HX641 02610 QP321 .R722 1881 General physiology o RECAP t I M'» i THE Scientific Series mmmmmmmmmmmmmmmimmmmm MSMMNKNiMPttnt j ra w w a ww w MOTWiff W HWwiiw i ts ^^ QT^sa 332? ColumWa 5flnttJerj^ftp College of S^tv^icmns anb burgeons! %ihvaxv Cy^4t^ te^t^'^.-c^ *-y,^X«.4a^ Digitized by the Internet Archive in 2010 with funding from Columbia University Libraries http://www.archive.org/details/generalphysioloOOrose THE miERNATIOML SCIE^^TIFIC SERIES. VOLUME XXXIL INTERNATIONAL SCIENTIFIC SERIES. KOW READY. In 12n-io, and bound in cloth. 1. FORMS OF WATKR. Kv Prof. John Ttndall. $1.50. 2. PHYSICS AND PULITICS ; or. The Application of the Principles of "Natural Selection" and ''Inheritance" to PoUtical Society. Bv W. Bageuot. $1.50. 3. FOODS. By Edward Smith, M.D., LL.B.. F. U.S. $1.75. 4. MIND AND BODY. By Ai,exander Bain, LL. D. $1.50. 5. THE STUDY OF SOCIOLOGY. By Heebrkt Spencer. $1.50. 6. THE NEW CHEMISTRY. By Prof. Josiau P. Cooke, Jr., of Harvard Uni- versity. .f^.llO. 7. 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Rood, Professor of Physics in Co- lumbia College. 130 hich the latter necessarily raised as soon as it strove to contract; but these weights did not act upon the muscle as long as it remained quiescent. It was, therefore, not weighted in the sense which has already been described ; for the weights at- tached were unable to extend the muscle. The com- paratively slight weight of the lever alone extended the muscle, and could be regarded as burden in the ordinary sense. In order to distinguish these weights, which are without effect mitil the muscle strives to contract from weight in the Ordinary sense, we will apply the term ' over-burden ' to them. The burden of a muscle may be great or small. In the experiments described above it was equal to the weight of the lever. Greater weightsmay be selected, a weight being placed upon the scale-plate and the muscle being then raised by means of the screw at the top of the apparatus, so long as the platinum point p still rests on the platinum plate. The muscle is then extended by the weight applied. If additional weight is added to that already on the scale- plate, the former acts as burden, the latter as over- burden. When a muscle thus circumstanced contracts, it has to lift both weights. Let us return to our first series of experiments, in which the weight = 0, or was at least very small. If more and more over-burden is gradually added, it is evident that a point wiU be reached at which the muscle will no lonsfer be able to lift the weight. This point may be very accurately 66 PHYSIOLOGY OF MUSCLES AND NERVES. determined by inserting a chain and an electro-magnet between the vices h and h' . The electric current then passes through the platinum point, the correspond- ing lever, the quicksilver capsule, and the coils of the electro-magnet. The latter becomes magnetic, and at- tracts an armature. As soon, however, as the current is interrupted by the contraction of the muscle, the electro-magnet sets the armature free, and the latter, striking against a bell, gives a signal which shows that the muscle has contracted. In this way even very minute contractions of the muscle are recognised. If the weights which act as over-burden, and counter- balance the tendency to contraction in the muscle, are gradually increased, a limit is reached at which, in spite of the irritation of the muscle, the current of the electro- magnet is no longer interrupted. The muscle is indeed irritated, and a tendency to contraction is generated within it ; but this is not sufficiently great to overcome the weight used ; and the muscle, therefore, remains uncontracted. In this way the extent to which the tendency of a muscle to contract — or its energy, as we called it, can increase — may be found. This extreme limit of its energy is called the force of a muscle. It is the same in amount as that which we theoretically inferred (p. 48) from the change in the elasticity of a muscle during contraction. Each muscle has a definite force dependent on the conditions of its nourishment and on its form. On comparing the muscles of the same animal, it appears that the force is dependent in no way on the length of the muscle-fibres, but on the number of these fibres, or, in other words, on the diameter of the muscle ; and that the force increases in exact pro- portion with the diameter of the muscle. So that a MUSCLE-rORCE. 67 muscle of double thickness therefore possesses double force. It is usual, therefore, to refer the force to units of diameter of the muscle, by dividing the force by the diameter of the muscle, and thus to calculate the force of a muscle of 1 square centimetre diameter.^ It has been found that in the muscles of the frog the force, for a diameter of I centimetre, is about 2*8 to 3 kilo- grammes ; that is to say, a muscle of 1 centimetre in diameter can attain a maximum tendency to contraction ■which a weight of 3 kilogrammes is capable of resist- ing-. This value of the force reduced to units of dia- meter is called the absolute force of a muscle. 6. An attempt has been made to determine the ab- solute muscular force in the case of man also. Edward Weber first tried to do this by an ingenious method. The muscles of the calf were chosen for the experiment. On standing upright and contracting these, the heels, and at the same time the whole body, are raised from the ground. Grymnasts call this balancing. The whole force of the calf-muscles of both legs is therefore greater than the weight of the body. If the body is weighted, a limit is reached at which it is no longer possible to balance. The total weight of the body together with that of all the weights applied, therefore, equals the force of the muscles of the calf; but in calculating this, however, attention must be paid to the fact that the force and the burden do not act on the same lever. ' The following method, adopted by Ed. Weher, is used to de- termine the diameter. The weight of the muscle, which is found by the use of scales, is multiplied together with the specitic weight of the muscle-substance, the result being the volume of the muscle. The length of the muscle is then measured, and the volume is divided by the length, which gives tl;e diameter. 68 PHYSIOLOGY OF MUSCLES AND NERVES. and that the force — the tension exercised bj the muscles of the calf — acts obliquely on the lever. It is of course impossible to determine the diameter in a living man ; it must be observed in a dead body of about the same size as that of the person experimented on. Henke also has lately determined the value of the absolute force of human muscle. He used the flexor muscles of the forearm (cf. fig. 23) to determine this. In the figure, a represents the upper arm, h the fore- arm—the former being in a ver- tical, the latter in a horizontal position ; c represents the muscles which raise or bend the forearm. (There are in reality two of these muscles, M. biceps and M. bra- chialis internus). Supposing that the muscles are stretched, and weights are placed on the hand till the muscles are no longer ca- pable of raising the hand, then, just as in the experiments with the muscles of frogs, equipoise is obtained between the tendency of the muscle to con- tract aud the weight carried. Care must, however, be taken that the muscles act on a long lever arm, the weight on a short one, and the weight of the forearm itself must also be taken into consideration. Due at- tention being given to all these circumstances, and to the diameter of the muscles when drawn into action, Henke calculated that the absolute force in human muscle is equal to from six to eight kilogrammes. Ex- perimenting in a similar way on the feet, he found somewhat lower figures in that case. Weber, however, Fig. 23. Diagram of the FLEXOu :mlscles of the FOREARM. MEASUREMENT OF MUSCLE-FORCE IN MEN. 69 Fig. 24. Dy.namomkter. in his results as regards the calf-muscles, found much lower figures. But in this case, errors in calculation evidently occurred, and explain the diiference. To determine the muscles of the forearm which bend the fingers, a dj'namometer, as represented in fig. 24, may be used. The strong spring handle of steel, J., being grasped with both hands, is pressed together with the whole strength. The alteration in the curves which is effected in the instrument at the points d and d', is trans- mitted by the lever a b a' to the index c, which indi- cates in kilogrammes the amount of force exercised on the graduated scale B. A somewhat elaborate calcu- lation would be necessary to find from this the absolute force of the muscles employed. If, however, the force which men are generally able to exercise with their hands is known, tTie apparatus may be conveniently used to detect occasional variations, such as occur, for in- stance, at the commencement of lameness and other diseases of the locomotive apparatus. The dynamo- meter has, therefore, become of importance in the in- vestigation of diseases. 7. We have already observed that a muscle dm-ing a single pulsation attains its full force, not at once, but only gradually, and we have seen the way in which the periods necessary for attaining the different values of ■ the energy may be determined by means of the electric method of measuring time. If the muscle contracts freely, little or no weight being attached, it exhibits 70 PHYSIOLOGY OF MUSCLES AND NERVES. tliis energy during each, instant in tlie form of increase in speed whicti it imparts to its lower end and to the slight weight attached to the latter. We may now raise the question as to the amount of force which the muscle when it has already accomplished part, say one half, of its contraction, can still evolve. Schwann, who first raised the question, fastened a muscle to one end of the beam of a scale and attached weights to the other end, but supported this end in such a way that the muscle was not extended. He was thus able to determine the force of the muscle in the same way as was described above with the apparatus shown in fig. 20, which depends on exactly the same principle. L. Her- mann repeated Schwann's experiment with this appa- ratus, which is more convenient for the purpose now under discussion. The unweighted, or, at least, "very slightly weighted, muscle having been inserted in the apparatus as accurately as possible, so that the platinum point 2J just rests on the plate, the muscular force is determined in the way described above (see pp. 65, 67). The ^ice which carries the muscle is then lowered to a certain definite extent, say 1 mm. If the muscle is then irritated it can become shorter by 1 mm. before it pulls the lever A ; if it becomes yet shorter it must raise the lever with the weights attached to it. The weight which it can still lift after it has become shorter by 1 mm. may thus be found. The muscle-"\ice is then again lowered — and this is again and again repeated. A series of weight-values is thus obtained which corre- spond with the force of the muscle dm'ing the different stages of its contraction. The result of the experiment is to show that the force of the muscle decreases, slowly at the commencement of contraction, but afterwards ALTEKATION IN MUSCLE-FORCE DURING CONTRACTION. 71 more rapidly. The muscle having contracted as far as possible without any weight, it can naturally no longer raise any weight — its whole energy is expended. The interest of this experiment lies in the fact that it shows in a different way that which we have already said (p. 48) as to change in elasticity during contraction. For these experiments determine the weight proper to each length of the active muscle, so that we can also directly deduce from these the curves of extension of an active muscle, which we had previously constructed only theoretically. The agreement of this deduction with the theory, found in a different way, is an impor- tant confirmation of the views which we have developed as to the bearing of the conditions of elasticity on the labom" accomplished by the muscle. 7'2 PUYSIOLOGY OF MUSCLES AND NERVES. CHAPTER V. 1. Chemical processes within the muscle ; 2. Generation of warmth during contraction ; 3. Exhaustion and recovery; 4. Source of muscle-force ; 5. Death of the muscle ; 6. Death-atiffening {Rigor moi'tis), 1. The relations just described between the elasticity and the work accomplished by the muscle have led us to suppose that a muscle has, as it were, two natural forms, one corresponding to its condition of rest, the other — a shorter form — corresponding to its active con- dition. Irritation induces the muscle to pass from one form into the other, and in so doing it contracts. This is, however, rather a description than an explanation of the fact of contraction. As the muscle on contraction is capable of raising weight, and thus of accomplishing work, it is necessary to inquire how this labour is effected. According to the law of the conservation of energy, the labour so accomplished can only come into existence at the expense of some other energy. Now, it can be proved that chemical processes proceed within the muscle during muscular contraction, while others, which proceed even in the quiescent muscle, are in- creased in degree during this same contraction. The mechanical work must, therefore, be accomplished at the expense of these chemical processes ; and it could CHEMICAL PROCESSES IN MUSCLE. 73 be proved that the amount of work accomplished corre- sponds exactly with these chemical changes. It is easy to show that chemical processes occur within the muscle ; but it is not so easy to determine these quantitatively, so that we are as yet imable to solve the question raised. Helmholtz long ago pointed out the fact that during muscular contraction such con- stituents of the muscle as are soluble in water decrease, while such as are soluble in alcohol increase. E. du Bois- Eeymond showed that an acid — probably a lactic acid {FleischmUchsdiire) — is generated in the muscle duriug its activity. Quiescent muscles also contain a certain amount of a starch-Hke matter called glycogen ; and, as Nasse and Weiss have shown, part of the glycogen is used up during the activity of the muscle, and is transformed into sugar and lactic acid. Finally, it can be shown that carbonic acid is generated in the muscle during its contraction. All these chemical changes are capable of producing warmth and work. In determining whether the whole amount of work accomplished is referable to this source, yet another special difficulty exists in the fact that, as in other machines, warmth is also produced as well as mechanical work. A muscle certainly grows warmer during its contraction, as Beclard and, with yet greater certainty, Helmholtz have shown. With suitable apparatus it is possible to indicate an increase in the warmth of a muscle even during a single contraction. Our knowledge of the chemical constituents of muscle is yet very incomplete. Not only is chemistry as yet unprovided with adequate means of examining albuminous bodies, which are the chief constituents of muscles, but a special difficulty also exists in the great tendency to change in the constituent matter of living 74 PHYSIOLOGY OF MUSCLES AND NERVES. muscle. The methods .usually employed in chemistry for the separation and isolation of different substances are of no avail in this case, since they essentially alter the nature of the muscle. We must, therefore, be satis- fied to assume as certain only that various albuminous bodies occur in the muscle, one of which, called myosin, appears to be peculiar to muscle, and of which others are the non-nitrogenous bodies glycogen and inosit, together with a certain amount of fat and a number of salts.' It appears somewhat doubtful whether lactic acid, which is always present in the muscle, if but in small 'quantities, is to be regarded as a normal con- stituent of muscle substance, or if it is not rather a product of decomposition. The same may be said of the gaseous carbonic acid which, like the- lactic acid, is probably only formed during the activity of the muscle, and also of the nitrogenous bodies, such as creatin, which are present in small quantities in muscle, and which must probably also be regarded only as the products of the dissolution of the albuminous bodies. 2. The only conclusion to be drawn from this frag- mentary information is that part of the muscle-substance unites during the activity of the muscle with oxygen, forming, partly carbonic acid, partly less highly oxidised products. That warmth is generated during these pro- jesses of oxidation, as we have above stated, is not sur- prising. To show this generation of warmth, Helmholtz employed the thermo-electric method. An electric cur- rent rises in a circle composed of two different metals, e.g. copper and iron, as soon as both points of contact — the points where the metals meet or are soldered together' — acquire unequal temperatures. The strength of this current is proportionate to the differeDce in temperature, GENERATION OF WARMTH DUEI^^G CONTRACTION. ID and thus, from the strength of the current, it is possible to determine the temperature of one point of contact if that of the other is known. In our case, in which it is not necessary to determine absolute temperatures, but only to show an increase in warmth, the method is more simple. It is only necessary to provide that the two points of contact have the same temperature at first, a cond,ition which can be recognised by the absence of any current, and the additional degree of warmth ac- quired can then be directly calculated from the strength of the current which is afterwards generated. Helmholtz performed the experiment by placing the two thighs of a frog which had been recently killed in a closed case, after he had so arranged the metals w^iich were to determine the warmth that one point of contact was inserted in the muscles of one thigh, the other in those of the other He then waited till the temperatures of both thighs became equal, so that, though the metals were connected with a sensitive mul- tiplier, no current was apparent. The muscles of one thigh were thrown into strong tetanus by introducing a suitable inductive current, while those of the other thigh remained at rest. The contracted muscles then became warmer and imparted their warmth to the soldered metals embedded in them ; the result was an electric current the strength of which was measured. The increase in the warmth of the muscle, thus de- termined, was about -15 of a degree. This warmth may seem slight, but it must be remembered that but a small mass of muscie was treated, and that this necessarily lost a considerable part of the w^armth gene- rated within it by radiation and by imparting it to the surroundino: substances. 76 PHYSIOLOGY OF MUSCLES AND NERVES. In order to form some conception of the amount of warmth thus generated, we will assume that the specific warmth of muscle is the same as that of water. As the greater part of muscle consists of water,' this assumption cannot he far ■wTong. By the specific warmth of a sub- stance is meant that amount of warmth which is neces- sary to warm one gramme of the substance exactly one degree, the amount necessary in the case of water being regarded as the unit. Therefore about one unit of warmth is requisite to warm one gramme of muscle substance one degree. According to our assumption, in each gramme of muscle substance at least "15 of a unit of warmth is generated. Now it is known that each unit of warmth is equivalent to 424 units of work, that is to say, when warmth is transformed into mechanical work, 424 grammes can be raised one metre by one unit of warmth. If, therefore, no warmth were set free from the muscle during tetanus, but if it were trans- formed into work, each gramme of muscle substance would be able to raise 424-^0*15 gramme to the height of one metre. This amount, therefore, represents the minimum of that which is accomplished as ' internal work ' in the muscle during tetanus. By soldering rods or strips of two metals alternately on to each other so that aR the points soldered are aiTanged in two planes, differences in temperature much more minvite than those which occur during tetanus may be measured. Such an apparatus is called a thermo- pile. Heidenhain had one of these made of rods of ' According to a recent statement of Dr. Adamkiewicz, the spe- cific warmth of muscle is even greater than that of water, though it had previously been assumed that the specific warmth of water is greater than that of any other known substance, with the excep- tion of hydrogen. GENERATION OF WAEMTH DURING CONTRACTION. 77 antimony and bismuth, and having covered the surface of each of the ends with a muscle from the lower leg of a frog, he waited until both had assumed an equal temperature. He then by irritation induced activity in one muscle, and owing to the sensitiveness of the apparatus he was not only able to determine the warmth arising during a single pulsation, but even to indicate differences in this according to the circumstances (burden, &c) under which the pulsation occrured. The law of the conservation of energy would lead us to expect that in cases in which the muscle ac- complished a greater amount of mechanical work, the production of warmth would be less, and vice versa. When weights are applied, as burden, to the muscle, the labour performed increases, as we found, up to a certain point with every increase in weight. The generation of warmth should accordingly decrease in this case. This was not, however, the case in the experiments made by Heidenhain. As we cannot sup- pose that the law of the conservation of energy,' whic^h is elsewhere throughout nature universally valid, is invalid as regards muscle, we can only suppose that the number of chemical modifications occurring at each muscular pulsation is not always the same, but that when greater weight is applied a larger amount of substances are consumed in the muscle, so that both the production of warmth and the work accomplished may, though the irritant remains the same, diflfer according to the degree of tension of the muscle. On the other hand, it is quite in accordance with the law of the conservation of energy that the muscle generates ' On this law sec the admirable work of Ealfoiir Stewart (Inter- national Scientific Series, vol. vi.). 78 THYSIOLOGY OF MUSCLES AND NERVES. the greatest amount of warmtli during tetanus, d\n'ing wMcIl no apparent labour is accomplished. The whole internal work of the muscle is in this case transformed into warmth, thus raising the temperature of the muscle- substance ; and the amount of this warmth may, as we have seen, be at least approximately measured and calculated. 3. One result of the chemical changes which occur Avithin the muscle dm-ing its activity, is naturally that part of the constituent matter of the muscle is expended, other matter being deposited in its place. As long as the muscle remains uninjured within the body of the animal, part of the matter thus formed is carried away, and fresh nutritive matter is brought to replace the expended material. The products which arise by decomposition during the activity of the muscle may therefore be indicated in the blood of the animal, and from the blood they are removed from out of the body by special excretory organs. Accordingly we find that the amount of carbonic acid excreted is considerably increased by muscular labour, and that the other products of muscular decomposition, such as creatin and the urea arising from the latter, lactic acid, &c., reappear in the m-ine. The more abundantly the blood-current flows through the muscles, the more quickly are the products of decomposition removed from the muscle. This is of course possible only in a very inferior degree when the muscle has been cut out from the body. This is the reason why an extracted muscle retains its power of activity for but a very short time. If, for instance, such a muscle is continuously tetanised, it will be found that the contraction, though it is at first very considerable, very soon decreases and EXHAUSTION? AND EECOVERY. 79 finally entirely ceases. The muscle is then said to be exhausted. But if it is allowed to rest it recovers itself so that it can again be induced to contract. This recovery is, however, never complete, and with each repetition of the experiment it becomes more defec- tive, the intervals requisite for recovery becoming continually longer, and the muscle finally remaining incapable of further contraction. If the muscle is not tetanised, but distinct pulsations are induced in it by separate irritants, it retains its power of activity for a very long time. From this it may be inferred that a portion of the products of decomposition perhaps re- form ; or it may be assumed that the muscle contains a large amount of matter capable of disintegration, but that this is capable of only gradual decomposition. So long as the blood continues to flow through the muscle, the products of decomposition are, as we have seen, soon carried away ; but as exhaustion occurs in this case also, we must draw the same conclusion, that the de- composable matter present can undergo decomposition only gradually, and that therefore in this case also intervals must necessarily occur between the separate exercises of activity. A muscle while undisturbed within the organism essentially differs from one that has been extracted in that in the former the expended material can be fully replaced. Accordingly, it is not only capable of again becoming active after an interval of rest, but, provided that the matter added exceeds that which was expended, it is afterward capable of performing more work than it was previously. To this is due the fact that the strength of muscle is increased by a proper alternation of rest and activity. 4. We have now to discover which of the substances 80 PHYSIOLOGY OF MUSCLES AND NEEVES. within the muscle are expended during its activity. As muscle consists principally of albuminous bodies, it has been assumed that it is to the decomposition of these that the labour accomplished is due. We have, however, seen that non-nitrogenous bodies, such as glycogen and muscle-sugar, are also contained in the muscle, and that lactic acid, which must originate from the latter, is formed during the active state. Although it is impossible to determine the products of decomposition within a single muscle, yet this may be done in the case of the whole mass of the muscles of the body during an activity of long continuance ; for the products of decomposition finally pass into the ex- cretions, and it is evident that the whole amount of addition to the excretions may be regarded as a measure of the decomposition in the active muscles. The nitrogenous constituents of muscle are almost without exception excreted in the form of urea with the urine. At least the amount of nitrogen contained in the other excretory products is so very small that it may safely be disregarded. Now, the amoxmt of urea contained in the urine may be determined with very great accuracy. Even when the body is in a state of complete rest — though even then a considerable amount of work is performed in the body, in the action of the heart and of the respiratory muscles — the excretion of urea depends entirely on the amount of nitrogen intro- duced in food. If entirely non-nitrogenous food is taken, then the excretion of urea decreases to a definite point, at which it remains constant for some time. If a larger amount of work is performed, a slight increase in the excretion of urea in fact usually occurs. The a.mount of albuminous matter which must be modified SOuRCE OF MUSCLE-FORCE. 