electrodes to the electrometer : each time the heart contracts there will be a diaphasic variation, the contracted portion at first becomes negative and then positive to the uncontracted part.
Fio. SS. — DLigram of tlie capillary elec- trometer.
CIRCULATION.
CHAPTER XXV.
THE ANATOMY OF THE FROG'S HEART AND ITS CONTRACTION.
Anatomy of the Frog's Heart. — The cerebrum can be destroyed by the application of a strong pair of Spencer-Wells forceps to the skull. The medulla oblongata and spinal cord are left intact, so that the vaso- motor control continues and the circulation is unimpaired. The frog is pinned on the corkplate belly uppermost. The skin over the abdomen is pinched up and slit up to the mouth. The abdominal wall is then divided slightly to one side of the mid line to avoid cutting the anterior
EI,EMENTAEY EXPERIMENTAL PHYSIOLOGY
93
abdominal vein. By a transverse cut the xiphisternum is divided and the junction of the anterior abdominal vein with the heart preserved. The pectoral girdle is next divided in the middle line. The inner blade of the scissors is kept hard against the sternum to avoid injuring tlie heart beneath. The divided halves of the pectoral girdle are pulled widely apart. The heart is now seen enclosed in a thin membrane — the pericardium. This is picked up with forceps and slit open. A slender band of connective tissue — the fraenum — connects the posterior surface of the heart with the pericardium. A thread is passed under the fraenum with fine pointed forceps and tied. The fraenum is then divided on the side of the thread remote from the heart.
Flo.
"The frog's heart.
A, Anterior view ; B, Posterior view ; section. (Mudge.)
C, Lonuitiidinnl
B3' means of the thread the heart can be lifted up and turned over for examination. In the front aspect of the heart a single blunt pointed ventricle is seen with the bulbus arteriosus and the two auricles — the bulbus ascends over the right auricle from right to left. It separates into two aortae. Each aorta is divided by longitudinal septa into three channels which soon separate and become the carotid, the aortic, and the pnlmono-cutaneous arches.^ The auriculo-ventricular groove separates the auricles from the ventricle. On turning the heart over the sinus venosus is seen, and the white crescentic line which marks the
' The frog respires both by Bkin and lungs.
94 PRACTICAL PHYSIOLOGY
junction of the sinus with the right auricle. Entering the sinus from below is a large vein, the vena cava inferior, into which open the hepatic veins. Above there enter the two smaller superior venae cavae. These are seen ou gently displacing the auricles. The small pulmonary veins enter the left auricle.
The Contraction of the Heart. — The venae cavae and sinus beat first, then the auricles, and lastly the ventricle and bulbus arteriosus. The blood is returned from all parts of the body to the sinus venosus, whence it passes to the right auricle. From the pulmonary veins the blood passes into the left auricle. The two auricles simultaneously contract and expel the blood into the ventricle. The two blood streams do not readily mix in the ventricle owing to the muscular meshwork within its cavity. When the ventricle contracts the venous blood on the right side is the first to enter the bulbus arteriosus. It is directed by a spiral valve within the bulbus into the pulmono-cutaneous arteries. The spiral valve is then driven over and closes the orifice of the pulmono-cutaneous arch, and the blood (partly arterial and partly venous) now passes into the systemic or carotid arch. The resistance is least in the systemic arch, so most of the blood at first takes this pathway. Finally, as the pressure increases in the systemic arch, the pure blood from the left side of the ventricle is expelled into the carotid artery. Between the auricles and ventricle there hangs the auriculo-ventricular valve. The bulbus arteriosus contains two sets of pocket-shaped valves in addition to the longitudinal spiral valve.
The ventricle becomes smaller, harder, and pale in colour during systole, as the blood is driven out of the muscular sponge-work of which it is composed. It reddens in diastole. Count the beats per minute.
The Tissue of the Heart possesses Automatic Rhythmic Power. — Excise the heart, cutting widely round the sinus venosus, and place it in a watch glass. Note the immediate effect and the after-eifect on the rhythm. The beats may at first intermit and then become more frequent, but quickly settle down to about the same rate as before.
The Effect of Temperature on the Rhythm.— Pour on the heart some normal saline solution which has been cooled in ice. The frequency becomes greatly lessened. Replace the cold with warm saline (25° C). The heart-beats become frequent as the temperature rises. If heated to 40°-43° 0. the heart stops still in diastole, but may recover if quickly cooled. If not cooled the heart passes into the condition of heat rigor.