81 within the body in order to afiford this increase in the amount of urea excreted may be calculated. Now, the equivalent in warmth of albuminous bodies is known ; that is, the amount of warmth produced by the com- bustion of a definite weight of albuminous matter is known. And, as the mechanical equivalent of warmth is also known, the amount of work which could be produced by these albuminous bodies under favourable circumstances may, therefore, also be calculated. "When this value in work is compared with the amount of work really accomplished, the figures found are always far too low. From this it may safely be inferred that the albuminous matter which undergoes combustion within the body is not capable of affording the work which is performed, and we must rather assume that other substances also undergo combustion, and con- tribute to the labour performed, contribute indeed even the greater part of such labour. If, on the other hand, the amount of carbonic acid excreted by a man during rest is compared with that excreted during greater labour, the increase is foimd to be very great indeed, and on calculating the amount of labour which should result from the combustion of a corresj)onding mass of carbon, the amount found corresponds nearly enough with that of the work really performed. This experiment, therefore, shows that the muscles generate their work not so much at the expense of albuminous bodies as by the combustion of non-nitro- genous matter. The addition of matter reqmred by the body if it is to remain in a condition capable of labour must, therefore, be regulated accordingly. Hence fol- ' lows the conclusion, of the greatest importance with reference to the question of diet, that men who have 82 PHYSIOLOGY OF MUSCLES AND NERVES. to perform a great amount of labom* require food abounding in carbon. The opposite was formerly as- sumed, the view being founded on the fact that English labourers, who are, as a rule, more capable of work than French peasants, eat more meat, which is a highly nitrogenous substance. It used also to be pointed out that the larger beasts of prey, which feed exclusively on flesh, are remarkable for their great muscular power. Neither instance really proves the conclusion which it was intended should be drawn from it. In the first place, as regards English labourers, more accurate ob- servation of the food usually consumed by them has shown that, in addition to meat, very considerable quantities of food abounding in carbon, such as bread, potatoes, rice, and so on, are taken. As regards the beasts of prey, it is impossible to deny that they are capable of very great labour; but in this case, also, closer observation shows that the whole amount of work accomplished by them is, at any rate, very small when compared with the constant work of a draught horse or ox. The relation of the food to the work performed by the muscles must evidently be regarded as similar to the relation borne by the fuel consumed by an engine boiler to the work performed by a steam-engine. Every- one knows that coal is burned under the boiler, and that this is finally transformed into work by the me- chanism of the machine. The same work might be produced by the combustion of nitrogenous matter; but it would be necessary to use considerably greater quantities. But the machine called muscle cannot be driven by pure carbon ; under the conditions presented' by the organism pure carbon cannot be applied to the SOURCE OF MUSCLE-FORCE. 83 production of work, as it cannot be digested, and, owing to the low temperature of the body, cannot be oxidised. But combinations abounding in carbon, such as are at hand in the carbon hydrates (starch, sugar, &c.) and in fats, are fitted for the purpose, and a given weight of these affords a considerably greater amount of work than can an equal weight of nitrogenous albumens. If, therefore, the muscle is capable, by the combustion of the non -nitrogenous bodies which it contains, of ac- complishing labour, it is evident that this relation is similar to that in the case of the steam-engine, in which the work is accomplished by the combustion of carbon. It Las been objected that the amount of non-nitro- genous substance within the muscle is very small, but the objection is scarcely tenable. If a whole steam- engine with its boiler and the coal in the furnace could be subjected to a chemical analysis, the percentage of coal in the whole mass would of course be found to be very small. But it is not by the amount of coal present at any given moment that the work is performed, but by the whole amount which in the course of a considerable time is added little by little by the stoker. In the case of muscle the blood acts the part of the stoker. It continually adds matter to the muscle,, and the products of combustion resulting from labour escape from the muscle, just as the carbonic acid does from the chimney of the steam-engine. It is evident that the amount of carbon consumed by a steam-engine might be accurately determined by collecting and analysing the carbonic acid which escapes from the chimney. We proceed in exactly the same way in the case of the muscle. The lungs represent the chimney ; the carbonic acid escaping from these may be collected, 84 PHYSIOLOGY OF MUSCLES AND NERVES. and from this the amount of carbon which must be con- sumed may be calculated. AYhatever does not escape in the form of gas during combustion remains behind as ash. The ash of the fire of the steam-engine is represented by the urea and other matter which passes from the muscles into the urine. The whole amount of both must correspond exactly with the whole amount of the products resulting from combustion within the muscle. Although the small amount of the non-nitrogenous substances present in the muscle does not, therefore, prevent us from regarding them as the main source of muscular labour, yet in one point the machine called muscle differs from the steam-engine, which it other- wise so strikingly rcisembles. We found that the ex- cretion of urea undergoes an increase, though this may not be very great, when the muscular labour is in- creased. It is, therefore, evident that there must be a greater destruction of the chief constituents of muscle- substance, of the tissue of which muscle is mainly formed, and which may be compared to the metallic parts of the steam-engine. Even in the latter a waste of the metalHc parts occurs ; but this is comparatively very small in degree. The muscular machine is not constructed of such durable material ; during its ac- tivity it, therefore, continually wastes a comparatively considerable amount of its own substance. As the matter leaves the body in a more highly oxidised form than it had when it was present in the muscle, warmth and work must also be freed during this partial com- bustion of the material of the machine. The muscle- machine works, therefore, partly at the expense of its own form-element ; and, if it is to work continuously, not SOURCE OF MUSCLE-FOKCE. 85 only must the main fuel, but also matter to replace the form-element must be constantly added. The more closely the composition of the food consumed corre- sponds with the material expended, the more complete will be the replacement which can occm-. The expen- diture of non-nitrogenous substance is, as we found, comparatively great, so that it would be entirely wrong to try to supply the loss merely with nitrogenous matter. All experience in the nourishment of labouring men and animals fully confirms this. The addition of nitro- genous matter is necessary, to keep the muscles in good condition ; but a yet more abundant addition of carbon compounds, such as are afforded by the non-nitrogenous food materials, is required, in order to supply the neces- sary amount of the chief producer of labom*. The wood-cutters of the Tyrol, who work exceedingly hard and with great expenditure of strength, accordingly con- sume an immense amount of food abounding in carbon in addition to a certain quantity of nitrogenous matter. They live almost exclusively on flour and butter. Only on one day in the week, Sunday, do they eat meat and drink beer. For six days they are limited to whatever they carry into the forests with them. The nature of the food may, therefore, be very accm-ately regulated in this case. Their power of enduring very great toil is principally due to the large amount of fat contained in their daily food. Chamois hunters and other moun- taineers take chiefly bacon and sugar by way of pro- vision on their laborious expeditions. Experience has taught them that these highly carboniferous com- pounds are especially suited to enable them to accom- plish great labour. Sugar is especially suitable for the purpose, because, being very readily soluble, it 86 PHYSIOLOGY OF MUSCLES AND NERVES. passes rapidly into the blood, and is, therefore, espe- cially capable of rapidly replacing the expended forces. It is not suitable for a sole or main food material durins' long periods, because when a great quantity of sugar is introduced into the stomach it is transformed into lactic acid and the digestion is injiured. 5. When muscles have lain by for some time after their extraction from the body, a change occurs in them which deprives them of their capacity for contracting when irritated. This change intervenes yet more rapidly when they are induced to pass into a state of activity by many repeated irritations. The time neces- sary for the intervention of this change varies much, and depends chiefly on the nature of the animal and on the temperatm'e. The muscles of mammals in a tem- perature such as that of an ordinary room lose their power of contraction in as little as from twenty to thirty minutes ; the muscles of frogs do not lose this power for several hours, and some from the calf-muscle of a frog have been observed to pulsate even for forty-eight hours in the temperature of an ordinary room. At a temperature of from 0° to 1° C. the same muscle may retain its power of contraction even for eight days. On the other hand, in a temperature of, or above, 45°, the contractile power is lost in a few minutes. Exactly the same happens in muscles yet remaining within the body of the animal if the blood-current ceases to pass through the body, either because of the death of the animal, or in consequence of the local application of ligatures to the vessels. This loss of contractile power is spoken of as the death of the muscle. Muscular death does not, therefore, correspond in time with the general death of the whole animal, but it follows this DEATH OF THE MUSCLE. 87 general death at a period varying from thirty minutes to several hours. 6. On looking at the dead muscle of a frog it will be noticed that its appearance differs essentially from that of a fresh muscle. It does not appear so transparent, is much duller and whiter in colour ; at the same time it feels harder, less elastic, but is capable of greater ex- tension, and, iinally, it is tender and easily torn apart, the more so the further the change has proceeded. Ex- actly similar changes affect the muscles of a dead body. This is called the death-stiffening (rigor mortis). E. du Bois-Keymond showed that on the occurrence of this death-stiffening the original alkaline or neutral reaction gives place to an acid reaction. This is probably due to the transformation of the neutral glycogen and inosit into lactic acid, which with the alkalis present forms acid-reacting salts. This change is the cause of the fact that butcher's meat, which remains hard and tough if it is cooked directly after death, becomes gi-adually more tender. If the meat is allowed to lie for a time after death, the death-stiffening again relaxes, the sepa- rate bundles of fibres no longer adhere so firmly to each other ; and when in this condition the meat is better adapted for preparation as food, because it is tender and may be more easily chewed, and because it offers less resistance to the digestive juices. The death-stiffening in its chemical nature, there- fore, bears a certain resemblance to the changes which occur during the activity of the muscle. In the latter case also an acid is formed, which is, however, again eliminated and carried away by the blood. In the death- stiffening this elimination cannot occur, the circulation of the blood having ceased. For this reason death- 88 PHYSIOLOGY OF MUSCLES AND NERVES. stiffening intervenes much more quickly in muscles which have been strongly irritated before death, as for instance in those of hunted animals. But while the formation of acid must always be very slight in active muscle, it increases greatly in muscles which have un- dergone death-stiffening, and the acid acts as a relax- ing agent on the connective tissue which holds the fibres together, so that the latter separate more readily. At the same time, however, another distinct change occurs within the muscle-fibre. If a fresh living muscle- fibre and one that has undergone death-stiffening are examined under the microscope, the latter appears dull and opaque ; the transverse striations are narrower and approach more nearly together, and the contents are not active and fluid, as in the living fibre, but are fixed and broken into fragments. When unextended muscles undergo death-stiffening, they usually become shorter and thicker. In the mobile facial muscles of a dead body the result of this is that the lines, which imme- diately after death were relaxed, again acquire a certain expression. The death-stiffening of the muscles is the cause of a certain rigidity in the limbs of corpses, so that the limbs are retained in the same relative posi- tion in which they were at death ; and it is to this circumstance that the name * death-stiffening ' {rigor mortis) is principally due. Moreover, this change does not occur simultaneously in the muscles of all parts of the dead body ; it usually begins in the muscles of the face and neck and passes gradually downward, so that the muscles of the legs are the last to be affected by it. The relaxation of the rigidity takes place in the same order. On account of the shortening undergone by muscles DEATH-STIFFENING. 89 during death-stiifness it was formerly believed that the latter was to be regarded as a true contraction, as a last exertion of muscular force in which the muscle took leave of its peculiar capacity. There is, however, nothing to show that this shortening which takes place at death, and which may moreover be hindered by the application of even a slight weight, corresponds in any way with the real state of activity. All the phenomena of mus- cular rigidity are, indeed, more fully explained by the assumption that some constituent part of the muscle Avhich is liquid in the living muscle becomes fixed or coagulates. Death-stiffening would accordingly be a process analogous to the coagulation of the blood, which after death or after it has been allowed to escape from the blood-vessels becomes firm, in consequence of the fact that one of its constituents, the blood fibrous matter, or fibrine, secretes itself as a solid. This view of death- stiffness was first expressed by E. Briicke and was after- ward confirmed by Kiihne. If the muscles of a frog are freed from all blood by injection with an innocuous fluid, such as a weak solution of common salt, and are then pressed, a fluid is obtained which represents part of the liquid contents of the muscle-fibres. If this fluid is allowed to stand for some hours in the ordinary tem- perature of a room, a flaky clot forms in it at the same period at which other muscles of the same animal undergo death-stiffening. The expressed muscle-fluid is originally quite neutral ; but while the clot is forming it becomes continually more acid. The resemblance of the process in this muscle-fluid to that in the muscle itself is, therefore, such as to justify the assumption that at the same time a coagulation, simvdtaneously with an acid-formation, takes pla:ce within the muscle DO PHYSIOLOGY OF MUSCLES AND NERVES. itself, and that this coagulation represents the essential fact in death-stiffening. Death-stiffening intervenes, as we found, earlier in proportion as the temperature is higher. Exactly the same is the case in expressed muscle-fluid. If it is heated to a temperature of 45° C. it coagulates in a few minutes, becoming acid at the same time. Muscles also, if they are heated to a temperature of 45° C, undergo death-stiffening in a few minutes. If they are still further heated, up to or above a tempe- rature of 73° C, they contract into shapeless lumps, become quite hard and white, and exhibit a Arm solid tissue resembling the white of eggs when cooked. From this it may be inferred that, besides the matter which coagulates during the death- stiffening, other soluble albuminous bodies are also present in muscle, and that these act as ordinary albunaen as it occurs in blood and in eggs ; for the latter also coagulates when heated to 73° 0. It therefore appears that various kinds of albumen occur in muscle. That which coagulates at 45°, or, though somewhat more slowly, in the or- dinary temperature of a room, is called myosin. It may be assumed that this albiuninous body is natu- rally soluble, but that it is rendered insoluble by the acids occurring within the muscle. Dea.th-stiffening Avould accordingly be the result of the formation of acid. Our knowledge on this point is, however, yet very incomplete, and must remain so until chemistry has afforded more full explanation of the nature of albuminous bodies. CHAPTEB VI. 1. Forms of muscle ; 2, Attachment of muscles to the bones; 3. Elastic tension ; 4. Smooth muscle-fibres ; 5. Peristaltic motion ; 6. Voluntary and involuntary motion. 1. In examining the action of muscle in the previous chapters we have invariably dealt with an imaginary muscle the fibres of which were of equal length and parallel to each other. Such muscles do really exist, but they are rare. When such a muscle shortens, each of its fibres acts exactly as do all the others, and the whole action of the muscle is simply the sum of the separate actions of all the fibres. As a rule, however, the structure of muscles is not so simple. According to the form and the arrangement of the fibres, anatomists distinguish short, long, and fiat muscles. The last- mentioned generally exhibit deviations from the ordinary parallel arrangement of the fibres. Either the fibres proceed at one end from a broad tendon, and are directed towards one point from which a short round tendon then effects their attachment to the bones (fan-shaped muscles) ; or the fibres are attached at an angle to a long tendon, from which they all branch off in one direction (semi-pennate muscles), or in two directions like the plumes of a feather (pennate muscles). In the radiate or fan-shaped muscles the pull of the separate parts takes efi"ect in different directions. Each of these 92 PHYSIOLOGY OF MUSCLES AND NERVES. parts may act separately, or all may work together ; and in the latter case they combine their forces, as is inva- riably the case with forces acting in different directions, in accordance with the so-called parallelogram of forces. As an example of this sort of muscle the elevator of the upper arm — which was before alluded to in the second chapter, and which on account of its triangular shape is called the deltoid muscle — may be examined. Contrac- tions of the separate parts really occur in this. When only the front section of the muscle contracts, the arm is raised and advanced in the shoulder socket ; when only the posterior part of the muscle contracts, the arm is raised backward. WTien, however, all the fibres of the muscle act in unison, the action of all the separable forces of tension constitute a diagonal which results in the lifting of the arm in the plane of its usual position. In some semi-pennate and pennate muscles the line of union of the t^^o points of attachment does not coincide with the direction of the fibres. When the muscle con- tracts each fibre exerts a force of tension in the direction of its contraction. All these numerous forces, however, produce a single force which acts in the direction in which the movement is really accomplished, and the whole action of the muscle is the sum of these separate components, each derived from a single fibre. In order to calculate the force which one of these muscles can exert, as well as the height of elevation proper to it, it would be necessary to determine the number of the fibres, the angle which each of these makes, with the direction finally taken by the compound action, as well as the length of the fibres — these not being always equal. This task if only carried out in the case of a single muscle would be a very great test of patience Fortu- ATTACmiENT OF MUSCLES TO BOXES. 93 natelj no such tedious calculations are requisite for our purpose. The force may be directly determined by ex- periment in the case of many muscles, by the method already described in Chapter IV. § 6 ; the height of elevation possible under the conditions present in the body may be yet more easily found ; and as regards the work which the muscle is able to perform, it makes no difference whether the fibres are all parallel and act in their own direction, or if they form any angle with the direction of work.^ 2. The direction in which the action takes effect does not, however, depend only on the structure of the muscle, but chiefly on the nature of its attachment to the bone. Owing to the form of the bones and their sockets, the points of connection by which the bones are held together, the bones are capable of moving only within certain limits, and usually only in certain direc- tions. For instance, let us watch a true hinge-socket, such as that of the elbow, which admits only of bending and stretching {cf. ch. ii. § 4). As in this case, the nature of the socket is such that motion is only possible in one plane, the muscles which do not lie in this plane can only bring into action a portion of their power of tension, and this may be found if the tension exercised by the muscle is analysed in accordance with the law of the parallelogram of forces, so as to find such of the component forces as lie within the plane. It is different in the case of the more free ball- sockets, which permit movement of the bone in any direction within certain limits. When a socket of this sort is surrounded by many muscles, each of the latter, if it acts alone, sets the bone in motion in the direction ' See Notes and Additions, No 2. 94 PHYSIOLOGY OF MUSCLES AND NERVES. of its own action. If two or more of the muscles as- sume a state of activity at tlie same time, then the action will be the resultant of the separate tensions of each, and this may also be found by the law of the parallelo- gram of forces. There is yet another way in which the work per- formed by the muscles is conditioned by their attach- ment to the bones. The latter must be regarded as levers which turn on axes, afforded by the sockets. They usually represent one-armed, but sometimes two- armed levers. Now, the direction of the tension of the muscles is seldom at right angles to that of the moveable bone lever, but is usually at an acute angle. In this case, again, the whole tension of the muscle does not take effect, but only a component, which is at right angles to the arm of the lever. Now, it is notice- able that in many cases the bones have projections or protrusions at the point of the attachment of the muscles, over which the muscle tendon passes, as over a reel, thus grasping the bone at a favourable angle ; or, in other cases, it is found that cartilaginous or bony thickenings exist in the tendon itself (so-called sesam- oid bones), which act in the same way. The largest of these sesamoid bones is that in the knee, which, in- serted in the powerful tendon of the front muscle of the upper thigh, gives a more favourable direction to the attachment of this tendon than there would other- wise be. Sometimes the tendon of a muscle passes over an actual reel, so that the direction in which the muscle- fibres contract is entirely different from that in which their force of tension acts. 3. The last important consequence of the attach- ELASTIC TENSION. 95 ment of the muscles to the bones is the extension thus effected. If the limb of a dead body is placed in the position %Yhich it ordinarily occupied during life, and if one end of a muscle is then separated from its point of attachment, it draws itself back and becomes shorter. The same thing happens during life, as is observable in the operation of cutting the tendons, as practised by surgeons to cure curvatures. The result being the same dming life and after death, this phenomenon is evi- dently due to the action of elasticity. It thus appears that the muscles are stretched by reason of their attach- ment to the skeleton, and that, on account of their elas- ticity, they are continually striving to shorten. Now, when several muscles are attached to one bone in such a way that they pull in opposite directions, the bone must assume a position in which the tension of all the muscles is balanced, and all these tensions must com- bine to press together the socketed parts with a certain force, thus evidently contributing to the strength of the socket connection. When one of these muscles con- tracts, it moves the bone in the direction of its own tension, but in so doing it extends the muscle which acts in an opposite direction, and the latter, because of its elasticity, offers resistance to the tension exerted by the first muscle, so that as soon as the contraction of the latter is relaxed the limb falls back again into its original position. This balanced position of all the limbs, which thus depends on the elasticity of the muscles, may be observed during sleep, for then all ac- tive muscular action ceases. It will be observed that the limbs are then generally slightly bent, so that they form very obtuse angles to each other. Not all muscles are, however, extended between 96 PHYSIOLOGY OF MUSCLES AND NERVES. bones. The tendons of some pass into soft structures, such as the muscles of the face. In this case also the different muscles exercise a mutual power of extension, though it is but slight, and they thus effect a definite balanced position of the soft parts, as may be observed in the position of the mouth-opening in the face. If the tension of the muscles ranged on both sides is not equal, the mouth opening assumes a crooked position. This happens, for example, when the muscles of one half of the face are injinred ; and it thus appears that in this case the elastic tension is too weak to allow of the retention of the normal position of the mouth. In muscles attached to bones the elastic tension is, however, much greater, a circiunstance which naturally exercises an influence on their action during contrac- tion. 4. As yet attention has only been paid to one kind of, muscle-fibre, that which from the very first we dis- tinguished as striated fibre. There is, however, as we have seen, another kind, the so-called smooth muscle- fibre. These are long spindle-shaped cells, the ends of which are frequently spirally twisted, and in the centre of which exists a long rod-shaped kernel or nucleus. Unlike striated muscle, they do not form separate mus- cular masses, but occur scattered, or arranged in more or less dense layers or strata, in almost all organs.^ Arranged in regular order, they very frequently form widely extending membranes, especially in such tube- shaped structures as the blood-vessels, the intestine, 1 An instance of a considerable accumulation of sfmooth muscle- fibres is afforded by tlie muscle-pouch of birds, which, with the ex- ception of the outer and inner skin coverings, consists solely of these fibres collected in extensive layers. SMOOTH MUSCLE-FIBRES. 97 &c., the walls of which are composed of these smooth muscle-fibres. In such cases they are usually arranged in two ]ayers, one of which consists of ring-shaped fibres surrounding the tube, while the other consists of fibres arranged parallel to the tube. When, therefore, these muscle-fibres contract, they are able both to reduce Fig. 25. Smouth .MLSCLii-Kiisiiiis (oOO toiks enlarged). the circumference, and to shorten the length of the walls of the tube in which they occur. This is of great importance in the case of the smaller arteries, in which the smooth muscle-fibres, arranged in the form of a ring, are able greatly to contract, or even entirely to close the vessels, thus regulating the current of blood through the capillaries. In other cases, as in the in- testine, they serve to set the contents of the tubes in motion. In the latter cases the contraction does net 98 PHYSIOLOGY OF MUSCLES AND NERVES. take place simultaneously througliout the length of the tube ; but, commencing at one point, it continually propagates itself along fresh lengths of the tube, so that the contents are slowly driven forward. The principal agents in this are the circularly arranged fibres, which at one point completely close the tube, while, by the contraction of the longitudinal fibres, the wall of the tube is drawn back over its contents, thus providing for the propulsion of the contents. This is called peri- staltic motion. It takes place along the whole of the digestive canal, from the throat to the other end, and in this case affects the forward motion of the food, as also, finally, the expulsion of the undigested residue. 5. Peristaltic motion may be very well observed by laying bare the throat of a dog, and then placing water in the mouth of the animal, so that the motion of swal- lowing takes place. It may also be seen in the intes- tines when laid bare, as also in the urinary duct, in which each drop of urine leaving the kidneys produces a wave which propagates itself from the kidneys to thj urinary bladder. Such movements may also be artifi- cially elicited by mechanically or electrically irritating some one point of the intestine, urinary duct, or other such part, or by irritating the nerves appropriate to these parts. The most striking feature is the slowness with which these motions take place. Not only does a long time, observable without any artificial aid, elapse after the application of the irritant before the motion begins, but, even if the irritation is sudden and in- stantaneous, the motion excited at one point passes along very gradually, slowly increasing up to a definite point, and then again gradually decreasing. This slow- ness of motion essentially distinguishes smooth from PERISTALTIC MOTION. 99 striated muscle-fibres. But, as we know, this is not a distinction of kind, but only one of degree; for we found that in the case of striated muscle also there is a stage of latent irritation, then a gradually increas- ing, and then again a gradually decreasing contraction. But that which in striated muscle occupies but a few parts of a second, in smooth muscle-fibres occupies a period of several seconds. No artificial aid is, there- fore, required in this case to distinguish the separate stages. At present, research into the nature of smooth muscle-fibre has not resulted in the acquirement of more than this somewhat superficial knowledge. Owing espe- cially to the difficulty of isolating the fibres, and to the rapidity with which they lose their irritability when separated from the body, it is very difficult to experi- ment with them. It is especially not yet clear by what means the transference of the irritation arising at one point to the other part is effected. The transference never occurs in the case of striated muscle. If a long, thin, parallel-fibred muscle is separated out on a glass plate, and a very small part of it is then irritated, the irritation immediately propagates itself in a longitudirsal direction in the muscle-fibre immediately touched. It is impossible to produce contraction in a striated muscle- fibre only at one point in its length, at least while the muscle-fibre is fresh. In dpng muscle-fibres such local contractions do indeed occur. Each separate muscle- fibre, therefore, forms a closed whole in which the con- traction excited at one point spreads over the whole fibre. The speed with which it spreads within the fibre has even been measured. As the striated muscle- fibre in contracting becomes also thicker, a small light lever, if attached to the fibre, is somewhat raised, 100 PHYSIOLOGY OF MUSCLES AND NERVES. and this rise can be indicated on a rapidly-moving myograph plate. If two of these small levers are placed near the ends of a long muscle, and one of the ends is then irritated, the nearer lever is first raised, the more remote not till later. This difference may be read off the plate of the myograph, and thus the speed of the propagation from one lever to the other may be calculated. Aeby, who first tried this experi- ment, found that the speed was from one to two metres in the second, or, in other words, that a contraction excited at one point of a muscle-fibre requires a period of from about -^-i^ to y^ of a second to advance one centimetre. More recent measurements by Bernstein and Hermann show the higher value of from three to four metres in the second. On the death of the muscle, the rate of propagation becomes continually less, finally ceasing entirely in muscles which are just about to pass into a state of death-stiffness, so that on irritation only a slight thickening is seen at the point directly irritated, and this does not propagate itself. Under all circum- stances, however, the excited contraction is confined to the fibres which are themselves actually irritated, the neighbouring fibres remaining perfectly quiescent. In smooth muscle-fibres, however, it is found that the contractions excited at one point propagate themselves in the adjacent fibres also. The marked distinction which thus appears to exist between smooth and striated muscles would, it is true, disappear if the views of Engelmann, resulting from his study of the urinary duct, are confirmed. According to that writer, the muscular mass of the urinary duct does not consist during life of separate muscle-fibre cells, but forms a homogeneous connected mass which only separates VOLUNTARY AND INVOLUNTARY MOTION. 101 into spindle-shaped cells at death. If this view could also be extended to the smooth muscle masses of other parts, a real connection would exist throughout the muscle-membranes, and the phenomena of the propaga- tion of irritation would admit of a physiological explana- tion. 6. As a rule, such parts as are provided only with smooth muscle-fibres are not voluntarily movable, while striated muscle-fibres are subject to the will. The latter have, therefore, been also distinguished as voluntary, the former as involuntary muscles. The heart, however, exhibits an exception, for, though it is provided with striated muscle-fibres, the will has no direct influence upon it, its motions being exerted and regulated inde- pendently of the will.' Moreover, the muscle-fibres of the heart are peculiar in that they are destitute of sar- colemma, the naked muscle-fibres directly touching each other. This is so far interesting that direct irritations, if applied to some point of the heart, are transferred to all the other muscle-fibres. In addition to this, the muscle-fibres of the heart are branched, but such branched fibres occur also in other places, for example, in the tongue of the frog, where they are branched like a tree. Smooth muscle-fibres being, therefore, not sub- ject to the will, are caused to contract, either by local irritation, such as the pressure of the matter contained within the tubes, or by the nervous system. The con- tractions of striated muscle-fibres are effected, in the natural course of organic life, only by the influence of ' Striated muscles also occur iu the intestine of the tench {Tinea vuhjaris), which in this differs from all other vertebrate ani- mals. It is doubtful whether tliis tissue is capable of voluntary motion, but it is very improbable. 102 PHYSIOLOGY OF MUSCLES AND NERVES. the nerves. We must now, therefore, examine the characters of nerves, after which we shall try to explain the nature of their influence on muscles. It must also be observed that the distinction between striated and smooth muscle-fibres is not absolute ; for there are transitionary forms, such as the muscles of molluscs. The latter consist of fibres, exhibiting to some extent a striated character, and, in addition to this, the character of double refraction. At these points the disdiaclasts are probably arranged regularly and in large groups, while at other points (as in true smooth muscle-fibres) they are irregularly scattered and are therefore not noticeable. CHAPTER VII. 1. Nerve-fibres and nerve-cells ; 2. Irritability of nerve-fibre ; 3. Transmission of the irritation ; 4. Isolated transmission ; 5. Irritability; 6. The curve of irritability; 7. Exhaustion and recoverj', death. 1. In the body of an animal nerves occur in two forms : either as separate delicate cords which divide into many parts and distribute themselves throughout the body, or collected in more considerable masses. The latter, at least in the higher animals, are enclosed in the bony eases of the skull and vertebral column, and are called nerve-centres, or central organs of the nervous system ; the nerve-cords pass from these centres to the most distant parts, and are spoken of as the ^peripheric nerve- system. When examined under the microscope these peripheric nerves are seen to be bundles of extremely delicate fibres united into thicker bands within a mem- brane of connective tissue. Each of these nerve-fibres when examined in a fresh state, and enlarged 250 or 300 times, is exhibited as a pale yellow transparent fibre in which no further difierentiation is visible. The appearance of the fibre soon, however, changes ; it be- comes less transparent, and a part lying along the axis becomes marked off from the circumference. This inner part is usually flat and band-like, and when seen under a higher power exhibits a very minute longitudinal 104 PHYSIOLOGY OF MUSCLES AND NERVES. striation, as though it were formed of very delicate fibrillse, or small fibres. It is called the axis-band, or axis -cylinder. The outer part has a crumpled appear- ance, and oozes at the cut ends of the nerve in drops which soon coagulate ; it is called the niedidlary, or marroiv-shexth. The medullary sheath entirely sur- rounds the axis-cylinder ; as, however, when in a fresh, uncoagulated condition, it re- fracts light in exactly the same way as the axis-cylinder, it is undistinguishable from the latter, nor do the two become really separately visible till after the coagulation of the marrow. The medullary- sheath and the axis-cylinder are further enclosed in a tough elastic tube, which is called the neurilemr,ia or nerve-sheath. These three parts are not present in all peripheric nerves. Some of the latter surrounded by the medullary sheath; j^^^.^ ^^ mcdulkry shcath, and are, therefore, axis-cylinders immediately sur- rounded by the nerve-sheaths. When many nerve- fibres are united into a bundle, these marrowless fibres are grey and more transparent, and are therefore some- times called grey nerve-fibres. Those nerve-fibres which have medullary sheaths appear more yellowish white. If the nerves are traced to the periphery, more and more nerve-fibres are continually found to branch off from the common stem, so that the branches and branchlets Fig. 23. Xerve-Fibres. a a, the axis-cylinder, still partially XERVE-FIBRES AND NERVE-CELLS. 105 gradually become thinner. At last only separate fibres are to be seen, these being, however, still in appearance exactly like those constituting the main stem. Such fibres as up to this point have had medullary sheaths ' now frequently lose them, and therefore become exactly like grey fibres. The axis-cylinder itself then some- times separates into smaller parts ; so that a nerve-fibre, thin as it is, embraces a very large surface. The ends of the nerve-fibres are connected sometimes with muscles, sometimes with glands, and sometimes, again, with peculiar terminal organs. In the central organs of the nervous system many nerve-fibres are found which are in appearance in- distinguishable from those of the peripheric system. There are fibres with axis-cyhnder, medullary sheath, and neurilemma, others without medullary sheuth, and, finally, others in which no neurilemma can be detected, and which may therefore be described as naked axis- cylinders. But, besides these, very delicate fibres, far finer than the axis-cylinders, occur. The central organs of the nervous system are however especially marked by the abundant occurrence of a second element, which, though it is not altogether unrepresented in peripheric nerves, yet is only found in the latter distributed in a few places, whilst in the central organs it constitutes an important portion of the whole mass. This consists of certain cell-like structures called nerve-cells, or gan- glian-cells. In each ganglion-cell it is possible to dis- tinguish the cell body, and a large kernel (^lucleus') within this; within the kernel, a smaller kernel (nu- cleolus^ may also frequently be distinguished. Some ganglion-cells are also surrounded by a membrane which occasionally passes into the neurilemma of 6 106 PHYSIOLOGY OF MUSCLES AlSfD NERVES. nerve-fibres, wliicli are connected with the cell. The kernel is finely granulated and is composed of a pro- toplasmic mass, which, when heated, or subjected to certain other influences, becomes dull and opaque, but which in a fresh condition is usually somewhat transparent. The form of the ganglion-cells is very variable. Sometimes they appear almost globular; in other cases they are elliptic ; others, again, are irregular, provided with numer- ous offshoots. Most ganglion- cells have one or more project- ing processes ; some are, indeed, found without processes, but it is certain that this condition is merely artificially produced, the processes having been torn off during the preparation of the ganglion - cell. Ganglion - cells are occasionally inserted in the course of the nerve-fibres, so that the processes differ in no way from other nerve-fibres, as is shown in fig. 27. In the gan- glion-cells of the dorsal marrow, which have many processes. Fig. 2/. Gaxglion-celi.s ^ j- wiTH NERVE-PROCESSES, somc of thcsc appear exactly like the rest of the cell body — that is to say, they are finely granulated ; these are called protoplasmic processes. On the other hand, in NERVE-FIBRES AND NERVE-CELLS. 107 almost every cell a process may be distinguished which is altogether distinct in appearance from the rest. The protoplasmic processes become gradually finer and sepa- rate into more parts, and the processes of neighbom-ing cells are partly connected together. But the one pro- cess which is distinguishable from the rest passes along for a certain distance as a cylindrical cord, and then, suddenly becoming thicker, it encases itself in a me- dullary sheath, and in appearance entirely resembles the medullary fibres of the peripheric system. It is extremely probable, although it is hard to prove it with certainty, that a fibre of this sort passing out of the dorsal marrow is directly transformed into a peripheric nerve-fibre, while the protoplasmic processes continu- incr on their course within the central orp'an serve to connect the ganglion-cells. The nerve-system, the main parts of which we have thus roughly examined, effects the motions and sensa- tions of the body. These qualities belong, however, mainly to the central parts, in which ganglion-cells occur. The peripheric nerve-fibres act merely as con- ducting or transmitting apparatus to or from the central organs. Before examining the peculiar action of the central nervous system, it is desirable to devote some attention to this conducting apparatus and to dis- cover its nature. 2. On exposing one of the peripheric nerves of a living animal and allowing irritants to act upon this, in the way which was described in the case of muscles, two effects are usually observable. The animal suffers pain, which it expresses by violent motion or cries, and, at the same time, individual muscles contract. On tracing the irritated nerve to the periphery, it will be 108 PHYSIOLOGY OF MUSCLES AND NERVES. found that certain of its fibres unite with those muscles ■which pulsated. We already know that the other end of the nerve is connected with the nerve-centre. If the nerve is cut at a point between the irritated spot and the nerve-centre, the muscular pulsation occurs as before on the re-application of the irritant, but the sensation of pain is absent. If, on the other hand, the nerve is cut at a point nearer the periphery, no muscular pulsation results from irritation, but pain is felt. It thus appears that the peripheric nerves, when irritated at any point in their course, are able to cause effects both at their central and peripheric ends, provided that the conductive power of the nerves re- mains uninjured in both directions. This enables us to study more closely the action of the nerves on the muscles, by extracting and preparing a portion of the nerve with its muscle, in an uninjured condition, and then subjecting this nerve to further research. That a nerve is irritable, in the same sense as we found that the muscle was, is already shown by these preliminary experiments. But while it was possible to observe the effects of the irritation on the muscle directly, the nerve does not exhibit any immediate change, either in form or appearance. Even under the strongest microscopic power nothing is discernible, and it would be impossible to know if a nerve is in any way irritable if the muscle which occurs at one end of it did not show by its pulsation that some change must have occiurred within the nerve. The muscle is there- fore used as a re-agent to test the changes in the nerve itself. The requisite experiments may be either with warm-blooded or with cold-blooded animals. As, how- ever, the muscles of warm-blooded animals, when with- IRKITABILITY OF NERVE-FIBRES. 109 drawri from the influence of the circulation of the blood, soon lose their power of activity, the nerves and muscles of frogs are preferable for these experiments. The lower part of the thigh of a frog, with a long portion of the sciatic nerve, which is very easily separable up to the point where it emerges from the vertebral column, is best suited for this purpose. In some cases it is better to use only the calf-muscle with the sciatic nerve ; the muscle must be fastened in the same way as in the former experiments, and its contractions must be made evident by use of a lever. If the muscle, thus fastened, is pinched at any point in its course it pulsates. The same result follows if a thread is passed round the nerve, and the latter is thiis constricted, or if a small piece is cut from the nerve with a pair of scissors. These are mechanical irrit- ants which act on the nerve. Pulsation will, however, also be seen if the nerve is smeared with alkaline matter, or acid — these are chemical irritants. A por- tion of the nerve may be heated ; that is, it may be thermically irritated. In all these cases, the nerve at the point irritated, immediately, or, at least very soon, loses its capacity for receiving irritation. But if the nerve is placed on two wires, by means of which an electric current is passed through one point in the nerve, it may, in this way, be repeatedly electrically irritated withoilt its irritability being immediately de- stroyed. It therefore appears that, in this respect, a nerve acts exactly as does a muscle. If a constant electric current is applied, the result is usually a pul- sation on the closing and the opening of the current, but sometimes a lasting contraction ensues while the current flows through the portion of the nerve. If 110 PHYSIOLOGY OF MUSCLES AND NERVES. inductive shocks are applied, each separate shock pro- duces a muscular pulsation, and if many separate in- ductive shocks are applied to the nerve, the muscle passes into a state of tetanus. These inductive shocks must be applied to the nerve at some distance from the muscle. Each inductive shock induces a muscu- lar pulsation. On cutting the nerve with a pair of scissors, between the point irritated and the muscle, all influence upon the miuscle ceases. It is useless to place two cut surfaces together, even with the greatest care; they may adhere, and the nerve, when super- ficially examined, may appear uninjured, but irritants applied above the point of section cannot act through the nerve upon the muscle. The same thing occurs if a thread, passed round the nerve, is drawn tight be- tween the point irritated and the muscle. The thread may be removed, but the crushed spot proves an im- passable barrier to all influence on the muscle. If, however, the wires are moved and the inductive cur- rents are applied to another point below the cut or the constriction, the action at once recommences. 3. The conclusion to be drawn from these experi- ments is, either that the nerve, even if only a small portion of it is irritated, passes at once into an active condition throughout its entire length as far as the muscle, or that the irritant acts directly only on the spot immediately irritated, and that the activity which is excited in the nerve at this point propagates itself along the fibres until it reaches the muscle in which it causes a contraction. If the latter view is correct, it must also be inferred that any injm'y to the nerve-fibre prevents the propagation of the activity in the latter ; and it may also be deduced from the experiments with TRANSMISSION OF THE EXClTEMf:NT. Ill the constricted nerves, that even if the nerve-sheath is in no way injured, the crushing of the contents of the nerve is in itself sufficient to prevent propagation of the activity. It can be showa that this latter view of the natiu-e of the case is actually correct. For it is possible to determine the time which elapses between the irritation of the nerve and the commencement of muscular pulsation. For this purpose the same methods are applicable a_s we employed in the case of muscles. Electric measurement of time, or the myograph represented in fig. 17, may be used for this purpose. As however in the present case the point to be determined is, not the form of the muscle-curve, but the moment of its commencement, duBois-Eeymond simplified the apparatus so that the curve is draAvn on a flat plate, which is pushed forward by spring power. Fig. 28 represents the apparatus. It stands on a strong cast-iron stand from which rise the two massive brass standards A and B. A light brass frame carries the indicating plate, which is of polished looking-glass, 1 GO mm. in length by 50 mm. in breadth. The frame runs with the least possible amount of friction on two parallel steel wires stretched between the standards. The dis- tance between the standards is equal to twice the length of the frame, so that the whole length of the plate passes across the indicating pencil when the frame is pushed from standard to standard. Eound steel rods are fastened to the short sides of the frame ; and these rods in length somewhat exceed the path along which the frame passes, and they then pass, with as little friction as possible, through holes in the standards A and B. The end b of one of these rods is surrounded by a steel spring. By compressing this between the standard B and a knob on 112 PHYSIOLOGY OF MUSCLES AND NERVES. the end of the rod, and thus driving- the frame witti the rods from B to A, in a direction opposite to that of the arrow on the indicating phite, a point is reached at which the ' trigger ' which is seen on the standard ^, and which acts upward, fits into a corresponding notch in the rod at a, thus preventing the re-extension of the spring. It therefore remains compressed till pressure Fig. 28. Spring Myograph, as used by du Bois-Reymond. on the trigger frees the frame, which then traverses the whole length of the wires at a speed depending on the strength of the spring, &c., in the direction from A to B, that indicated by the arrow. In order to describe the miiscle-pulsation on this plate, side by side with it there is a lever with an indicating pencil, such as was used in the former ex- periment, to indicate the height of muscular elevation TRANSMISSION OF THE EXCITEMENT. 113 and the elastic extension (see fig. 8, p. 26). This part is omitted in fig. 28, in order to make the indicating plate more visible. The rate at which the plate flies from A to B at first increases up to the point at which the spring exceeds the position in which it was when at rest. When the frame is in the position corresponding with this point, a projection d, which is situated on the lower edge of the frame, strikes the lever h and thus opens the main current of an inductorium, by which an inductive cm'rent is caused in the secondary coil of the inductorium ; and this traverses and irritates the muscle. The result of this is that the muscle is irritated exactly at the moment at which the glass plate assumes a definite position relatively to the indicating pencil of the lever. If the glass plate is first pushed toward A, and is then slowly pushed toward 5, until the projection d just touches the lever, and if the muscle is then caused to pulsate, the indicating pencil, being raised by the pulsation, describes a vertical line, the height of which represents the height of elevation of the muscle. If the glass plate is agajn brought back to A, and, by pressing the trigger, is then caused to fly suddenly and with great speed toward B, then the irritation of the muscle will occur when the glass plate is in exactly the same position, the indicating pencil standing exactly at the vertical stroke before described. The muscular pulsation thus produced will, however, in this case be indicated on the rapidly moving glass plate, with the result of giving, not a simple vertical stroke, but a curved line. The distance of the point of commence- ment from the vertical stroke expresses the latent irritation. If, instead of irritating the muscle itself, a point 114 PHYSIOLOGY OF MUSCLES AND NERVES. ' in the uerve is exposed to the irritation, the muscle in this case also describes the curve of its pulsation on the rapidly moved plate of the myograph. Arranging matters so that two curves of pulsation are allowed to describe themselves in immediate sequence, but with the difference that the nerve is irritated in one case at a point near the muscle, but in the other case at a point far from the muscle, two curves will be obtained on the plate of the myograph, which will appear ex- actly alike but yet will not cover each other. On the contrary, they are everywhere somewhat separated from each other, as is shown in figure 29.^ In this Fig. 29. Pi;opagation of the excitement within nehves. figure, a 6 c is the curve first described, on irritation of the nearer portion of the nerve ; in order to dis- tinguish it from the other it is marked by small nicks ; a' h' c' represents the curve indicated immediately after the former, but obtained as the result of the irritation of a portion of the nerve remote from the muscle. The second curve is seen to be somewhat separated from the other ; it does not commence so soon after the moment of irritation (which is indicated by the vertical stroke o) ; that is, a longer time elapsed between the moment of ' The curves in fig. 29 wore described when the g]ass plate moved more rapid! j'-. so that they appear more extended than those represented in figure 18. TKAKSMISSION OF THE EXCITEMENT. 