Rhythmic Contraction — the function of the Heart Muscle. — Talcing another heart, cut away the sinus at the sino-auricular junction. After
ELEMENTARY EXPERIMENTAL PHYSIOLOGY 95
a short period of inhibition both parts begin to beat, but with a different rhythm. The sinus is the more injured, and beats at a slower rate. If the cut be made through the auricles, the sinus beat continues and is unaffected by the injury. Cut off the ventricle just above the auriculo-ventricular groove. After a period of inhibition both auricles and ventricle beat. The auricles recover first. Cut through the ventricle below the auriculo-ventricular groove. The apex preparation does not beat spontaneously. It responds to a prick by a beat, and may in some cases be taught to beat rhythmically by rhythmic stimulation. A crystal of common salt placed on the apex or the passage of the galvanic current through the apex preparation provokes its rhythmic contraction.
Cut out small pieces of the bulbus arteriosus, and place them under the microscope in a watch glass containing Einger's fluid. The pieces will beat rhythmically. There are few if any nerve cells in the bulbus, and there are certainly none in some of these pieces, so the rhythm is probably the function of the heart muscle. In support of this are the following facts :
A frog's heart painted with nicotine (1 per cent, solution) continues to beat. Nicotine paralyses nerve cells.
Isolated portions of the mammalian heart will beat rhythmically for hours if fed through their nutrient arteries with oxygenated blood.
The structural elements of the heart are nucleated, branched, and cross-striated cells. The muscle-cells are joined together into net- works and bands, so as to form one functional whole, and hence excitation of any one part leads to the contraction of the whole. The first part to begin to functionate in the embryo is the venous end. In the mammalian heart it has been shov.'n that muscle fibres of a peculiar type connect the auricles with the ventricles ; they form the auriculo ventricular bundle.
The above experiments suggest that rhythmic contractility is the inherent function of the cardiac muscle. The muscle of the sinus and auriculo-ventricular junction is more embryonic in structure and possesses greater power of initiating rhythm. It is less excitable, and conducts a stimulus less rapidly than the muscle of the auricles and ventricle. The auricular and ventricular muscle is more differentiated in structure. The cross striae are more marked. It does not so easily initiate rhythm. Owing to its greater excitability and conductivity it follows the lead of the sinus.
During the period of systole the heart is refractory to artificial excitation. The excitability returns with diastole, increasing as
96 PRACTICAL PHYSIOLOaY
diastole proceeds. The energy of any cardiac contraction depends on the previous activity of the heart, on the pressure of the diastolic filling, on the resistance to systolic outflow, temperature, nutrition, etc. It is indepefadent of the strength of the stimulus so long as the latter is efficient. Owing to the refractory period, the slow rate of contraction, and the independence of the amplitude of contraction on the strength of stimulus, the heart cannot be tetanised.
By the study— with the aid of the capillary electrometer or string galvanometer — of the electrical current of action which accompanies the systole, it has been shown that the contraction of the heart is a simple twitch, and not a tetanus. The current of action is triphasic in the mammal— (1) base negative, ("i) apex negative, (3) base negative. The excitatory wave travels from base to apex and from apex to base, following the course of the muscle-bands, which start from the base, run to the apex, and, turning in there, ascend on the inner wall of the ventricle. The current of action travels at the same rate as the excitatory state.
The power of slow, sustained contraction seems to depend on the richness of the heart-muscle in sarcoplasm. The heart-muscle possesses tone, and this varies with the temperature and nutrition. Muscarine, acids and chloroform weaken, while digitaline, caflfeine, and alkalies increase the tone of the heart. The auricular muscle of the toad exhibits rhythmic alterations in tone.
Antiperistalsis is difficult to produce because the excitatory process in the ventricle is slow, and does not easily affect the more rapidly contracting auricle. The refractory period which persists during systole also prevents antiperistalsis. The excitatory wave is delayed in passing through the more embryonic type of muscle in the sino-auricular and auriculo-ventricular junctions, and therefore the auricle beats in sequence to the sinus and the ventricle in sequence to the auricle. By cooling the sinus and warming the ventricle the sequence of the heart can be reversed, for the excitability of the ventrical is by these means raised, while that of the sinus is lowered.