115 irritation and the pulsation of the muscle in the latter case than in the former ; and this diflference evidently depends only on the fact that in the latter case the excitement within the nerve had to traverse a longer distance, and therefore reached the muscle later, so that the pulsation did not begin till. later. This time may be measured, if the rate at which the plate moved is known ; or if simultaneously with the muscle-pulsation the vibrations of a tuning-fork are allowed to indicate themselves on the plate. From the time thus found and from the known distance between the two irritated points of the nerve, the rate at which the excitement propagates itself along the nerve may be calculated. Helmholtz, on the ground of his experiments with the nerves of frogs, found it to be about 24 m. per second. It is not, however, quite constant, but varies with the temperature, being greater in higher and less in lower temperatiu-es. It has also been determined in the case of man. If the wires of the inductive apparatus are placed on the uninjured human skin, it is possible, as the skin is not an isolator, to excite the underlying nerves, especially where they are superficially situated. On thus irritating two points in the course of the same nerve, the resulting pheno- mena are exactly the same as those just observed in the case of the nerves of frogs. In order to determine the commencement of the muscle pulsation in the un- injured human muscle, a light lever is placed on the muscle in such a way that it is raised by the thickening of the latter. Experiments of this kind were made by Helmholtz with the muscles of the thumb. The appro- priate nerve (n. rtiedianus) may be irritated near the wrist and near the elbow. From the resultinfy difference J 16 PHYSIOLOGY OF MUSCLES AND NERVES. in time and from the distance between the two irritated points the rate of propagation of the excitement was found to be 30 m. per second. The high figure as com- pared with that found with the nerves of frogs is ex- plained by the higher temperature of human nerves. The rate of propagation would indeed be much lowered if the temperature of the arm were considerably de- creased by the use of ice. The above calculation of the rate of propagation is made on the assumption that this rate is constant throughout its duration. There is, however, nothing to shoAV that this is the case. On the contrary, it is more probable that the propagation proceeds at first at a greater and afterwards at a less speed. This may be inferred from an experiment arranged by H. Munk. If three pairs of wires are applied to a long nerve, one close to the muscle, another at the centre, and the third considerably above, and then causing three con- secutive curves to describe themselves on the myo- graph plate by irritating these three points, it will be found that the three curves are not equally removed from each other ; on the contrary, the first and second stand very near together, while the third is far from the two former. More than double the time was re- quired for the excitement to traverse the full distance from the upper to the lower end than it took to traverse the half-distance from the middle of the nerve to its lower end. The simplest explanation which can be given of this phenomenon is that the excitement during its propagation is gradually retarded, just as a billiard ball moves at first very quickly but afterward at a gradually decreasing speed. The retardation of the billiard ball is due to the friction of the underljdng surface. From ISOLATED TRANS]VUSSION. 117 this it may be inferred that a resistance to the trans- mission exists within the nerve, and that this gradually retards the rate of propagation. Such a resistance to transmission is also probable on certain other grounds, to which subject we shall presently revert. 4. If the main stem of a nerve is irritated by elec- tric shocks, all the fibres are invariably simultaneously irritated. On tracing the sciatic nerve to its point of escape from the vertebral column, it appears that it is there composed of four distinct branches, the so-called roots of the sciatic plexus. These rootlets may be separately irritated, and when this is done contractions result, which do not, however, affect the whole leg but only separate muscles, and different muscles according to which of the roots is irritated. Now as the fibres contained in the root afterward coalesce in the sciatic nerve within a membrane, it follows from the experi- ment just described that the irritation yet remains isolated in the separate fibres and is not imparted to the neighbouring fibres. This statement holds good of all peripheric nerves. Wherever it is possible to irri- tate separate fibres the irritation is always confined to these fibres and is not transmitted to those adja- cent. We shall afterwards find that such transmis- sions from one fibre to another occur within the cen- tral organs of the nervous system. But in these cases it can be shown with great probability that the fibres not only lie side by side, but that they are in some way interconnected by their processes. In peripheric nerve-fibres the irritation always remains isolated. Their action is like that of electric wires enclosed in insulating sheaths. One of these nerves may indeed be compared to a bundle of telegraph wires, which are 118 PHYSIOLOGY OF MUSCLES AND NERVES. protected from direct contact with each other by gutta- percha or by some other substance. The comparison is, however, but superficial. No electrically-isolating membrane can. really be discovered in any part of the nerve-fibre, but all their parts conduct electricity. When, as we shall presently find, electric processes occur within the nerve, these standing in definite re- lation to the activity of the nerves, we must assume that isolation as it occurs in the nerves is not the same as in telegraph wires. We cannot here trace the matter further, but must accept the fact of isolated conduction as such, reserving its explanation for a future occasion. 5. On irritating the nerves by means of currents from an inductive apparatus, it is found that the pulsa- tions which occur are sometimes strong, sometimes weak. All nerves are not alike in this respect, and even the parts of one and the same nerve are often very different. We must accordingly suppose that nerves are variable in the degree in which they receive irritation. This is spoken of as the excitability of the nerve, to express the greater or less ease with which they may be put in action by external irritation. Two ways may be adopted to measure the excitability of a nerve or of a certain point in a nerve. Either the same irritant may always be used, and the excitability may be determined by the strength of the muscular pulsation evoked by this irritant ; or the irritant may be altered until it just suffices to evoke a muscular pulsation of a definite strength. In the former case it is evident that the excitability must be estimated as higher in proportion as the muscular pulsation pro- duced by the irritant is stronger; in the latter case the excitability is said to be greater in proportion EXCITABILITY. 119 as the irritant wHch is able to evoke a pulsation of definite strength is weaker. Each of these methods when practically applied has advantages and disad- vantages. The former is capable of detecting very minute differences in the excitability, but it can only do this within certain narrow limits; for when the excitability sinks, the limit for a definite irritant is soon reached, after which no further pulsation at all results ; and when the excitability rises, the muscle attains its maximum contraction, above which it is incaj)able of fiuther contraction. Changes above or below either of these limits are, therefore, beyond observation so long as the irritant remains the same. The best way to apply the second method practically is to find that strength of irritant which exactly suffices to produce a just observable contraction of the muscle. This assumes the power of graduating the strength of the irritant at pleasure. If inductive, currents are used to effect irritation, this graduation may be made with the greatest precision by altering the distance between the primary and secondary coils of the apparatus. In du Bois-Eeymond's sliding inductive apparatus, repre- sented in fig. 13, p. 35, the secondary coil is, there- fore, attached to a slide which may be moved forward in a long groove. This arrangement is used in order to find the particular distance of the secondary coil from the primary which results in a just observable contraction of the muscle ; and this distance, which can be measured by means of a scale divided into millimetres, is regarded as the measure of excitability.' 6. If a recently prepared nerve, as fresh as possible, is placed on a series of pairs of wires, and the excita- ' See Notes and Additions, No. 3. 120 PHYSIOLOGY OF MUSCLES AND NEEVES. bility at the various points of the nerve is consecutively determined in the way described above, it is generally found that the excitability of the upper part of the nerve is greater than that of the lower. There is, how- ever, no great regularity in this character. Sometimes a point is found in the centre of the nerve which is less irritable than those immediately above and below it. Very frequently the most excitable point occurs, not immediately at the cut end, but at some little distance from this ; so that, on proceeding downward, it is found to increase at first, and then, at a yet lower point, to decrease again. If such a nerve is observed for some, little time, its excitability at the various points being tested every five minutes, it is found that the excita- bility alters especially soon at the upper end; it de- creases, and in a short time is entirely extinguished, so that no muscular pulsations can afterwards be elicited from the upper parts even by the most powerful currents. The nerve is then said to be dead in its upper parts, and this death proceeds gradually down- ward in the nerve, so that pulsations can only be obtained by irritating the part situated nearest the muscle, and at a little later period even this part becomes dead. After the whole nerve is dead, pul- sations may yet always be obtained for a time by direct irritation of the muscle. The muscle does not usually die until much later than the nerve Yet in a quite fresh preparation of the nerve and muscle, the latter is always less excitable than the former, and a much stronger irritant is required to excite the mascle directly, than indirectly through the nerve. In all these experiments the nerve must be care- fully protected from drying up, as otherwise its excita- CURVE OF EXCITABILITY. 121 bility is very soon destroyed, and in a very irregular manner. We have seen that the nerve dies gradually fiom the top downward. This death does not, however, consist in a simple falling off in the excitability from its original degree till it completely dies out. If the excitability is tested from time to time at a point some distance from the cut end, it is found to increase at first until it reaches a maximum, at which it remains for some time stationary, and it is not till after this that it gradually decreases and finally expires. The further the point experimented on is from the point which has been cut, the more slowly do all these changes occur ; but their sequence is in all cases essen- tially alike. The explanation of this may be that the upper parts of the nerve, which directly after the pre- paration is made usually exhibit the highest degree of excitability, are really already changed. It must be assumed that these changes intervene very quickly at a point close to the section, so that it is impossible to submit these points to observation until they are al- ready in the condition which does not intervene till later at the lower points — in the condition, that is, of increased excitability. This view is confirmed by the following experiment : if the excitability is determined at a lower point of the nerve, and the latter is then cut through above this point, the excitability increases at the point tested, and this takes place more quickly in proportion as the cut was made nearer to the tested spot. Each of the lower points may, therefore, be artificially brought under the same conditions under which only the upper parts of the nerve usually lie, that is, it may be arranged that they are near the 122 PHYSIOLOGY OF MUSCLES AND NERVES. point of section. These chang-es in the excitability may, therefore, he thus conceived : that when the nerve is cut some influence makes itself felt from this cut, and that this first increases the excitability of the nerve, then decreases, and then extinguishes it. If this view is right, we must assume that the high degree of excitability of a freshly cut nerve is also only the result of the incision which is made. This is not, however, exactly the case. The nerve with the muscle of a living frog may be freed and prepared np to the vertebral column without separating it from the dorsal marrow. On irritating the various points in such a nerve, differences, slight indeed but yet observable, are noticed in the excitabiKty, the upper parts being always more excitable than the lower. Uniujured human nerves may also, as we have seen, be irritated at various points in their course, and in this case also it is found that irritation is invariably more easily effec- tive in the upper than in the lower parts. Pfliiger, who first called attention to the differences of excitability at the various points of the nerve, thought that the explanation of this is that the irritation evoked at one point in the nerve, in propagating itself along the nerve, gradually increases in strength; he spoke of it as an avalanche-like increase in the exciternent tvithin the nerves. This explanation appears to contra- dict the above-mentioned fact as to the effect of cutting on the nerve, for in such cases it appears that the irri- tation is strengthened by the cutting away of the higher portion of the nerve, even though the length of that portion of the nerve which is traversed by the irritation remains unaltered. It must at any rate be admitted that at one and the same point in the nerve DEATH OF THE ISERVE. 123 the excitability may vary in degree, and it is therefore simpler to assume that the difference in the results of irritating the nerve at various points depends directly on differences in the excitability at those points, instead of being in the first place dependent on changes caused by transmission ; it can even be shown to be probable on various grounds, as indicated above, that the excite- ment in propagating itself through the nerve meets with resistance, and is therefore rather weakened than strengthened. Why the excitabihty diflfers in different parts of the same nerve we cannot explain. As long as we are ignorant of the inner mechanism of nerve- excitement, we must be satisfied to collect facts and to draw attention as far as may be to the connection of details, but we must decline to offer a full explanation of these.^ 7. The phenomena of exhaustion and recovery may be exhibited in nerves as in muscles. If a single point in a nerve is frequently irritated, the actions become weaker after a time, and finally cease entirely. If the nerve is then allowed to rest for a time, new pulsations may again be elicited from the same point. It is not known whether this exhaustion and recovery corresponds with chemical changes in the nerve. We are almost entirely ignorant of the whole subject of chemical changes within the nerve. Some observers maintain that in the nerve, as in the muscle, an acid is set free duriug the active condition, but this is denied by others. The generation of warmth in the nerve during its activity has also been asserted, but this is also doubtful. If any chemical changes do take place within the nerve, they are extremely weak and ' See Notes and Additions, No. 4. 124 Pm'SlOLOGrY OF MUSCLES AND NERVES. cannot be shown with our present appliances. As motions of the smallest particles (molecules) probably take place in the nerve, though the external form remains unaltered, and therefore no work worthy of consideration is accomplished, it is easily intelligible that these processes may be accompanied only by ex- tremely slight changes in the constituent parts. The speed with which death and the changes in excitability connected with death take place mainly depends, apart from the length of the nerve, on the temperature. The higher the temperature the more quickly does the nerve die. At a temperature of 44° C. death occurs in from ten to fifteen minutes ; at 75° C. in a few seconds ; and in the average temperature of a room the lower ends of a long sciatic nerve may re- tain their excitability for twenty-four hours or longer after extraction and preparation. Drying at first in- creases the excitability, but afterwards rapidly decreases it. Chemical agents, such as acids, alkalis and salts, destroy the excitability the more rapidly the more concentrated they are. In distilled water the nerve swells and rapidly becomes incapable of excitement. There are, therefore, certain densities of salt solutions in which the nerve remains excitable longer than in thinner or in more dense solutions. A solution of com- mon salt of 0'6 to 1 per cent., for instance, has almost no effect on a nerve submerged in it, and preserves the excitability of this nerve about as long as damp air. Pure olive oil, if not acid, may also be regarded as innocuous. These are, therefore, used when the in- fluence of different temperatures on the nerve is to be studied. CHAPTER VIII. I Electrotonus ; 2, ilodifications of excitability : 3. Law of pulsa- tions ; 4. Connection of electrotonus with excitability; 5. Trans- mission of excitability in electrotonus; 6. Explanation of the law of pulsations ; 7. General law of nerve-excitement. 1. It has already been observed that a constant elec- tric current, if transmitted through the nerve, is able to excite the latter ; but that this exciting- influence takes eifect especially at the moment at which the cur- rent is closed and opened, and that it is less effective during the course of the current's duration. As yet it has been desirable for our purpose, that of studying the process of excitement in nerves, to make use of induc- tive currents, which are of such short duration that the closing and the opening, the beginning and the end, immediately follow each other in quick succession. Without now entering into the question, to be dis- cussed later, as to why the exciting action of the cur- rent is less during the steady flow of the latter than at the moments of closing and opening, we will now ex- amine whether the electric currents which traverse the nerves do not act on the nerves in some other way, distinct from their exciting influence. Let us suppose that the current traverses either the whole or a portion of a nerve. At the instant at which the current in the nerve is closed, the appropriate muscle 126 THYSIOLOGY OF MUSCLES AND NERVES. pulsates, thus indicating that something, which we have called excitement, has occurred within the nerve. WTiile, however, the current flows steadily through the nerve, the muscle remains perfectly quiescent, nor is any change apparent in the nerve itself. Yet it may easily be proved that the electric current has effected a com- plete change in the nerve, not only in that part traversed by the current, but also in the neighbouring parts above and below the portion of the nerve subjected to the electric cm-rent. The great importance of this Kes in the fact that it reveals relations between the forces prevailing in the nerves and the processes of the elec- tric currents, which relations are of great importance in the explanation of the activity of nerves. Our knowledge of nerves has not as yet reached a point at which it is possible to understand all the changes which occur within them under the influence of electric currents. Indeed, but one set of these changes can as yet be described : these are the changes in the excitability. Of all the vital phenomena of nerves, their capacity of being brought into an active condition by irritants has at present alone been studied by us. This, as has been said in the previous chapter, may be quan- titatively determined. Experiment shows that the ex- citability may be altered by electric currents. If a small portion of a nerve is placed on two wires in such a way that an electric current may be caused to traverse this portion, it appears that not only the portion actually traversed by the current, but the nerve beyond this, also suffers changes in its excitability. In order to study these, let us imagine several pairs of wires ap- plied to the nerve n oi' (fig. 30). Through one of these pairs of wires, c d, let a constant current be ELECTROTONUS. 127 "conducted ; by means of proper apparatus the current may be strengthened or weakened, and may be closed and interrupted by means of a key at s. Let a current from a sliding inductive apparatus pass through another portion of the nerve, e.g. a h, and let us find that posi- tion of the secondary coil at which the muscle exhibits marked pulsations of medium strength. The changes which occur in these pulsations when the current in the portion c d is alternately closed and interrupted Fig. 30. Electrotoxls. must now be observed. It is found that thepe changes depend on the direction of the current within the nerve. If the current passes in the direction from c to d, then the action of the same irritant is weakened in the por- tion a h as soon as the current is closed, but regains its former strength as soon as the current is interrupted. In this case, therefore, the excitability in the contiguous portion a b was lowered or hindered by the influence of the constant current traversing the portion c d. If, however, the constant current is reversed, so that it 128 PHYSIOLOGY OF MUSCLES AND NERVES. passes from d to c, the influence of the irritant seems, on the contrary, to increase in a b when the current is closed, and to resume its original strength "when the current is interrupted. In this case, therefore, it ap- pears that the action of the current tends to increase the excitability. If the wires e f are next connected with the secondary coil of the inductive apparatus, and if the irritants are again applied in such a way that weak but noticeable pulsations occur, these latter are strengthened when the current in the portion c d passes from c to d; and are, on the contrary, weakened when the current is in the opposite direction. In these two series of experiments the irritant was applied in one case above, in the other case below, the constant cur- rent. Both cases showed consistent results. As soon, that is, as the irritant acted on the side of the positive electrode or the anode, through which the current entered the nerve, the excirability was in both cases lowered. But when the irritant was applied on the side of the negative electrode or the kathode, through which the current emerged from the nerve, the irritant being strengthened, the excitability increased. These changes in the excitabihty may be shown throughout the whole length of the nerve; but they are strongest in the immediate neighbourhood of the portion traversed by the constant current, gradually decreasing upward and downward from the electrodes. In order to find whether a change in the excitability also occurs within the electrodes, the current must be made to traverse a longer portion of the nerve, and the irritant must then be applied to a point within the electrodes. According to the point at which the elec- trode is applied, various changes maybe shown to occur ELECTROTONUS 129 here also. If the irritant is near the positive electrode, the excitability is lowered ; near the negative electrode it is increased ; and between the two occurs a point at which no noticeable change in the excitabiUty takes place under the influence of the constant current. From all these experiments we may infer that a nerve, one part of the. length of which is traversed by a constant current, passes throughout its whole length into an altered condition, and that this is expressed in the excitability. One part of the nerve, that on the side of the positive electrode, exhibits decreased excita- bility ; the part of the nerve corresponding with the negative electrode exhibits increased excitabiHty. This altered condition is spoken of as the electrotonus of the nerve, the condition which exists on the side of the anode being distinguished as anelectrotonus ; that on the side of the kathode as katelectrotonus. Where the anelectrotonus approaches the katelectrotonus, a point occurs between the electrodes at which the excitability remains unchanged ; this is called the neutral point. The neutral point does not, however, always lie exactly between the electrodes ; but its position depends on the strength of the applied currents. When the cur- rents are weak, it lies nearer the anode ; when they are stronger, it is situated nearer the kathode ; and when the currents are of a certain medium strength, the neutral point is exactly midway between the two electrodes. This electrotonic condition of the nerve may be ex- hibited as in fig. 31. In this n n' indicates the nerve, a and h the electrodes, a signifying the anode, h the kathode. The direction of the current within the nerve is, therefore, that indicated by the arrow. In order to 7 130 PHYSIOLOGY OF MUSCLES AND NERVES. indicate the change which the excitability undergoes at any definite point in the nerve, let us suppose a straight line drawn at this point at right angles to the longitu- dinal direction of the nerve, and let this line be made longer in proportion as the change is greater. In order, moreover, to show that the changes which occur toward the anode are of an opposite tendency to those toward the kathode, let the line on the anode side be drawn downward, that on the kathode upward. By connecting together the heads of these lines a curve is obtained which diagrammatically represents the changes at each Fig. 31. Electrotonts under the influence ok currents of varying strexgth. point. Of the three curves, the middle represents the condition under the influence of a current of medium strength ; the other two curves, indicated, the one by short lines, the other by a dotted line, represent the conditions under the influence of a strong and of a weak current respectively. These curves show that the changes are more marked in proportion as the cur- rent is stronger ; that they are most strongly developed exactly at the electrode points ; and, finally, that the neutral point, under the influence of currents of dif- ferent degrees of strength, assumes a variable position between the electrodes. MODIFICATION OF THE EXCITABILITY. 131 2. Apart from these clianges in the excitability which are thus observable while a continuous current passes through the nerve, others can also be shown to occur immediately after the opening of the current. Indeed, the excitability altered in electrotonus does not immediately revert to its normal value when the cur- rent is interrupted, but only regains this after the lapse of a short time. The duration of the changes in the excitability observable after the opening of the current is greater in proportion as the current is stronger and its duration is longer. These changes, which, to dis- tinguish them from the electrotonic changes, are called modifications of the excitability, are not merely the continuance of an electrotonic condition, but are some- times completely different from the latter. If, for in- stance, the experiment is tried at a point near the anode, at which the excitability is decreased during the continuance of the current, the excitability is found to be increased immediately after the opening of the current, and it is not till after this that the original normal excitability is regained. Similai'ly, in the neigh- bourhood of the kathode, the excitability decreases for a short time after the opening of the current, after which it again increases, and only gradually regains its normal condition. As a rule, these modifications do not last more than a few parts of a second. If, however, the constant current has been long present in the nerve, these modifications may endure for a somewhat longer period. On account of their transient nature it is diffi- cult to observe and test them. The change of condi- tion which follows the opening of the current within the nerve may, moreover, lead to excitement in the latter ; so that, on the opening of a cux'rent which has been 132 PHYSIOLOGY OF MUSCLES AND NERVES. present in the nerve for some time, a series of pulsa- tions or an apparent tetanus is occasionally observed. This phenomenon has long been known as an opening tetanus, or as Rittefs tetanus. The connection existing between these changes in the excitability, and the fact that the nerve may be excited by electric currents, has led to the adoption of a view of the electric excitement in nerves which we shall not be able to develop until we have more closely studied electric excitement itself. 3. If a continuous current is passed through a nerve, and is alternately closed and opened, the excitement appears to occur irregularly, sometimes at the closing, sometimes at the opening of the current, and occasion- ally even at both. Closer observation has, however, shown that very definite laws control this, provided that attention is paid to the strength of the current and its direction within the nerve. Let us first examine these phenomena as they occur in fresh nerve, and, as we found that the conditions in the nerve change very rapidly in the neighbourhood of the cut end, let us commence our observations at a low point in a fresh nerve, of which as great a length as possible has been extracted. For this purpose it is especially necessary to possess a convenient means of graduating at will the strength of the applied currents. Various methods have been used for this prupose. The best is that which is based on the distribution of the currents in branching conduc- tors. The electric current, on being made to traverse a conductor which separates at any point into two branches, divides, the strength of the currents distri- buted into these two branches not being always equal, but being in each branch in inverse ratio to the resis- tance offered in that branch. Supposing that the nerve LAW OF PULSATIONS. 133 is inserted in one branch, and that the resistance of the other branch is altered, then the strength of the cur- rent passing through the nerve will change, although the conductor which contains the nerve remains un- altered ; the current within the nerve will increase in strength when the resistance in the other branch is increased, and it will decrease when the resistance in this branch is decreased. The resistance of a wire being proportionate to its length, it is only necessary to arrange, as the conductor Fig. 32. Eheoci:oi;i). A S, a wire the length of which can be in some way altered. The simplest way of doing this is by extend- ing the wire in a straight line and moving a slidino-- piece along it, so that any required length of the wire may be brought into the conductor. Such an apparatus is called a rheochord, from pFos, a current, and x^P^V, a chord— because the current is conducted along a wire extended like a chord. A rheochord of the simplest kind is represented in fig. 32. The current of the chain P Z traverses the wire A B. From A a branch con- 134 PHYSIOLOGY OF MUSCLES AND NERVES. ductor passes to the nerve, and returns from there to the shde S, which sKps along the wire A B. The branch-current traversing the nerve is strengthened or weakened according as this slide is placed further from or nearer to A. By means of a rheochord of this sort there is no difficulty in making the currents within the nerve so weak that they exercise no influence at all. If their strength is then gradually increased, a pulsation is always first seen to occiu* in the fresh nerve when the current is closed, whatever the direction of the current within the nerve. In order to be able to indicate the direction, it has become customary to speak of such a current, when it passes within the nerve from a central to the more peripheric parts, as descending, and when it passes in the opposite direction, as ascending. Ascending and descending currents, therefore, when they are weak, afford pulsations only on the closing of the current. If the strength of the current is in- creased, pulsations gradually begin to occur also on the opening of the current, at first usually with the descending current, though, when the strength is in- creased yet more, they occur in connection with the ascending current also. Finally, the pulsations in all four cases are of equal strength. If, however, the strength of the current is yet further increased, two of these four pulsations again become weaker — the closing pulsation with the ascending current, and the opening pulsation with the descending current. A strength of current is at last reached at which these two pulsations entirely cease, so that pulsations occur only on the closing of the descending, and on the opening of the ascending cuiTents. These phenomena, LAW OF PULSATIONS. 135 which represent the dependence of the excitement of the nerve on the strength and direction of the current, are spoken of as the law of pulsations. This law is represented in the following table, in which S signifies closing, opening, Z pulsation, and E rest — i.e. no pulsation — the duration of the currents being indicated by the arrows. Law op Pulsations in the case op Fkksh Neeve. Current Weak Current of Medium strength Current Strong 1 a, Z 0, E, S, Z 0, z S, Z 0, R t y, z 0, R S, Z 0, z S, R 0, Z As soon as the nerve dies, the phenomena under the law of pulsations change. If weak currents are applied to a fresh nerve, which in either direction produce pulsations only on the closing of the current, and if then, the currents remaining entirely unaltered, their influence on the nerve is tested from time to time, it will be found that pulsations gradually begin to occur on the opening of the current; these are at first weak, but they continually become stronger till they are fully equal in strength to the pulsations resulting on the closing of the current. This condi- tion is retained for some time, after which the closing pulsations of the ascending current and the opening pulsations of the descending current become weaker, and finally entirely disappear, so that the descending current produces only closing pulsations, and the ascending current only opening pulsations ; and this condition endures until the excitability at the points examined is entirely expended, the pulsations be- 136 PHYSIOLOGY OF MUSCLES AND NERVES. coming gradually weaker, and finally disappearing en- tirely. The law of pulsations in the case of dying nerve may also be represented in tabular form, three stages of excitability being distinguished ; the signs remain the same as in the former table. Law op Pulsations in the case op Dying Nerve. (Under the Application of Weak Currents.) First Stage Second Stage Third Stage i S, Z 0, R S, Z 0, z S, Z 0, R t S, Z 0, R S, Z 0, z S, R 0, Z It is at once apparent that these two cases of the law of pulsation, occurring in different circumstances, entirely agree. The sequence of the phenomena which occur at the death of the nerve on the application of cur- rents of little power is exactly the same as that which may be elicited from a fresh nerve by gradually increas- ing the strength of the current. In other words, if the nerve is irritated with weak, unvaried currents, these act on a fresh nerve, after a time, in exactly the same way as currents of medium strength, and, after a somewhat longer time, as powerful currents would have acted. In order to understand this, it is necessary to recall our previous experiences of the changes in the excitability at the death of the nerve. We found that in that case the excitability at first rises and attains a maximum before it again falls. Supposing, therefore, a fresh nerve is irritated by means of currents of definite but weak strength, and supposing that this nerve is ex- amined after the lapse of a short time, during which its excitability has risen, it is evident that these weak cur- LAW OF PULSATIONS. 137 rents must already act as would stronger, and that, when the excitability has risen yet further, that they will act as very strong currents. The expressions weak, strong, and medium currents bear no absolute meaning, the same in the case of all nerves, but must always be under- stood relatively to the excitability of the nerve. That which in the case of one nerve is a weak current may evidently act as much stronger in the case of another nerve the excitability of which is much greater ; and, moreover, one single nerve, at different times, may be conditioned in this respect as though it were two diffe- rent nerves, if its excitability has in the interval under- gone considerable changes. There can, therefore, be no difficulty in understanding how, as the excitability gradually rises, the action of weak currents gradually becomes equal to that of medium and strong currents. One striking fact must, hoAvever, be observed. As the excitability after it has reached its highest point begins to fall again before it entirely disappears, it might be supposed that the same currents which at the extreme height of the excitability acted as strong currents, would now act again as currents of medium strength, and then as weak currents, before they entirely lose their power. According to this, the third stage of excitability, in which a closing pulsation is observable in the case of the descending current, an opening pul- sation in the case of the ascending current, should be succeeded by a fourth and a fifth stage, of which the fourth should resemble the second, and the fifth the first. This has indeed been said to occur by some observers, but it does not appear as a rule. In explana- tion of this, it has been assumed that no real, but only an apparent decrease of the excitability takes place after 138 PHYSIOLOGY OF MUSCLES AND NERVES. it has reached its highest point. It must, moreover, be remembered that it is never merely a single cross-section of a nerve which is irritated, but always a portion of greater extent, and that the excitability measured by us is in reality only the average excitability of the various points within the irritated portion. It may further be assumed that the excitability at each point, when it has reached its height, is very rapidly, if not instan- taneously, destroyed. As this, however, occurs sooner at the higher than at the lower points, it follows also that the excited portion, beginning from the top, gradually becomes a powerless thread, which is, how- ever, still capable of transmitting electricity. The ex- citement occurs in reality only in the lower division of the portion irritated, and this, as long as it retains any power of action, must remain at the highest point of excitability.^ 4. In studying the law of pulsations we attended only to the closing and opening of the current, entirely disregarding the period during which the continuous current flowed through the nerve. In reality, the nerve, as a rule, remains unexcited during this period. Sometimes, however, especially on the application of but moderately powerful currents, an enduring excite- ment expressing itself as a tetanus in the muscle is observable while the current lasts. Ascending and descending currents do not behave quite alike in this matter. The latter are followed by tetanus, even in the case of currents of somewhat high power, while the ascending currents are only followed by tetanus when they are weak. In all cases this tetanus is, however, but slight, and cannot be compared with that which ' See No'.es and Additions, No. 5. RELATION OF ELECTROTON'US TO EXCITEMENT. 139 may be induced by repeated separate irritations, for instance, by inductive shocks, or by jErequently and repeatedly closing and opening a current. It thus appears that variable currents are better adapted for effecting the excitement of a nerve than are con- stant currents. Inductive currents, though their dm-a- tion is extremely short, may be regarded as similar to constant currents which are re-opened immediately after being closed. True pulsations may indeed be un- failingly elicited, even with constant currents, if, by using suitable apparatus, they are but momentarily closed, and are then again reopened. But experience of the law of pulsations shoAvs that either the closing or the opening are under certain circumstances alone sufficient to elicit pulsations. As we know that the altered condition called electrotonus is produced in the nerve by closing the current, and that on the opening of the current this condition gives place, if not im- mediately, yet after a short time, to the natural con- dition, we may, therefore, assume that the excitement of the nerve is actually due to the fact that the nerve passes from a natural into an electrotonic condition, or back again from this into its natural state. We may suppose that the smallest particles of the nerve are transferred, on the intervention of electrotonus, from their normal into changed positions, and that this mo- tion of the particles is under certain circumstances con- nected with excitement. We have, however, found that a nerve, when electrotonus intervenes, is distin- guishable into two parts, the conditions of which evi- dently differ ; for in the one, that of kat electrotonus, the excitement is increased, while in the other, that of anelectrotonus, it is decreased. It might, therefore, 140 PHYSIOLOGY OF MUSCLES AND NERVES. be possible that these two conditions differ in the re- lation which they bear to the excitement. Indeed, Pfliiger supposed that excitement occurs only at the commencement of katelectrotonus and at the cessation of anelectrotonus. On the basis of this hypothesis the phenomena of the law of pulsations may be explained ; and it becomes intelligible why on the closing and opening of the current pulsations sometimes occur and are sometimes absent. In order, however, fully to Fig. 33. Electrotonus. understand this hypothesis and the law of pulsations based upon it, we must study the phenomena of elec- trotonus more closely than we have yet done. 5. We have already seen that the excitability is in- creased on the side of the kathode during the closing of the current, and is decreased on the side of the anode. Easy as it is to prove this law under the appli- cation of weak, or medium currents, it is sometimes very hard to do so when the current causing the elec- trotonus is strong. Let us again imagine that the TRANSmSSION OF EXCITEMENT DUKING ELECTROTOXUS. 141 nerve, n n' (fig. 33) is traversed between c and d by an ascending current, and that it is irritated between the points e and/, above the portion traversed by the current. The muscle is accordingly at n\ as in our previous ob- servations. Irritation takes place on the side of the kathode. An increase in the excitability should there- fore occur. This may easily be shown when the cur- rents used for effecting electrotonus are weak. If, however, the current used for this purpose is somewhat strengthened, no increase in the excitability is ob- servable ; and, indeed, if the ciurrents are sufficiently strong, it becomes quite impossible to effect contrac- tion in the muscle by irritation at ef. This may seem to afford an exception to the law of the electrotonic changes in the excitability. But from the previous experiments it is evident that this must not be in- ferred. Possibly the excitability is in reality increased at e / in entire accordance with the law ; but in order that the action of the excitement at this point should become visible, the excitement must pass through the portion under the influence of electrotonus, as well as through the an electrotonic portion lying below the latter, and it may be supposed that this propagation of the excitement meets with an insuperable obstacle in the condition of strong anelectrotonus which prevails there. It can indeed be shown that this is the case. If the current is reversed, so that it flows in a descend- ing direction through the nerve, then irritation at the portion a b will invariably show the existence of heightened excitement, however strong the current may be. But the portion a. & is now under exactly the same conditions as was the portion e/ previously. It is in itself very improbable that the nerve acts differently 142 PHYSIOLOGY OF MUSCLES AJ^D IS^EEYES. in two sucli entirely similar cases. The difference between the two cases consists solely in the fact that in the latter the kateiectrotonic point examined is situated immediately nest to the muscle, so that its condition of excitability can be indicated directly by the ninscle ; while in the case first observed, the con- dition of excitability at the point e f, before it can find expression in the muscle, must find means of passing througli the otherwise altered portions c d and a h. Now it may, on the other hand, be shown that transmission in a nerve under the influence of electrotonus really takes place at an altered speed. In the kateiectrotonic portion the rate of propagation is but little altered — is, perhaps, slightly increased; but in the anelectro- tonic portion it is markedly decreased. From this it may be inferred that anelectrotonus not only decreases the excitability, but also hinders the propagation of the excitement; and that where the anelectrotonus is strong, propagation is even entirely prevented. 6. This not only exj)lains the apparent exception to the laws of electrotonus, but also affords explanation of the feet that strong ascending currents, when closed, are followed by no pulsations. "VTe know that a strong electric current induces katelectrotonus in the upper half, anelectrotonus in the lower. According to Pfliiger's hvpothesis, excitement occurs in the nerve only at the point at which katelectrotonus intervenes; that is, on the closing of the ascending current, in the upper por- tion of the nerve. In order to reach the muscle, this excitement must pass through the lower portion of the nerve, and as this is strongly anelectrotonic, it presents an obstacle to the further passage of the excitement. The excitement which occurs in the upper half is, there- EXPLAXATIOX OF THE LAW OF PULSATIONS. 143 fore, unable to reacli the muscle, so that pulsation is necessarily absent on the closing of the current. In order to apply the corresponding case to the opening of a descending current, the help of another hypothesis is required, according to which the great modification which follows the disappearance of katelec- trotonus, and which so greatly decreases the excitability, also involves a hindrance to transmission. This assump- tion has not yet been experimentally proved ; proof is indeed difficult, on account of the ephemeral charac- ter of the modifications. The similarity of necrative modification to anelectrotonus, both decreasing the excitability, favours the hypothesis that in negative modification also an obstacle is afibrded to transmission. According to this view, the case is the same on the opening of a descending current as on the closing of an ascending current. According to Pfliiger's hypothesis excitement occurs on the opening of a current only in that portion of the ner\ e at which anelectrotonus dis- appears. This, in the case of a descending current, is the upper portion of the nerve. In order to reach the muscle thence, the excitement would have to tra- verse the lower portion, which is at the same time taken possession of by a strong negative modification, and this prevents propagation of the excitement ; no opening pulsations, therefore, occmr in the case of the descend- ing current. Pfliigiir supported his hypothesis by the following experiment. Mention has already been made of the so-called Ritter's tetanus, which intervenes when a current which has traversed a nerve for some time is interrupted. According to Pfliiger's hypothesis, this excitement should also be located on the side of the 144 THYSIOLOGY OF MUSCLES AND NERVES. anode. If an ascending current is passed through a nerve, the anode side is situated in its lower portion ; but if the current is descending, then it is situated in the upper portion. If Eitter's tetanus is induced by means of a descending current, and if the nerve is bi- sected between the electrodes immediately after the opening of the current, the tetanus at once ceases. If the same experiment is ti-ied with an ascending current, then the cutting of the nerve in no way influences the tetanus. Yet another proof of the truth of this hypothesis is afforded by Pflager's study of the excitement of the sensory nerves by an electric current. As the terminal apparatus of sensory nerves, by the action of which the irritation is recognised, is situated at the opposite end of the nerve, it seems that the law of pulsations should prevail in an opposite way to that in which it pre- vails in the case of the motor nerves. Pfliiger as- certained that in reality strong ascending currents induce sensation only when closed, strong descending currents only when opened. The explanation is the same in this case as in that of the motor nerves. On the closing of the descending current, excitement oc- curs in the lower portion of the nerve. In order to effect sensation the excitement must pass to the spinal marrow and the brain ; it would have, therefore, to pass through the upper parts of the nerve, where it would be checked by the strong anelectrotonus which prevails there. The opening of the ascending current has a similar irritating effect on the lower parts of the nerve. In order to reach the spinal marrow and brain, this excitement would have to pass through the upper parts, where, in this case, it would be checked by the strong negative modification. GENEEAL LAW OF KEKVE EXCITEMENT. 145 The only explanation of the fact that weak currents, whatever their direction, act only on being closed, is that the changes in the nerve probably begin more quickly than they disappear on the closing of the cur- rent. The differences are, however, very slight; and a very slight strengthening of the current suffices to elicit opening pulsations of the nerve also. This is especially true- of the descending current ; if the nerve is not quite fresh, opening pulsations may occasionally be observed even in the case of very weak currents which do not as yet afford any closing pulsations. This is connected with the circumstance that the ex- citability is somewhat greater in the upper than in the lower portions of the nerve. The natural superiority of the closing pulsation is thus cancelled in the case of the descending current, and opening pulsation is con- sequently rendered more easy. 7. From what has been said it seems very probable that every excitement in the nerve is due to a change in its condition, which might be directly shown in the case of the electric current by the electrotonic change in the excitability. The more quickly these changes occur, the more easily are they able to excite the nerve. This law is exhibited even in the case of non-electric excitement. It is, for instance, possible by gradually increasing pressui'e on the nerve entirely to crush the latter without producing any excitement, though every sudden pressure is, as we have seen, inseparable from excitement. A similar fact may be observed in the case of thermic and chemical irrita- tion. From this it may be inferred that the excitement in the nerve is due to a certain form of motion of its smallest particles, and that a sudden blow is better 146 PHYSIOLOGY or MUSCLES AND NERVES. adapted for exciting this motion than is slow action. That even slight mechanical disturbances are capable of producing excitement, although the nerve is not crushed, has been proved by Heidenhain. He attached a small ivory hammer to the instrument which we have already described under the name of Wagner's hammer, and, having laid the nerve on a small ivory anvil, placed the latter under the hammer in such a way that the latter tapped gently on the nerve. The result of this was strong tetanus lasting for several seconds. To obtain a more accia-ate conception of the mechanism of nervous excitement, it would be necessary first to learn accurately the arrangement of the smallest par- ticles in the quiescent nerve. Now we shall later on examine certain behaviour of the quiescent nerve from which conclusions may be drawn as to the regular arrangement of the smallest particles. While postpon- ing the closer examination of these details, we may at present try to explain the facts of excitement as clearly as circumstances permit. For this end we will assume that the particles of the nerve are retained in an en- tirely definite relative position by molecular forces. Excitement can, accordingly, only intervene when the particles are displaced from this position and are set in motion. The more powerful are the forces which retain the particles in their balanced position, the greater must be the forces which move them, and, therefore, the smaller is the excitability. It must also be ex- plained that the separate particles of the nerve mutu- ally influence each other, each particle influencing the ol her and helping to retain it in its relative position. A comparison drawn by du Bois-Reymond may be used to make this somewhat involved explanation more GENERAL LAW OF NERVE EXCITEMENT. 147 intelligible. It is a well-known fact tliat a magnetic needle suspended by a thread assumes such a position, in consequence of the magnetic attraction of the earth, that one of its ends points to the north, the other to the south. Now, su2Dj)osing a series of many magnetic needles, all suspended one behind the other in the same meridian line, as in fig. 34, then each of these needles NS NS NS NS NS ~J~ ~Y~ ^ "T" ~5~ Fig. 3L A siiuiES of siagnktic nkedles arranged as a diagram OF THE PAUTICLES OF A NERVE. will be yet more firmly retained in its position by its neighbours, for the adjacent north and south poles of the needles mutually attract each other. If, for ex- ample, we wish to move the middle needle. No. 3, more force must be used to do this than would be necessary if the needle were alone. But when the centre needle is turned, the immediately adjacent needles cannot re- main at rest, but are similarly deflected ; these exercise a similar deviating influence on their neighbours ; and so on. So that the disturbance created at one point in this series of magnetic needles passes like a wave through the whole series. This evidently bears much resemblance to that which takes place in nerves. It explains not only how a disturbance commencing at any point in the nerve propagates itself, but also how each separate part of the nerve is able to influence the other parts. We have already found that the excitability of any point of the nerve increases if the immediately superior portion of the nerve is cut away. The magnetic needles show that just in the same way each is more readily move- 148 PHYSIOLOGY OF MUSCLES AND NERVES. able when some of its neighbours have been removed. Without, therefore, assuming other resemblances be- tween the forces which act on the magnetic needles and those present in the nerve, we may accept the comparison so far that we may imagine the nerve to consist of separate minute particles, arranged one behind the other in the longitudinal direction of the nerve, and mutually retaining each other in their position. Now, if there are forces which retain the particles in this relative position yet more firmly, it is evident that they must lessen the excitability ; while, on the other hand, such forces as tend to move the nerve-particles from their relative positions must at the same time decrease the strength of their connection, and must therefore render the nerve more excitable. As regards the electric current, we have seen that the two poles act on the nerve in opposite ways. We may, therefore, assume that by one pole, the positive, the nerve par- ticles are retained in their quiescent position, while by the negative pole, on the other hand, they are disturbed from this position. If this is the case, it explains the fact that excitement occurs only at the negative pole when the current is closed. The excitability is in- creased at the positive pole on the opening of the cur- rent; here, therefore, there occurs a movement of the particles such as follows the closing in the negative pole, so that in this case the excitement can occur on the opening of the current. The fact that the nerve remains unexcited by changes in its condition, although these same changes if they occur suddenly do induce excitements, bears so sio-nificantly on the explanation of the nervous processes, that we must study it in yet greater detail. The fact GENERAL LAW OF NERVE EXCITEMENT. 