By gently clamping a strip of tortoise auricle muscle between two little bits of. cork an artificial block can be created, and the piece of auricle below the clamp then beats in sequence to the piece above the clamp. The natural delay in conductivity at the auriculo-ventricular junction is thus imitated (Gaskell). The conductivity is decreased by the clamp.
The nerve cells of the heart are placed in the least differentiated parts : in the sinus (Eemak's ganglion), in the inter-auricular septum
ELEMENTAEY EXPERIMENTAL PHYSIOLOGY
97
(v. Bezold's ganglion), and in the auriculo-ventricular groove (Bidder's ganglion). The nerve cells are the cell stations of the vagus nerve. The nervous system regulates, but does not initiate either the rhythm or sequence of the heart. The maintenance of the rhythm depends on the blood, and there is evidence to show that it especially depends on the oxygen, and on the mineral salts which are in solution in the blood.
The chief mineral salts, chlorides and phosphates of sodium, potassium, and calcium, are dissolved in the blood in minute traces, and are in a state of ionisation. The presence of these ions seems to be absolutely necessary for the production of the excitatory state. As the mineral salts in the serum, with a due supply of oxygen and water, are sufficient to maintain the heart in rhythmic activity for hours, it is clear that the heart muscle contains a large supply of contractile material in its sarcoplasm.
CHAPTER XXVI.
METHODS OF RECORDING THE HEART.
The Suspension Method of Recording the Heart-beat. — The frog is placed on a cork plate which is fixed to the stand beneath the lever.
Pig. so.— Suspension method of recording the contraction of the frog's heart, with
use of rubber thread as a spring.
G
98 PRACTICAL PHYSIOLOGY
A fine pin is bent into the shape of a hook and passed through the tip of the apex of the ventricle. A thread is attached to this hook and to the lever. The tissues round the base of the heart are pinned down to the cork plate on which the frog rests.
Pio. 91.— Contractions of the frog's neart. A=auricular, V=ventricular con. traction. The time is marked in seconds. The curve should be read from left to right. (L.H.)
Flo. 92. — Contraction of the frog's heart. The curve should be read from right to left. The effect of rendering heart bloodless. Note the plateau on the top of the normal ventricular curve, and the pointed top after the blood has escaped at the point marked by the star. Time marked in fifths of seconds. (L.H.)
The lever is provided with a long light straw. A fine wire spring is attached to the lever, and the heart pulls slgainst this. Adjust its tension so that the lever is horizontal,^ and record the heart-beats on a
^ With the form of heart-lever (Fig. 90) the contraction is represented by the down-stroke ; with the lever (Fig. 93) the contraction is indicated by the up- stroke. The curves obtained with the former lever can be best compared with those made with the latter by turning the tracing upside down and reading from right to left.
ELEMENTARY EXPERIMENTAL PHYSIOLOGY
99
drum (slow rate). Note the auricular and ventricular curves, and the rounded top or plateau of the ventricular curve. Kender the heart blood- less by opening an auricle. The apex of the ventricular curve becomes pointed. Internal ten- sion excites the muscle of the heart to more prolonged and sustained contractions.
Another method of re- cording the heart is shown in Fig 93. A long light straw lever is taken, and a needle is passed through it. The needle plays in holes in the brass upright as shown.
Effect of Heat and Cold on the Excised Frog-heart. — Expose the heart of a pithed frog. Pass a small hook attached to a thread through the tip of the ventricle. Excise the whole heart, cutting widely round it, and pin the tissues surrounding the base of the heart to a cork which is attached to the bottom of the vertical limb of a T-piece. The T-piece is placed beneath the recording lever, and the thread which
Fia. 93.-
- Lever, for recording the frog's heart. (Pembrey and Phillips.)