149 may be most easily and surely sliown in the case of electric excitement, as there is no difficulty in allowing the strength of the currents to increase or decrease more or less gradually. Let the apparatus be arranged as in fig. 35 in which the nerve is traversed bv a Fig. 35. Riieochoud. current, the strength of which may be altered by moving the slide S. Let a key be inserted in the circle, and let the slide be so placed that pulsations occur on the closing and the opening of the current. On placing the slide S close to A (in which position the resistance in the branch J. ^ is nil, so that no current passes through the nerve), and pushing it slowly forward to its former position at S, the current within the nerve slowly in- creases from zero to its former strength : on again push- ing the slide slowly back till it touches A, the strength of the current again slowly decreases to 0. In neither of these cases is the nerve excited. As soon, however, as the movement of the slide is in any way effected * E. du Bois-Eeymond has described apparatus of this sort ur.der the name of ScJt?i'ankit»ff,vJteocJiord. 150 PHYSIOLOGY OF MUSCLES AND NERVES. with great speed,' the nerve is ezcited and the muscle pulsates. When, therefore, the current being closed or opened by means of the key, the nerve is excited, this is due to the fact that the strength of the current increases with great rapidity from zero to its full strength, or sinks from the latter to zero. The facts thus observed explain why inductive shocks, which are of but very short duration, and in which closing and opening follow each other in such rapid succession, are so especially capable of exciting the nerve. All inductive shocks are not, however, equally adapted for this purpose. When, making use of the inductive apparatus already described, the current in the primary coil is closed and then interrupted, the result is the creation of two currents differing in their direction in the secondary coil, these being the closing inductive current and the opening inductive current. If these are made to pass through a nerve, the exciting influence of the latter is alwa;ys much greater than that of the former. This can be very plainly shown by placing the secondary coil at a distance from the pri- mary. By this means, a distance may always be found at which the opening inductive current is active, while the closing inductive current as yet exercises no in- fluence ; if the coils are then brought nearer to each other, the latter also becomes active. If, however, when the coils of the inductive apparatus are in any position, the secondary coil is connected with a multi- plier, then the deflections of the magnetic needle are always of equal strength in the case of both inductive currents. The nerve, therefore, exhibits a difference which the multiplier is incapable of indicating. It has, however, been shown that the two inductive currents GENERAL LAW OF NERVE EXCITEMENT. 151 differ entirely in duration. The closing inductive cur- rent increases slowly, and decreases just as slowly, while, on the other hand, the opening inductive current very rapidly attains its full strength and ends just as quickly. It is to this difference that the latter evi- dently owes its greater j)hysiological effect.^ Let us return to the experiment as first arranged with the rheochord. Instead of pushing ah>ng the slide between A and *S^^ it may be moved backward or forward between any two points. The current in the nerve, in this case, never ceases, but is either strength- ened or weakened according to the direction in which the slide is moved. If the latter is moved suddenly and with great speed, it may produce excitement ; but the nerve always remains unexcited when the move- ment is- gradual. It therefore appears that it is not the actual closing and opening of a current which is required to excite the nerve, but that any change, whether it strengthens or weakens the current, is suffi- cient to effect this, provided that the alteration is sufficiently great and sufficiently rapid. Closing and opening are but special cases of alteration of the cur- rent in which one of the limits to the strength of the current = 0. The following law regarding the electric excitement of nerve may therefore be stated: any change in a current traversing a nerve may excite the latter if it is sufficiently strong^ and if it occurs luith sufficient speed. We have however seen that this law has very many exceptions. For under certain circum- stances a greater alteration (the closing of a strong ascending current) may appear to be without effect, al- though one less strong takes effect. If, however, it is ' See Notes aud Additions, Xo 6. 152 FHYSIOLOGY OF MUSCLES AND NERVES._ admitted that in such, cases excitement does in reality take place, but that it is not observable on account of external circumstances (hindrance to the propagation tp the muscle), then these exceptions may be said to be merely apparent. Moreover, assuming that the changes in the strength of the currents within the nerve only excite in consequence of the fact that they bring about changes in the molecular condition of the nerve, and combining with this all that we know of the effect of other forms of nerve irritation, the following law regard- ing nervous excitement may be regarded as the final result : — Excitement of the nerve depends on a change in its molecular condition. It occurs as soon as such a change is effected with sufficient speed. It may be added that this law is in all essential points true also of muscle. But it appears that the molecules of muscle are more sluggish than are those of nerve, so that in the former very transient influences may more easily be without effect.^ ' See Notes and Additions, Nos. 7 and 8. CHAPTEE IX. i. Electric phenomena; 2. Electric fishes; 3. Electric organs; 1. Multiplier and tangent galvanometer ; 5. Difficulty of the study; 6. Homogeneous diverting vessels ; 7. Electromotive force ; 8. Electric fall ; 9. Tension in the closing arch. 1. As yet in examining the essential qualities of muscles and nerves we have disregarded a series of important phenomena common to both, in order that we may now treat them as a whole. We refer to the electric actions which proceed from these tissues. Muscles and nerves are especially distinguished among all other tissues of the animal body by the fact that they exercise very regular and comparatively powerful electric action ; and from the relation existing between electric currents and the excitability of muscles and nerves it may be inferred that these independent elec- tric actions bear some relation to the essential qualities of muscles and nerves. It is true that electric action is exhibited in other animal, as well as vegetable tissues ; but these are very slight, and are apparently insignificant.' Electric cur- rents are so easily generated under all circumstances that it is not very surprising that traces of them are ' An exception is perhaps afforded by the electric phenomena of the leaves of Lioiuea miturijwla which will presently be men- tioned. 154 PHYSIOLOGY OF MUSCLES AND NERVES. everywhere to be found. In the researches in which we are aboiit to engage, we must always endeavour as far as possible to exclude these accidental currents, or at least to distinguish them froixi those currents which it is our task to examine, and the causes of which lie in the animal tissues themselves. Apart from muscles and nerves, but one tissue seems endowed with some- what strong electric action ; this is that of the glands. This has, indeed, not as yet been fully proved, but it has been shown to be in a very high degree probable. In connection with this it is a very interesting fact that the glands are in some phj^siological respects very similar to the muscles, and that they bear the same relations to nerves as do muscles. 2. There is, on the other hand, a tissue in which electric action is exhibited in far greater strength, so that its nature was known long before it was recog- nised that muscles and nerves possess the same capa- city. This tissue does not, however, occur in all animals, but only in a few fishes, which on this account are called electric fishes. In these animals special organs of peculiar structure occur, in which, as in an electric battery, currents of \ery considerable strength arise, the discharge of which is caused by the influence of the will, the animal using this power to frighten its enemies, or to benumb and kill its prey. Long before the world knew anything accurately as to the physical nature of electric phenomena, such powerful influences as are exhibited in electric fishes did not fail to attract the attention of chance observers. Notices of these remarkable phenomena are actually found in ancient writers ; and the Koman poet Claudius Claudianus ' ' He lived in Alexandria toward the end of the fourth century. ELECTRIC FISHES. 155 has given a very vivid description of these actions in the following lines : — ' Who has not heard of the power of the dreadful ray, of the benumbing force to which it owes its name.^ Formed only of gristle, it swims slowly against the waves or creeps sluggishly on the waterwashed sand. Nature has armed it with an icy poison, has poured into its marrow coldness to freeze and stiffen all living- things, and has filled it with everlasting winter. To these gifts of nature it adds craft, and, conscious of power, it remains quietly stretched among the sea- grasses ; yet when some animal, swimming upward to the sea-top, passes near, unpunished it fearlessly feeds on the living limbs. Nor when, having carelessly bitten at some bait, it feels the line, the bent hook in its mouth, does it attempt flight, biting itself free, but craftily creeping yet nearer to the dark hair-line, conscious of its power, it pours the electric breath from its poison- ous veins far and wide o^'er the water. The electric fluid flashes along hook and line, harming even the fisherman where he stands above the water ; from the lowest depth the dreadful lightning flashes, and passing along the hanging line, by the magic of its power carries cold as of ice through the rod, wounding the strong arm and curdling the blood of the fishermen, who, terror-struck, throws away the baneful prey, and, careless of his line, hurries homeward with dismay.' After the theory of electricity had received a new development in consequence of the discoveries of Galvani and Volta, these fishes were frequently studied Older notices of tlie Torpedo occur in Plin}% J^llian, Opjiian (wliosc poem on fishing Claudianus appears to have known), and in Aristotle. ' Torpedo, from ^yr/-' — J Fig. G3. DiAGRA.M OF THE EI.ECTKIC ACTIOX IS AX AGGREGATION OF JICSCLE-ELEMENTS. must prevail in the centre of the longitudinal section ; the greatest negative tension in the centre of the cross section ; and both must decrease in a regular way toward the edges. We now take a bundle of muscle-fib] es, the ends of which are formed by two artificial straight cross- sections, in other words, a regular muscle-prism. The separate muscle-fibres, which constitute the bundle, are surrounded by sarcolemma, held together and en- veloped by connective tissue. JMoreover, the outer- most strata must obviously become subject sooner than the inner to the unfavourable influences of mortifica- tion, which, as we have seen, finally lead to the entire loss of electric qualities ; these outermost strata there- 234 PHYSIOLOGY OF MUSCLES AND NERVES. fore become quite inoperative, or less operative than the inner. This injurious influence must be yet more strongly developed on the cross-section, where a layer of crushed, that is, dead muscle-substance, overlies the parts which yet remain operative. Owing to all these circumstances, a coating of inoperative but con- ducting substance envelopes the operative muscle- elements, and the distribution of the tensions on the regular muscle-prism is fully explained. And when such a muscle-prism is divided, the conditions always remain unaltered. Each part of a muscle-prism must act as would the whole. Fig. 64. Diagram of ax oblique ceoss-sectiox. Our hypothesis is therefore quite able to explain the electric phenomena of a regular muscle-prism. We must now see how it stands in relation to the other facts which we have learned. If the artificial cross- section is made obliquely to the axis of the muscle- fibres, as in a regular or irregular muscle-rhombus, then our assumed muscle-elements, at the cross-section, will be arranged one over the other like steps, and are clothed by a layer of crushed, and therefore inopera- tive tissue, as is represented in fig. 64. On such a cross-section it is evident that separate currents must circulate from the positive longitudinal section to the negative cross-section of each individual muscle- element, and these combine with the current circu- NEGATIVE VARIATION AND PARELECTRONOMY. 235 lating from the longitudinal to the cross-section of the entire prism, to make the obtuse angle more positive than negative. 5. We must next inquire how the negative varia- tion of the muscle-current during activity can be ex- plained in accordance with our hypothesis. We have already found reason to believe, from the phenomena of muscle-tone, that the contraction of the muscle depends on a movement of its smallest particles. Mi- croscopic observation of muscular contraction shows that the movement takes place within each muscle- element, for the change in form may be detected in each muscle-element just as in the whole muscle-fibre. It is therefore not difficult to conceive that, in con- nection with these movements of the smallest particles within each muscle-element, the electromotive opposi- tion between the longitudinal and cross-sections of that element undergo a change. It is of little importance whether we conceive the matter as though the mo- lecules of the muscle undergo vibratory motion during contraction, or whether we give the preference to some other theory. Where facts are wanting to support or contradict certain assumptions, the imagination may have free play, and may picture any process by which changes of the kind under consideration might pos- sibly be brought aboiit. But the discreet man of science, while allowing himself this liberty, ever re- members that such free play of the imagination is of no real scientific value, either didactically, as explain- ing known facts, or temporarily as leading anti inciting to new researches. Good hypotheses are always avail- able in both these ways, and the scientific man uses only such. lie may perhaps amuse himself in a leisure 236 PHYSIOLOGY OF MUSCLES AND NERVES. quarter of an hour by allowing his imagination to carry the hypotheses further than the point up - to which they are based on known facts ; but he does not presume to urge the results on others. Finally, we have to examine how far the hypothesis to which we have given the preference is confirmed by the phenomena observable in entire muscles. The tendonous covering on the ends of muscle-fibres may be regarded as a layer of non-active conducting sub- stance. In so far as the same phenomena are ex- hibited in the uninjured muscle, as in the muscle- prism or muscle-rhombus with its artificial cross section, nothing need be added to the previous ex- planations. But this is, as we have seen, though generally, yet not always the case. The natiual cross- section of a muscle is generally very slightly negative, sometimes not at all, as compared with the longi- tudinal section ; but the negative character becomes marked as soon as the natural cross-section has been destroyed in any way, either mechanically, chemically, or thermically. In explanation of this condition of the natural ends of muscle-fibres, we may assume that the arrangement of the molecules in the latter or in the terminal muscle-elements in each muscle-fibre may sometimes be different from that at all other points. If, for example, the cross-section in the terminal muscle-element were not negative, the muscle-fibre could atford no current, though such a current would arise as soon as this terminal muscle-element was re- moved or was transforme.d into a non-active conductor. E. du Bois-Eeymond has lately succeeded in discover- ing a very probable reason for this abnormal condition of the ends of muscle-fibres ; but without entering too THE NERVES. 237 deeply into details we should not be able to explain this here.^ 6. We will now turn our attention to nerves. The resemblance of the phenomena in the case of muscles and of nerves is so great that it is natural at once to transfer the hypotheses assiimed for the former to the latter. It is true that in nerves there are not the microscopically visible particles (the so-called muscle- elements) on which we based our theory in the case of muscles, and in which we recognised the presence of electromotive forces. But from what we have already seen of the processes of excitement in the nerve, it is at least evident that in the nerve also separate par- ticles, with independent power of movement and inde- pendent forces, must be arranged in sequence in the longitudinal direction of the nerve. If, without being able to say anything further of their nature, but be- cause of the analogy, we call these particles nerve- elements, and if we assume that each of these nerve- elements is the seat of an electromotive force, in consequence of which the longitudinal section exhibits positive tension, the cross-section exhibits negative tension, then the phenomena in the quiescent nerve and the negative variation of the nerve-current during activity are explicable exactly as were the correspond- ing phenomena in muscles. The entirely similar be- haviour of nerves and muscles when irritated is alone sufficient to show satisfactorily that the two must be very much alike in their physical structure ; and the similarity of their behaviour, in point of electromotive activity is such as to lend weight to our assumption of ' See Notes and Additions No. li. 238 pm'SioLOGY of muscles and nerves. the similarity in the arrangement of their smallest particles. But together with many points of resemblance, nerve and muscle exhibit some points of difference. The muscle during activity changes its form and is able to accomplish work ; the nerve is incapable of this. The nerve, on the other hand, under the in- fluence of continuous electric currents, exhibits those changes in excitability which we observed under the name electrotonus, and which, as we have seen, corre- spond with changes in the distribution of the tensions on the outer surface of the nerve. No correspond- ing phenomena have been shown in muscle. Other changes which effect these changes in tension must, therefore, occur within the nerve-element. It is a well-known fact that all substances occupy- ing space are regarded as composed of small particles, to which the name molecules is given. In a simple chemical body, such as hydrogen, oxygen, sulphur, iron, and so on, all these molecules consist of homogeneous atoms ; in a chemically compound body, such as water, carbonic acid, and so on, each molecule is composed of several atoms of different kinds. A molecule of water, for instance, consists of an atom of oxygen and two atoms of hydrogen ; a molecule of carbonic acid con- sists of an atom of carbon and two atoms of oxygen ; a molecule of common salt consists of an atom of natron and an atom of chlorine, and so on.* A piece of salt contains a very large number of such atoms composed of chlorine and natron, but each of these • Details of the atomic and molecular theory will be found in 'The New Chemistry.' Cooke (International ScientlSc Series, vol. ix.). THE NERVES. 239 (in pure cooking salt) is like every other. But a muscle, a nerve, or any other organic tissue, is much more complex in structure. Molecules of albumen, of fats, of various salts, of water, and so on, are mingled in it. A very small piece of such a tissue must be regarded from a chemical point of view as a compound of very many diJSerent substances. To avoid confusion, the name ' muscle-element ' or ' nerve- element ' has been given to these particles, in which we assume the existence of all the quaHties of muscle or nerve, but this name expresses nothing further than a fragment of a muscle or nerve. Even such a frag- ment must be regarded as of very complex structure. Very complex physical and chemical processes may take place within it ; and the processes of muscle and nerve activity, the actual nature of which is as yet quite unknown to us, are certainly connected with such chemical and physical processes. If electric forces also occur in such a nerve- or muscle-element, it is not sur- prising that these also undergo various changes. Of this sort must be the changes which occur during ac- tivity and during electrotonus. In speaking, as we have occasionally done, of nerve- and muscle-molecules, we have, therefore, not used the term molecule quite in the clear and fixed sense in which the term is used in chemistry. Our conception was rather of something which, itself composed of va- rious chemical substances, forms a unit of another order. For the sake of brevity we shall still sometimes use the expression in this sense, as, after the explana- tion Avhich has now been given, we may do this without fear of being misunderstood. A muscle- or a nerve- molecule accordingly means a group of chemical mo- 240 PHYSIOLOGY OF MUSCLES AND NERVES. lecules combined in a particular way, many of which, in combination, form a muscle-molecule or a nerve- element respectively. We have learned to regard the negative variation of the muscle- or nerve-current as a movement of these muscle- or nerve-molecules respectively, in consequence of which the differences in tension between the longi- tudinal and cross-sections become less. In explanation of the electric phenomena of electrotonus, we may now assume that under the influence of continuous electric currents the nerve-molecules assume a different relative position by reason of which the distribution of the tensions on the outer surface of the nerve is changed. This changed position is retained as long as the electric current flows through the nerve, and disappears more or less rapidly after the opening of the current. At first it takes effect only within the electrodes, but it propagates itself through the extrapolar portions, be- coming gradually weaker the further it is from the electrodes. In illustration of this conception, we may avail ourselves of the comparison which we have already made of the nerve-molecules with a series of magnetic needles. When the position of some of the needles in the centre of such a series is changed, owing to some external influence, those needles which lie more on the outside of the series must be turned to an extent de- creasing with their distance from the centre. Or we may also refer to the conception which physicists have formed of the so-caUed electrolysis, the analysis of a fluid by an electric current. All these analogies can only explain the process in so far that we recognise how an electric current is capable of causing a change in the relative position of the muscle- and nerve-molecules, GLANDS AND ELECTRIC ORGANS. 241 at first only between the electrodes, but afterward beyond these, which change then corresponds with a change in the distribution of tension on the surface. 7. We have yet to consider how far the hypothesis under discussion explains the electric phenomena in electric fishes and in the glands. The electric shock of the torpedo must evidently be regarded as analo- gous to negative variation in muscle- and nerve- currents. The apparently great difiference that in the latter a current present during a state of quiescence becomes weaker during activity, while in electric fishes an organ which is entirely inoperative during the state of quiescence generates a current when it becomes active, appears, when closely examined from the point of view afforded by our hypothesis, to be of no account. For, from the fact that no current in an organ can be externally shown, it by no means follows that no elec- tromotive forces are present within the organ. A piece of soft iron is in itself entirely non-magnetic ; but as this may at any time bo transformed into a magnet by bringing a magnet into its neighbourhood, or by the influence of an electric current, we suppose that mole- cular magnets are present even in the soft iron, though these a,re not regularly arranged as in a regular magnet, such as that represented in fig. 61, p. 230. The action of the magnet which is brought near, or of the electric current, therefore consists solely in the fact that it ar- ranges the irregularly placed molecular magnets within the soft iron, and thus allows thek action to appear externally. If no magnetic action were known in soft iron, no one would ever have had an idea that magnetic forces were present within it. But comparison with the permanent magnet, and the possibility that thoroughly 242 PHYSIOLOGY OF MUSCLES AND NERVES. non-magnetic iron may at any time be transformed into a magnet, makes the involved conception quite natural. It is exactly the same in the case of the electric organs of the torpedo. The fact that they, though in them- selves electrically inoperative, become electrically oper- ative under the influence of the nerves, when combined with what we know of nerves and muscle, naturally leads us to suppose that electromotive forces are pre- sent in the electric plates, but that they are so ar- ranged as to cause no observable differences of tension on the outer surface. Under the influence of the ac- tive nerves, the particles endowed with electric forces undergo a change in their relative position, differences of tension between the two surfaces of the electric plates intervene, and, as all the electric plates in an organ act in the same way, the result is a powerful electric shock, which, in spite of its powerful efifect, differs from the negative variation of the m.uscle- and nerve-currents only as does the powerful current of a many-celled galvanic battery from the weak current of a small apparatus. In order to make the similarity between the electric organ on the one hand, and muscles and nerves on the other, yet more prominent, we will carry the compari- son with magnetic phenomena yet further. In fig. 65, A B lY s Fig. 65. Magnetic ixductiox. A B isa, piece of soft iron, JV S a. magnet which we bring from some distance toward the iron rod A B. The result is to evoke magnetism inAB,A becoming a north pole, and B a south pole. Now, let us suppose that the non- magnetic iron rod AB i?. replaced by an entirely similar, GLANDS AND ELECTRIC ORGANS. 243 but magnetic rod iV, S^ (fig. 66). At the moment at which the magnet NS is brought near, the magnetism of iVj S^ becomes weaker, ceases entirely, or is even ,S-, jV, A'' S Fig. 66. Magnetic induction. reversed. The same process of magnetic induction is concerned in both cases. The only difference is that in one case the induction seizes on an iron rod the mole- cular magnets of which are irregularly arranged, and which therefore appears non-magnetic; while in the second case the iron rod is in itself magnetic. So that in one case magnetism is evoked by induction, in the other, magnetism which was already present is weak- ened ; but the induction is the same in both cases. In just the same way electric tensions are induced in the electric plate by the influence of the nerves, while the tensions present in the muscle are weakened ; but the process in the electric plate and in the muscle is the same. We have now only to say a few words about the glands. The phenomena in these are, so far as we can infer from the few known facts, so entirely like those in muscles, that it is only necessary to transfer the expla- nation which we have given in the case of the muscles to the glands. In each gland-element electric forces are present which make the base of the gland positive, the mouth-opening negative. When the gland becomes active, these differences in tension become less. There is no occasion to speculate as to how far this affects the process of secretion, as it could not further explain the process. 244 ■ PHYSIOLOGY OF MUSCLES AND NERVES. CHAPTEE XV. 1. Connection of nerve and muscle; 2. Isolated excitement of individual mnscle-fibres ; 3. Discharge-hypothesis; 4. Principle of the dispersion of forces; 5. Independent irritability of muscle- substance; 6. Curare; 7. Chemical irri'ants; 8. Theory of the activity of the nerves. 1. In the foregoing chapters we have examined the characters of muscles and nerves separately. The ni.uscle is distinguished by its power of shortening and thereby accomplishing work. The nerve has not this power : it is only able to incite the muscle to activity. We must now inquire how this incitement, this trans- ference of activity from the nerves to the muscles, occurs. ■ To understand the action of a machine, of any piece of mechanism, it is necessary to learn its structure and the relative positions of its separate parts. In our case, microscopic observation can alone afford the explana- tion. If we trace the course of the nerve within the muscle, we find that the separate fibres, which enter the muscle in a connected bundle, separate, run among the muscle-fibres, and spread throughout the muscle. It then appears that the single nerve-fibres divide, and this explains the fact that each muscle-fibre is eventu- ally provided with a nerve-fibre — long nerve-fibres even with two — although the number of nerve-fibres wkich enter the muscle is generally much less than the COXXECTIOX OF XERVE AXD MUSCLE. 245 number of the muscle-fibres which compose the muscle. Till the nerve approaches the muscle-fibre, it retains its three characteristic marks — the neurilemma, medullary sheath, and axis-cylinder. When near the muscle-fibre, the nerve suddenly becomes thinner, loses the medul- lary sheath, then again thickens, the neurilemma co- FlG. 67. TKRMI>fATIONS OF XERVES IX THE MUSCLES OF A GL'ISEA-PIO. alesees with the sarcolemma of the muscle-fibre, and the axis-cylinder passes directly into a structure which lies within the sarcolemma pouch, in immediate con- tact with the actual muscle-substance, and is called the terminal nerve-plate. Fig. 67 represents this passing of the nsrve into the muscle as it occurs in mammals. In other animals the form of the terminal plate is some- 246 PHYSIOLOGY OF MUSCLES AND NEKVES. what different ; but the relation between the nerve and the muscle is the same. The essential fact is the same in all eases : the nerv^ passes into direct contact with the muscle- substance. All observers are now agreed on this point. Uncertainty prevails only as to the further nature of the terminal plate. In the frog, for instance, there is no real terminal plate, but the nerve separates within the sarcolemma into a net-like series of branches, which can be traced for a short dis- tance from the point of entrance in both directions. Professor Gerlach has recently declared that this net, as well as the terminal nerve-plate, are not really the ends of the nerves, but that the nerve penetrates throughout the muscle-substance, and that throughout the whole muscle-fibre there is an intimate imion of nerve and muscle. 2. However this may be, the fact that the nerve- substance and the muscle-substance are in immediate contact must serve as the starting-point from which to attempt an explanation. When it was thought that the nerve remained on the outer surface of the muscle- fibre, there was difficulty in explaining how a pulsation of individual muscle-fibres within a muscle could be elicited by irritation of individual fibres of a nerve. For the nerve-fibres, in their course within the muscle, touch externally many muscle-fibres, over which they pass before they finally end at another muscle-fibre. In the case of flat, thin muscles, it may be shown con- clusively that such a nerve-fibre may be irritated in such a way that those muscle-fibres over which it passes remain quiescent, and only those pulsate at which the nerve-fibre ends. As soon, however, as it is understood that the excitement present in the nerve- THE DISCHARGE HYPOTHESIS. 247 fibre cannot penetrate through the sheaths, it is clear that the excitement can only act on the muscle- substance where the nerve-substance and the muscle- substance are really in immediate contact — that is, only within the sarcolemma pouch. The nerve-sheath is, as we already know, a real isolator as regards the process 'of excitement within the fibre ; for an excitement within a nerve-fibre remains isolated in this, and is not trans- ferred to any neighbouring fibre. It is quite impos- sible, therefore, that it can transfer itself to the muscle- substance, since it is separated from the latter not only by the nerve-sheath, but also by the sarcolemma. But if the nerve-fibre penetrates the sarcolemma, as appears from the microscopic observations above de- scribed, and if nerve-substance and muscle-substance are in immediate contact, then the transference of the excitement present in the nerve to the muscle substance is intelligible. The argument holds good whether we assume that the nerve, directly after its entrance within the sarcolemma, ends in a nerve -plate or a short nerve- net, or whether, as Gerlach says, it spreads further. All that is needed to make the process of transference in- telligible is that the two substances should be in imme- diate contact, and so much is granted, whichever view is preferred. But the process, if intelligible, is yet not explained. An attempt at explanation must be based on, and have regard to, all the established facts. 3. It is natm-al to think of the electric characters of nerves and muscles, and to seek the explanation in these. In nerves electric tensions prevail which dur- ing the activity of the nerve undergo a sudden decrease, a so-called negative variation. Such sudden variations of electric currents are, we know, able to excite the 248 PHYSIOLOGY OF MUSCLES AND NERVES muscle. We may, therefore, conceive the process som.e- what as follows. The excitement in the nerve, however caused, propagates itself along the nerve-fibre until it reaches the end of the latter. Connected with it is an electric process, by which a sudden electric variation is caused in the terminal apparatus of the nerve- fibre, and this excites the nerve-substance, just as a shock acting externally immediately on the muscle would excite it. Following du Bois Eeymond, the above conception may be called the discharge-hypothesis {Entladivngs- hypothese). According to it, the muscle end of a nerve- fibre must be regarded as similar to an electric plate in the pecuhar organs of electric fish. Indeed, in the latter, an electric discharge is effected by the influence of nerve-excitement, which is able to cause other excit- able structures, such as muscles and glands, to contract. We do not attach any weight to the accidental external resemblance of the terminal nerve-plate to the electric plate. In frogs and many other animals there are no terminal plates, and yet the conditions are the same in their case also. And even if the view upheld by Gerlach is confirmed, and it is shown that nerve-substance comes into more intimate contact with muscle-substance than merely at the point at which it enters the muscle- pouch, our explanation will be unaffected. All that we claim is that an electric discharge, by which the muscle- substance is irritated, takes place in the terminal expan- sions of the nerves, of w^hatever form these expansions may be. Against the acceptance of this view a difficulty at first seems to present itself in the fact that such an electric shock, taking place in the end of a nerve, would THE FREEING OF FORCES. 249 excite not only the muscle-fibre in whicli the nerve ends, but the adjacent fibres also. For in the muscle and its envelopes no electric isolators are present, and an electric shock, occurring at any point, can and must spread throvighout the whole muscle mass. But from the law of the distribution of currents in irregular con- ductors, the essential outlines of which are given in the twelfth chapter, it apjjears that the strength of the cur- rent in the immediate neighbourhood of the spot at which the discharge actually takes place may be con- siderable, though it decreases so rapidly with increasing distance, that it is easy to believe that it may be quite unnoticeable, even in a muscle-fibre which stands side by side with the fibre directly irritated. It is this very circumstance which lends especial weight to the fact that the nerve penetrates within the muscle-fibre, and there comes into immediate contact with the muscle- substance. Only in this way is it intelligible that a discharge occurring in the nerve can irritate the muscle. When the excitement has once arisen at any point within the muscle-substance, it can, as we have seen, spread within the muscle-fibre. It is possible that this may result without any co-operation of the nerve-sub- stance ; so that the spreading of the nerve within the muscle-substance, as claimed by Gerlach, is not required to explain the processes within the muscle.' 4. We therefore assume that the excitement aris- ing in the nerve itself becomes an irritant, which then irritates the muscle. The forces which are gene- rated, in consequence of this, in the muscle are, as we know, able to accomplish considerable labour, which bears no relation to the insignificant forces which act ' See Notes and Additions, No. 15. 12 250 PHYSIOLOGY OF MUSCLES AND NERVES. on the nerve and which are active in the nerve itself while the latter transmits the excitement. To use a common but appropriate simile, the nerve is but the spark which causes the explosion in the powder-mine ; or, to carry the simile further, the sulphur train which, being fired at one end, carries the fire to the mine, and there causes the explosion. The forces which are set free within the muscle are chemical, due to the oxida- tion of its substances ; the irritant originating from the nerve is only the incitement in consequence of which the chemical forces inherent in the muscle come into play. Physicists call such processes the freeing of forces. The nerve-irritant, therefore, frees the muscle- forces, and these translate themselves into warmth and mechanical work. In every such freeing, the freeing force is generally very small when compared with the forces set free, and which may be dormant for incalcu- lable periods ; though when they are once set free, they are capable of enormous effects. A huge block of stone may for years hang in unstable equipoise on the edge of a precipice till some insignificant disturbance makes it fall, carrying destruction to all in the way of its de- scent. It is even supposed that the slight disturbance caused in the air by the sound of a mule-bell is suf- ficient to start the ball of snow which at last thunders down into the valley in the form of a mighty, all- destroying avalanche. This freeing by small forces is only possible in the case of unstable equipoise. But there is also a chemical unstable equipoise. Carbon and oxygen may lie for thousands of years side by side without combining. Closely mingled, as in gunpowder, or still more closely, as in nitro-glycerine, they are in unstable equipoise ; the slightest blow suffices to cause INDEPENDENT lERITABILIT i' OF MUSCLES. 251 their combination, which by their expansion is able to accomplish such gigantic work.^ In muscle, too, carbon and oxygen lie side by side in chemical unstable equi- poise ; and it is the irritation of the nerves which effects the solution which destroys the equilibrium. An arrange- ment such as that just described is called sensitive, because even an insignificant disturbance is sufficient to disturb the unstable equipoise and to develop force. The muscle is therefore a sensitive machine. But the nerve is in a yet higher degree sensitive, for the smallest dis- turbance of its equipoise gives play to the forces within it. But these forces are in themselves incapable of any great effects. They would hardly be indicable, were not this sensitive machine, which we call the nerve, connected with the machine, also sensitive, which we call muscle, in such a way that the activity of the one sets free the forces within the other. 5. A sensitive machine is not equally sensitive to all possible disturbances. Dynamite ^ may be placed on an anvil and hammered without exploding ; or, if lighted with a cigar, it burns quietly out like a fire- work. But when it comes in contact with the spark of a percussion cap, it explodes, and develops its gigantic forces. A nerve is sensitive to electric shocks, and to certain mechanical, chemical, and thermic influences. It is not sensitive to many other influences. The in- fluences to which the nerve is sensitive we have called irritants. A muscle is sensitive to electric shocks, to certain mechanical, chemical, and thermic influences ; ' On these processes see Balfour Stewart * On the Conservation of Energy ' (International Scientific Series, vol. vi.) ; and Cooke on ' The New Chemistry ' (same series, vol. ix.). 2 Dynamite is a mixture of nitro-glycerine wilh 'kiesclguhr,' an earth consisting of the shells of infusoria. 252 Pm^SIOLOGY OF MUSCLES AND NEEVES. and, above all, to the influence of the active nerve. The latter may perhaps, as we have explained in the foregoing paragraphs, be refeiTed back to electric irri- tation. It is thus apparent that muscle and nerve behave essentially in the same "way towards irritants. But, remembering that nerves run for part of their course within the muscle, between its fibres, and even penetrate within the very muscle-fibres, the thought now suggests itself, that perhaps the muscle is in no way electrically, chemically, thermically, or mechani- cally irritable ; perhaps, when these irritants are allowed to act on the muscle, it is only the intra-muscular nerves which are irritated, and which then in turn act on the muscle-fibres. In other words, we have to determine whether the muscle is only irritable mediately through the nerves, or whether it is also immediately irritable, independently of the nerves, by any irritants. The question is not a new one. Albert von Haller, poet and physiologist (1708-77), asked it, and even he was not the first to do so. Haller declared himself in favour of the second of the two above-mentioned possi- bilities. He called this capacity of the muscles to re- ceive independent irritation (Irritabilitat), and the name has been retained. Haller met with much opposition from his contemporaries ; and a dispute arose which has lasted to the present time. In Haller's days, of course, only the larger nerve-branchings were known. The fiurther the nerves can be traced by means of the micro- scope, the harder does it evidently become to determine the question under discussion. 6. In the year 1856, the French physiologist Claude Bernard made experiments with a poison brought from Guiana, which the Indians of that region use to poison CUEAEE. 253 their arrows. It is called curare, ourari, or Avurali, and is a brown, condensed plant juice, which is brought over in hollowed, gourd-like fruits called calabashes. He found that animals poisoned with this curare are dis- abled, and that in animals thus disabled, irritation of the nerve- trunks, even with the strongest electric or other irritants, is entirely ineffective, though the muscles are yet easily irritable. This was indeed no new phenomenon. Harless, at ]\Iunich, had already observed something similar in strongly etherised ani- mals. But soon afterwards, Koelliker, at Wiirzburg, and, simultaneously, Bernard himself, in extending the experiments of the latter, found something new. If ligatures are applied to the hough of a frog, and the animal is then poisoned with curare, the lower leg is not disabled. By irritation of the sciatic nerve the muscles of the lower leg may be induced to contract where the poison could not penetrate, the appropriate vessels being tightly constricted. Curare, therefore, does not disable the muscles, for these always and everywhere remain irritable ; nor does it disable the nerve-trunks, for these remain irritable if the poison cannot reach the muscles. There is but one other thing possible : the poison disables something which is be- tween the nerve-trunk and the muscle-fibre, so that the nerve-trunk can no longer act on the muscle. If that which is disabled is the end of the nerve, then the im- mediate irritability of the muscle-substance, without the participation of the nerves, about which there has been so much strife, is proved. This striking phenomenon is not solitary. The action of some other poisons, such as nicotine and coniue, is entirely like that of curare. These also dis- 254 PHYSIOLOGY OF MUSCLES A]S"D NERVES. able, not the nerve-trunks or the muscle-substance, but some part intermediate between these two. The diffi- culty is to prove that this part is exactly the final termi- nation of the nerves. Assuming that these poisons disable some part which lies between the nerve-trunk and the muscle, but not the very end of the nerve, then, though all the phenomena explained above are quite intelligible, yet no answer has been gained to the ques- tion of irritability, which we are discussing. Considering now the characters of the nerve, and of its passage into the nerve-fibre, it is easy to understand why the poison does not take effect on the nerve-trunks. The nerve-fibres receive but few blood-vessels, so that the poison in solution in the blood can only reach them slowly, and in very small quantity. Moreover, the fatty medullary-sheath probably forms a sort of protec- tive envelope round the axis-cylinder. But where the nerve enters the muscle-fibre it loses the medullary sheath : and just at this same point a very complex net of blood-vessels is present. Probably, therefore, it is exactly the terminal nerve-plate (or the corresponding nerve-branchings in the naked amphibia) which is most exposed to the attack of the poison. So long, however, as it is impossible to prove that this is really the actual end of the nerve-fibre, a chance is left open to the op- ponents of the theory of irritability. Great pains have been taken to settle this point with certainty. If a muscle poisoned with curare is compared with a similar but unpoisoned muscle, it ap- pears that the former is less excitable ; that is, that stronger irritants are needed to cause it to pulsate. The explanation of this may be that the muscls-sub- stance is excitable, but not so much so as the intra- CHEMICAL IRRITANTS. 255 muscular nerves. The following reasons may also be given for the probability of the independent irritability of muscle-substance. A nerve is, as is known, strongly excited by short, sudden variations of a current, and an unpoisoned muscle behaves in the same way; but a muscle poisoned with curare is less sensitive to current, shocks of short duration than to such as take place more slowly. If we ascribe independent irritability to muscle-substance, then greater sluggishness prevails in muscle-substance than in nerve-substance, so that the irritating influences require longer time to take effect in the former. In the case of nerves it has, moreover, been shown that currents which pass at right angles to the longitudinal direction of the nerve-fibre are entirely ineffective. In muscles under the influence of curare no difference in this point can be shown. If the independent irritability of muscle-substance is de- nied in spite of this, it must be assumed that in these experiments the point lies in differences between the nerve-fibres and their real ends. But nerves and muscles are evidently very similar, and it might evidently be possible to assume considerable difference between nerve-fibres and nerve-ends, and that these ner\e-ends differ from the muscle-substance in , nothing but that the power of being irritated is ascribed to the former, while it is denied to the latter. It appears then, that the whole dispute resolves itself into an empty word- strife as to whether this thing which lies between the nerve-fibres and the muscle-substance is to be reckoned as part of the nerve or as part of the muscle. 7. The much-discussed question of the independent irritability of muscle-substance is, as appears from what has now been said, due principally to the fact that the 256 PHYSIOLOGY OF MUSCLES AND NEEVES. same irritants wtiich act on the nerve are also able to act on a muscle, and even on a muscle poisoned with curare. We have, however, found slight differences, and, if it were possible to show the existence of greater differences, especially if irritants were found which act on muscle-substance but not on nerve-substance, a new point of departure would be gained for this theory of independent irritability. Chemical irritants are beyond all others capable of variation. From the endless num- ber of chemical bodies we may choose such as irritate the nerve or muscle in general, and we may try each of these in every degree of concentration. If differences between nerve-substance and muscle-substance really exist, it is probable that we shall find them by these means. Starting from these premisses, Kiihne experi- mented on the condition of nerves and muscles ; and he was so far successful that he discovered some dif- ferences. In studying the character of nerves and muscles relatively to chemical irritants, it is best to make a cross-section, and to apply the substance which is to be tested to this section. It is best to apply the test to a thin parallel-fibred muscle, usually to the musculus sartoriiis of the upper leg. It is suspended upside down from a vice, which holds fast its lower pointed tendon; and its upper end, which now hangs down- ward, is then cut. The liquid which is to be tested is then brought in contact with the cross-section thus made, and care is taken to observe whether a pulsa- tion takes place or not. The short, used portion having then been cut off, the experiment can be repeated, and so on till the whole length of the muscle has been used. The nerve is treated similarly ; the sciatic nerve CHEMICAL IRRITANTS. 257 is, as in all experiments by irritation, used for the pur- pose, either in connection with the whole lower leg, or only with the calf-muscle. If the effect of volatile bodies — vapom-s or gases — is to be tested, the muscle must be shut off from the nerve in an adequate manner. The muscle is extraordinarily sensitive to certain substances. One part of hydrochloric acid in from one thousand to two thousand parts of water affords strong pulsations. The smallest trace of ammonia is enough to cause strong contraction. The observer must there- fore abstain from smoking whilst experimenting, for the slight amount of ammonia in tobacco-smoke is suf- ficient to elicit continued pulsations. The nerve, on the contrary, is much less sensitive towards hydro- chloric acid, and is not at all sensitive towards am- monia. If the nerve is immersed in the strongest solution of ammonia it very soon dies, but is not at all irritated. These are the most marked differences. But it must also be mentioned that glycerine and lactic acid in concentration exercise an irritating effect on the nerve, but not on the muscle ; and that when many other substances (alkalies, salts) are applied, small dif- ferences are exhibited, in that sometimes the nerves, sometimes the muscles, contract in response to a some- what thinner concentration. It thus appears that the differences are extremely slight. Kiihne, however, attaches weight to these, and interprets them as favourable to the theory of the in- dependent irritability of muscle-substance. He sup- ports this conclusion by the following observations. In the case of specific muscle-irritants (ammonia, greatly diluted hydrochloric acid) the result is the same whether the experiment is tried on an ordinary muscle, or on 258 PHYSIOLOGY OF MUSCLES AND NERVES. one poisoned witli curare. Nor does it make any dif- ference whether a strong ascending current is passed through the nerve of a sartorius thus conditioned, thus inducing strong anelectrotonus in the intra-muscular nerve-branchings, so as to disable it. He sees in this a proof that the nerves which spread through the muscle do not share in this form of irritation. He has, more- over, discovered that the nerves are not equally dis- tributed throughout the sartorius. They enter at a point somewhat below the middle of the muscle, and distribute themselves upward and downward between the muscle-fibres ; but they cannot be traced to the ends of the muscle, and there are at these ends regions of from 2 to 3 m. in length, in which at least the larger muscle-fibres are wanting. (Whether the nerve- net which, according to Gerlach, lies within the sarco- lemma, extends to these regions, is another question with which we have nothing here to do.) The specific muscle-irritants affect these regions exactly as they do the rest of the muscle ; while the specific nerve-irritants (concentrated lactic acid and glycerine) are never able to affect these ends, though they elicit single pulsa- tions in the parts containing nerves. These nerve- containing parts are also more electrically excitable than are the ends ; by curare and by anelectrotonus their excitability is decreased, though that of the nerveless ends remains unaltered. Many objections have been brought forward against these conclusions. For my part, in the very insignifi- cance of the differences between nerve and muscle in this point also, I am inclined to see new reason to believe that these two organs, so similar in all points (as yet we know only two important differences, which THEORY OF XERVE-ACTIVITY. 259 are, that the muscle is contractile, which the nerve is not, and that electrotonus, which intervenes in nerve, cannot be shown in muscle), may also be entirely simi- lar in the matter of irritability, and that those who dis- pute this quality are forced to assume the existence of a substance intermediate between that of the nerve and of the muscle, and which diifers almost more from the nerve than from the muscle. 8. Summing up, it appears that the independent irritability of muscle-substance has not been proved ; nor has it been disproved. To understand how the nerve acts on the muscle one must assume that the latter is irritated by the former, and therefore there is no sufficient reason, remembering the similarity in all other points between nerve and muscle, to dispute that it may also be ii'ritated by other irritants (electric, chemical, mechanical, or thermic). In the theory above explained as to the nature of the influence on the muscle, we have assumed that this irritation takes place electrically. We have therefore tacitly presup- posed that the muscle is electrically excitable. Except on this assumption, all that can be said is that the molecular process originating in the nerve is trans- ferred to the muscle : which explains nothing, but rather renounces all explanation. Our hypothesis, on the other hand, has the undeniable advantage that it is based on the well-known process of the negative variation of the nerve during its activity. That the negative varia- tion, when it has once originated in the nerve, propa- gates itself to the nerve-ends, can only be regarded as natural, and, pro\dded that it is of sufficient strength, it can then act as an ii-ritant on the muscle. We Lave already seen that the nerve must be 260 PHYSIOLOGY OF MUSCLES AND NERVES. regarded as composed of many particles arranged one behind the other, each of which is retained in a defi- nite position by its own forces and by the influence of the neighbouring particles. Whatever acts as an irritant on the nerves must displace these particles from this position, and must cause a disturbance, which then propagates itself, owing to the fact that a change in the position -of one particle causes a disturbance in the equilibrium of the adjacent particles, in consequence of which the latter are set in motion. Negative varia- tion must be regarded as a result of this movement of the nerve-particles, in that the electrically acting parts are arranged in different order by the movement, and therefore must exercise a different external influence. But just as this change in the position of the nerve- particles is able to set the needle of a multiplier, if it is properly connected with the nerve, in motion, so the electric process originating in the nerve must act on the muscle, if the latter is sensitive to electric varia- tions. This was the assumption from which we started, and which, after the above explanations, will be regarded as thoroughly trustworthy. To enter further into the details of the activity of nerves and muscles, and to substitute more definite conceptions for such as are at present often indefinite, is impossible in the present state of knowledfife. CHAPTER XVI. 1. Various kinds of nerves; 2. Absence of indicable differences in the fibres ; 3. Cliaracters of nerve-cells ; 4. Various kinds of nerve-cells ; 5. Voluntary and automatic motion ; 6. Eeflex motion and co-relative sensation ; 7. Sensation and conscious- ness ; 8. Retardation ; 9. Specific energies of nerve-cells ; 10. Conclusion. 