The thread from the heart is attached
FiQ. 94. — Heart chamber.
was attached to the ventricle is fastened to the lever (Fig. 94). Take a tracing of the heart when immersed in a beaker of Ringer's fluid at room temperature (12-15° C). Ringer's solution is made by saturating 0"65 % NaCl with calcium phosphate and adding to each 100 c.c. of this solution 2 c.c. of 1 % KCl. Next fill the beaker with Ringer's
100 PRACTICAL PHYSIOLOGY
fluid which has been kept in broken ice, and take another record. The cooled heart gives slow and forcible beats. The periods of con- traction and relaxation are prolonged, the frequency greatly diminished. Now fill the beaker with Einger's fluid at 25° 0. The frequency becomes greatly increased, and the period of contraction and relaxation
Fio, 95. — Contraction of the frog's heart recorded by the suspension method 15" C and tiiva immersed in saline at 25** O. The curve should be read from left to right. The time is marked in secouds. (L.H.)
greatly shortened. A temperature of about 35° C. causes diminution of the tone of the heart. The ventricle ceases to follow the auricular rhythm, although it still responds to excitation. At 38° to 43° C. the whole heart ceases to beat, and gradually passes into the condition of heat rigor. The heat contraction, when once fully established, is not set aside by cooling.
CHAPTER XXVII.
THE STANNIUS HEART.
The Stannius Heart. — The heart of a pithed frog is exposed and a thread is tied to the fraenum which is then cut away from the posterior surface of the pericardium. Pass a ligature under the two aortae and then by means of the thread attached to the fra«num gently pull the heart towards the mouth of the frog. The dorsal aspect of the heart is now readily seen. Draw the ligature round the white crescentic line
ELEMENTARY EXPERIMENTAL PHYSIOLOGY 101
riAJUiJVAAJU^
./iyvA-Ay>vi/i./iAMjVi
83P
43"
FiQ. 96. — Contraction of the frog- s heart recorded by the suspension method. Efifect of pouring over the heart normal saline at the temperatures indicated. The water cools rapidly when this method is used, aud the heurt is not heated throughout its mass to the temperature indicated. (Pembrey and Phillips.)
102
PEACTICAL PHYSIOLOGY
Pig. 97. — Stanniua heart. The first and second
ligatures (Hedou).
1, Auricles ; 2, Sinus ; 3, Ventricle.
which marks the sino-auricular junction and tie it exactly over this line. The sinus continues to beat, while the auricles and ventricle, after giving a few rapid beats, stand still. The sinus, with its more embryonic type
of muscle, possesses the greatest power of initiating rhythmic con- traction. The more specialised muscle of the auricles and ven- tricle is more excitable, and conducts an excitatory wave more rapidly, but is less capable of initiating rhythm. The ex- citatory wave which is started from the sinus is blocked by the ligature; thus the auricles and ventricle cease to beat. Prick the ventricle; it will respond by a single beat to each stimulus. The Stannius preparation is like a muscle preparation, and can be used to record the contraction of the heart and the latent period. Tie a second ligature just above the auriculo- ventricular groove. Both auricle and ventricle are excited by the ligature and start beating. The rhythm is no longer the same in the three chambers of the heart. The mere contact of the lever or elec- trodes resting on the Stanniused heart sometimes evokes rhythmic contractions. The inhibitory effect of the first ligature has been attributed by some authors to excitation of the vagus nerve.
The Heart cannot be thrown into Complete Tetanus. — Set up a circuit for giving single induction shocks (see Fig. 16, p. 9). Apply
Pio. 98.— Contraction o£ the frog's heart recorded by the suBi)ension method. The effect of tightening the first Stannius ligature at first gently and then firmly. The curve should be read from right to left. The time is marked in seconds. (L.H.)
the electrodes to the Stanniused heart and record the effect of rapidly repeated excitations. The heart gives an incomplete tetanus curve. Owing to the refractory period it cannot be completely tetanised.
ELEMENTARY EXPEEIMENTAL PHYSIOLOGY 103
The Extra-systole and Compensatory Pause.— Excite with a single induction shock a rhythmically beating heart. The heart is recorded as in Fig. 90 or 93. An extra contraction excited during the diastolic
Fig. 99.— Effect of tetanising the Stanniused heart. The curve should be read from left to right. The time is marked in seconds. The third line shows the period of stimulation. (L.H.)
period of the rhythmically beating heart is followed by a compensatory pause. Note that the heart does not respond when excited during systole — the refractory period (Fig. 150).
This period of inexcitability is seen in skeletal muscle (p. 42), but is of much shorter duration than the refractory period of the heart. The difference probably depends upon a slower metabolism in the cardiac muscle.
CHAPTER XXVIII. THE CARDIAC NERVES AND GANGLIA.
The Intra-cardiac Ganglia, and Nerves. — The vago-sympathetic nerves enter the sinus with the