1. At present we have paid attention only to such nerve-cells as are in connection Tvith muscles, and bj the activity of which the appropriate muscles are ren- dered active. We have referred only incidentally to other kinds of nerves. The difficulty due to the cir- cumstance that a suitable reagent is necessary for the study of such nerve-activity as does not express itself in any visible change in the nerve, compelled us to con- fine our studies in the first place to muscle-nerves or 'motor nerves, in which the muscle itself acts as the required reagent. We now have to discover how far the experiences which we have gained of motor-nerves, and the views which we have based on these experiences, are applicable to other nerves. Besides the real motor nerves, we may distinguish those which act on the smooth muscle-fibres of the blood-vessels, through these effecting a decrease in the diameter of the smaller vessels, and thus regulating the circulation of the blood. These are called vaso-motor nerves. They are, however, in no way different from 262 PHYSIOLOGY OF MUSCLES AND NERVES. other motor nerves. "But a difference is observable even in the case of the secretory nerves or gland-nerves, of which we have already had occasion to make mention. When these nerves are irritated the appropriate nerves begin to secrete. The connection of these nerves with the glands must from a physiological point of view be entirely similar to that of the motor nerves with the muscles. When the latter are irritated the muscles connected with them at once pass into a state of activity. Just in the same way the gland-nerves, when they are irritated, cause the glands connected with them to pass into a state of activity. That this activity is quite different from that of the muscles, is obviously due to the entirely different structure of the glands and the muscles. A gland, unlike a muscle, cannot contract; when it become.s active, it secretes a liquid, this being its activity. There is therefore no reason to assume any difference in any of these nerves, the difference in the terminal apparatus, in which the nerves end, being sufficient fully to explain the difference in the pheno- mena. But there are other nerves the action of which is much harder to understand. Among these are the sensory nerves. When these are irritated, they effect sensations of different kinds, some being of light, others of sound, and so on. Moreover these nerves are capable of receiving irritation in a peculiar way, some by waves of light, others by sound vibrations, and others again by heat-rays ; but in all cases, only when these influ- ences act on the ends of the respective nerves. It is not self-evident that these nerves are homogeneous in themselves or with the previously mentioned kinds. Finally, it is yet harder to understand the action of SIMILARITY OF NERVE-FIBRES. 263 another, and the last class of nerves, which are called retardatory nerves (Hemmungs-nerven). It is com- mon knowledge that the heart beats ceaselessly during life. Now, if a certain nerve which enters the heart is irritated the heart ceases to beat, recommencing when the irritation of the nerve is discontinued. This remarkable fact was discovered by Edward Weber, who spoke of the phenomenon as retardation. It is curious that a nerve can by its activity still a muscle which is in motion. 2. Before we endeavour to determine this and the other points raised, we must note whether any differ- ences can be shown in these various nerves, which act in such entirely different ways. In the previous chap- ters we have observed so many peculiarities in nerves, and among these, qualities which can be examined without the intervention of the muscle, that it seems not altogether unjustifiable to hope that we may be able to observe differences also in nerves if any such occur. But if this is impossible, if all nerve-fibres, though examined in every possible way, seem to be quite homogeneous, then we shall be justified in con- sidering them really homogeneous, and must look for an explanation of the variety in their actions in other circumstances. It may at once be said that it is quite impossible to show differences in the different kinds of nerves. Microscopic observation shows no differences ; for the difference, to which allusion has already been made, between medullary and medulla-less fibres does not affect the point in question. We are obliged to infer that the medullary sheath is of entirely subordinate significance in the activity of the nerve. At any rate, 264 PHYSIOLOGY OV MUSCLES AND NERVES. the presence or absence of this medullary sheath does not correspond with differences in the physiological actions of nerves. Nor are the small differences in diameter of the separate nerve-fibres of greater import- ance. Nor do experimental tests bring any differences to light. The bearing of nerves to irritants does not vary: the electromotive effects are the same in all. In all these points we need simply refer to the previous chapter, for the explanations there given are equally true of all kinds of nerve-fibres. If, therefore, all kinds of nerve-fibres are alike, we can only explain the difference in their action as due to their connection with terminal organs of various form. We have already made use of this principle in explanation of the difference between motor and secretory nerves, and we must now endeavour to ex- tend it to all other nerves. 3. While the motor and secretory nerves have their terminal organs in the periphery of the body, the sensi- tive or sensory nerves act on apparatus which are situ- ated in the central organs of the nervous system. An irritant which affects a motor nerve, to become appa- rent, must propagate itself toward the periphery, till it reaches the muscle situated there ; an irritant, on the other hand, which affects a sensory nerve, must be pro- pagated toward the centre before it sets free any action. Nerves of the former kind are therefore called ceMtrlfu- gal, those of the latter centrijpetal. We have, however, already found that this does not depend on a difference in the nerve itself, but that each nerve-fibre, when it is affected at any point in its course, transmits the ex- citement in both directions ; and we therefore presumed that the fact that action takes place only at one end CAPACITIES OF JMEEVE-CELLS. 265 must be due to the nature of the attacbment of the fibres to the terminal apparatus. {Cf. chap. xiii. § 3, p. 217.) After we had carefully examined the peripheric ter- minal apparatus of the motor nerves, that is to say, the muscles, we were in a position to study the processes in motor fibre. In order now to understand the action of sensory fibres, it will be therefore necessary first to obtain further knowledge of the central nervous organs. The central organs of the nervous system, in ad- dition to nerve-fibres, include, as we have seen (chap, vii. § 1, p. 105 et seq.), also cellular structures, called ganglion-cells, nerve-cells, or ganglion-halls. They are not always globular, but are generally irregular in form. Beside the forms represented in fig. 27 (p. 106), which occur scattered here and there in the course of the peripheric nerves, forms such as those represented in fig. 68 occur much more abundantly in the central or- gans. They generally have many processes (four, six and even up to twenty), which branch and unite together like network. Many cells exhibit one process, difi'er- ing from the others, which passes into a nerve-fibre (nerve-process : cf. fig. 68,, la and 3c). These nerve- processes pass out from the central organ and form the peripheric nerves. Within the central organ the processes of the ganglion-cells form a very involved network of fibres ; between these there are, however, other fibres which completely resemble the peripheric nerve-fibres. There is no reason for ascribing to these fibres of the central organ qualities other than those of the peripheric fibres. When in the central organ phe- nomena are observed which never occur in the peri- 266 PHYSIOLOGY OF MUSCLES AND NERVES. pheric nerve-tibres, it is natural to refer these to the presence of the ganglion-cells. As a matter of fact, all organs which contain nerve- cells, the central organs as well as the peripheric Fig. 68. Ganglion-cells from the human brain. 1. A eaiigUon-cell, of which one process,, a, becomes the axis-cj'linder of a nerve- fibre, 6. 2. Two cells, a and b, interconnected. 3. Diagrammatic representa- tion of three connected cells, each of -which passes into a nerve-fibre, c. 4. Ganglion-cell partly filled ^^•ith black pigment. organs, in which they are present, though not so abun- dantly, exhibit certain peculiarities, which we must re- gard as caused by the nerve-cells, /.nd as we are in no case able to examine the nerve-cell by itself, but must always examine it in connection with, and mingled with the nerve-fibres, we can but carefully determine the dif- CAPACITIES OF NERVE-CELLS. 267 ference in the behaviour of these organs from that of ordinary nerve-fibres, and then regard all not appertain- ing to the nerve-fibres as peculiar to the nerve-cells. We know that the nerve-cells are irritable, that thej transmit the excitement which arises in them, and transfer it at the terminal orofan. The excitement can never occur of itself in a nerve-fibre, but it always re- sults from an irritant acting externally, and can never pass from one nerve-fibre to another, but always remains isolated in the excited fibre. But where nerve-cells occur, the case is different. As long as a nerve-fibre passes uninjured from the brain and spinal-marrow, or from one of the accumulations of nerve-cells situated in the periphery, to a muscle, ex- citement arises without externally visible cause, and this acts through the nerve on the muscle, sometimes at regular intervals independently of the will, sometimes from time to time at the instigation of the will. Again, where nerve-cells occur, we find that excitements which are transmitted to the central organ by a nerve-fibre may there be imparted to other nerve-fibres. Thirdly, we find that excitements which are transmitted to the central organ by nerve-fibres there elicit a peculiar process, which is called sensation and consciousness. Fourthly and finally, the remarkable phenomenon, mentioned above, of retardation, only occiu-s where nerve-cells are present. The four following qualities, which are entirely absent in nerve-fibres, must there- fore be attributed to nerve-cells : — (1) Excitement Tnay arise in them independently, i.e. luithout any visible external irritant. (2) They are able to transfer the excitement from one fibre to another. 268 THYSIOLOGY OF MUSCLES AIS'D NERVES. (3) They can receive an excitement transmitted to them and transmute it into conscious sensation. (4) They are able to cause the suppression (retar- dation) of an existing excitement. 4. From the above it must not be supposed that all ganglion-cells possess all these qualities. On the con- trary, it is to be supposed that each nerve-cell per- forms but one of these functions, and .even that there are more minute differences in them, so that, for in- stance, the nerve-cells which accomplish sensation are of various kinds, each of which accomplishes but one distinct kind of sensation. This is no mere hypo- thesis, for there are established facts which confirm the view. Conscious sensations occur only in the brain, and the various parts of the brain may be separately removed or disabled, in which case individual forms of sensation fail, while others remain undisturbed. If the whole brain is removed, the nerve-cells of the dorsal marrow suffice fully to accomplish the pheno- mena of the transference of excitement from one nerve- fibre to another. Again, there are certain regions of the brain which separately are able to give rise to inde- pendent excitement in themselves ; and certain accumu- lations of nerve-cells which lie outside the actual central nervous organs have the same power. The forms which nerve-cells assiune being very varied, it often happens that the cells of certain regions, where only certain capa- bilities can be shown, are alike in form, and differ in this respect from the cells of other regions, where the capa- bilities are different. As yet, however, it has not been found possible to distinguish differences in form suffi- ciently characteristic, and relations between the form and the function of nerve-cells sufficiently characteristic to FORMS OF NERVE-CELLS. 269 make it possible definitely to infer the function of a cell from its form. On the contrary, it is better, by experi- ments with animals and experiences with invalids, to determine step by step what functions belong to the cells of a given region. Considering the complex and yet very imperfectly known structure of the central nervous organs, it is not svu'prising that this task has by no means yet been fully accomphshed. As in the present work we are not treating of the physiology of the separate parts of the nervous system, but are only concerned with the general characters of the elements which constitute the nervous system, we must not enter into details ; but we must be satisfied to show what the nerve-cells in general are able to accomplish and to give due prominence to the fact that each separate nerve-cell is probably always able to accom- plish only one definite thing. We will now run through these capacities and show the facts which serve as proof of these. 5. The natural rise of excitement takes place either voluntarily or involuntarily. We are always able voluntarily to contract our muscles, though not all of these, for many, especially the smooth forms, are not subject to the will, but contract only as the result of other causes. Sometimes, moreover, the want of power to contract certain muscles is to be ascribed only to want of use, as is shown by the fact that some men are able voluntarily to contract the skin of their scalps or their ear-muscles, though this is impossible to most men, or is possible only in a very restricted degree. Similarly, it is a matter of use how far the will is able to effect a limited con- traction of separate muscles or parts of a muscle. 270 PHYSIOLOGY OF MUSCLES AND NERVES. Those beginning to play the piano find it difficult to move individual fingers apart from the others, though by practice they soon learn to do this. \Yhenever an intended contraction of a muscle is accompanied by another unintended and simultaneous, the latter is called a co-relative movetnent. Such co-relative movements sometimes accompany illness. Stammerers, for instance, when they speak, twitch the face muscles or even those of the arm. It has also been observed that in the case of injuries, after blood has been lost from the brain, movements of the injured limbs not voluntarily possible occur involuntarily as co-relative motions. Some co-relative movements are natural in the organism ; for instance, when the eye is turned inward, the pupil simultaneously decreases in size, and a contraction of the adjusting muscle occurs, by which the eye is enabled to see at a short distance. This co-relative motion has been regarded as a case of the transmission of the excitement from one nerve- fibre to another; but it seems to me that this is incorrect. For there is nothing to show that the excitement originated in one fibre and was then transferred to other fibres, and it is more simple to assume that the various fibres were excited simultaneously by the will, either because isolated excitement of these fibres sepa- rately is really impossible on account of the anatomical structure of the nerve, or because of an insufficient specialisation of the influence of the will, resulting from want of exercise — that is, it is due to unskilful- ness on the part of the will. If it is asked how the voluntary excitement of the nerve-fibres is caused in the nerve-cells, an answer is yet to be sought in physiology. Into the question VOLUNTARY AND AUTOMATIC MOVEMENTS. 271 wliether there is actually a purely voluntary excite- ment, that is, that no incitement acted externally on the brain but that the excitement originated quite spontaneously, we will not enter further here. All that is certain is that in many cases an action appears to be voluntary which, if the process is more closely analysed, is found to result from external influences. But the physiological process by which (whether externally influenced or not) excitement arises in the nerve-cells, which excitement is then transmitted through the nerve-fibre to the muscle, is as yet ex- tremely obscure ; and if it is said that it is a molecular motion of the constituent particles of the nerve-cell, this explains nothing, but merely expresses the convic- tion that it is not a supernatural phenomenon, but merely a physical process analogous to the process of excitement in the peripheric nerves. Involuntary movements occur sometimes irregularly, as twitchings, spasms ; sometimes regularly, as in the case of respiratory movements, the movements of the heart, the couti'actions of the vascular muscles, of the intestinal muscles, and so on. The latter, which occur with more or less regularity while life lasts, and are for the most part of deep significance as regards the normal condition of the vital phenomena, have natu- rally been especially subjected to thorough research. They are called automatic nnovements, that is, they occur independently of the co-operation of the will, and apparently without any incentive. But notwith- standing this, it is chiefly in such cases that the causes which effect the excitement of the nerves concerned have been to a certain extent established. Automatic movements may be distinguished into 272 THYSIOLOGY OF MUSCLES AND NERVES. such as are rhythinic, in which contraction and relaxa- tion of the muscles concerned take place in regular alternation, as in respiration and in the movements of the heart ; such as are tonic, in which the contractions are more constantly enduring, even if the degree of contraction varies, as in the contraction of the vascular muscles, and of the rainbow membrane of the eye ; and such as are irregular, i.e. the peristaltic movements of the intestine. Our knowledge of automatic movements is based principally on those connected with respira- tion ; but the conceptions gained in this case may be directly applied to the other cases. It will be suffi- cient therefore to speak of respiratory motion only. Eespiration begins immediately after birth, and its movements continue from that time throughout life. In the higher animals (mammals and birds) they are unconditionally necessary for the preservation of life, for only by their means is sufficient oxygen conveyed to the blood to provide for all the vital processes. On the other hand, when the organ from which the ex- citement of the respiratory muscles proceeds is in any way insufficiently nourished or is otherwise in- jured in condition, respiratory action ceases and life is threatened. This organ is a limited point in the Tnedulla oblongata, formed of a mass of nerve-cells, in which the excitements originate, and from which they are conveyed by the nerves to the respiratory muscles. This is called the respiratory centre {Lehenshnoten of the Grermans, noeud vital of the French), because of its importance to life. It is the spot which the matador in bull-fights must reach by a skilful blow with his knife, to bring the enraged animal to the ground ; it is the spot which, if crushed between the VOLUNTARY AND AUTOMATIC MOVEMENTS. 273 first and second vertebrae, the result is instant death by the so-called dislocation of the neck. It has been shown that the cause which induces this ceaseless activity in the nerve-cells of the respiratory centre lies in the character of the blood. When the blood is quite saturated with oxygen, then the activity of the respiratory centre commences.^ When the blood becomes freer from oxygen, the respiratory motions become stronger. Far from being necessarily active, independently and without external incentive, the nerve-cells of the respi- ratory centre are also rendered active by external cir- cumstances. But they are much more sensitive than the nerve-fibres, so that they are influenced even by slight changes in the gaseous contents of the blood which plays over them. And the other automatic nerve-cells behave exactly as do the cells of the respiratory centre. Yet small differences in sensitiveness occur among them, so that some are excited even when only the average amount of oxygen is contained in the blood, others when a point lower than this average has been reached, as happens only occasionally during life. It would take too long to apjDly this theory, now • Experimental proof of this may alwa3's be tried by anyone on himself. Attention must be given for a time to the respiratory movements, their depth and number being noted. From eight to ten inspirations and expirations are then drawn slowly one after the other. By this means much more air is introduced into the lungs than by ordinarj' respiration, and the blood can therefore thoroughly saturate itself with ox3'gen. If, after this, voluntary respiration is ceased, it will be found that twenty seconds or more elapse before a respiration again occurs, long enough that is for the consumption of the introduced oxygen. Only after this do respira- tions begin, at first weakly, but always increasing in strength, until the former regular respiration again prevails. 13 274 PHYSIOLOGY OF MUSCLES AND NERVES. briefly explained, to each of the other processes of automatic motion. We must content ourselves with the remark that an analogous conception of the nature of the movements of the heart is probable, though no experimental proof of its correctness has yet been achieved. The cause of movements of the intestine is not quite so difficult to understand ; at any rate, the main principles found in the case of the nerve-cells of the respiratory centre are valid in the case of all other automatic centres.^ Mention must still be made of the fact that in the heart and intestine the nerve-cells from which the automatic action proceeds are situated within the respective organs themselves. For this reason these organs can yet exhibit movements after the nerve-centres have been destroyed, or the organs have been cut from the body. 6. The transference, by means of the nerve-cells, of an excitement from one nerve-fibre to another is most clearly shown in that which is called reflec- tion. By this term is meant the passage of an excite- ment, which having acted on a sensory fibre has been transmitted by it to the nerve-cells, to a centrifugal fibre, by which it is conducted back from the centre (as a ray of light is reflected from a mirror) and makes its appearance at another point. The reflection can occur either in a m,otor fihre, in which case it is called a reflex action, or in a secretory or retarda- tory fibre. The. former case is more common and better known. As examples of such reflex actions, I may mention the closing of the eyelids on the irrita- ' Those who wish to obtain further information as to these cir- cumstances may be referred to my work BemerTiimgen iiier die Thdtigheit dcr autoviatischen A^erven-centra, &c. Erlangen, 1875. REFLEX MOTIONS. ^ 275 tion of the sensory nerves of the eye, sneezing on irritation of the mucous membrane of the nose, cough- ing on the irritation of the mucous membrane of the respiratory organ. Wherever sensory nerves are con- nected by nerve-cells with motor nerves, these reflex actions may occur. If an animal is decapitated and its toe is pinched, the leg is drawn up and contractions occur in it. The reflex actions are here accomplished through the nerve-cells of the spinal marrow, and the removal of the brain favours the action, while it at the same time excludes the possibility of the intervention of voluntary movements. There is no doubt that in this process the nerve- cells play a part, and that the process does not depend solely on the direct transference of the excitement from a sensory nerve-fibre to an adjacent motor nerve-fibre. Apart from the fact that the transference never takes place except where nerve-cells can be shown to be pre- sent, this is confirmed by the fact that the process of reflex transference occupies a very noticeable time, much longer than that required for transmission through the nerve-fibres. With the knowledge which we have now gained of the structm'e of the central nervous organs, it may be considered established, that nowhere is there immediate connection between sen- sory and motor nerve-fibres, but a mediate connection through the nerve-cells. This allows the possibility of the propagation of an excitement from a sensory nerve- fibre, through a nerve-cell, to a motor nerve-fibre. It is thus intelligible how, owing to the interconnec- tion of the nerve-cells, the passage of the excitement from any sensory nerve-fibre to any or every motor nerve-fibre is possible, for the excitement advances 276 PHYSIOLOGY OF MUSCLES AND NERVES. from nerve-cell to nerve-cell, from each of which it can repass into a motor fibre. From the length of the time occnpied by the reflex irritant, it is to be inferred that the transmission of the excitement has to meet considerable resistance in the nerve-cells. This resistance naturally increases with the number of nerve-cells to be traversed, so that the transference of a reflex action from a definite sensory fibre to different motor nerve-fibres is not always equally difficult, and is the more difficult the greater is the number of the cells which lie between the two. All this agrees with the facts found by experiment. It also explains why, by certain influences, not only is the reflex trans- ference rendered easier, but the passage of the excite- ment to the most remote motor fibres is also rendered peculiarly possible. The best known case of this is poisoning by strychnine. This so greatly facilitates the reflex transference that the slightest touch on any point of the skin, or even the disturbance caused by a breath, is sufficient to throw all the muscles of the body into violent reflex tetanus. As each excitement of a sensory fibre which reaches the nerve-centre can give rise to a conscious sensation, the spread of the excitement within the centre must have the same effect as would be the case if a larger number of excitements of several sensory fibres reached the centre simultaneously. This process, vvhich, how- ever, only occurs in the case of strong excitements, is called co-relative sensation. Sensation is caused not only by the excitement of the nerve-cell directly concerned, but also by the spread of the excitement to the other nerve-cells. It may also be spoken of as the radiation of the sensory irritant, because the excite- SENSATION AND CONSCIOUSNESS. 277 ment seems to spread within certain limits from the point directly touched. 7. These phenomena will become more evident when we have more accurately learned the origin of conscious sensations in general, and the conceptions which depend on this. In order that such conscious sensations should result it seems absolutely necessary that the excitement should reach the main brain {cerehriiiii). Whether other parts of the brain, or even the spinal marrow, are able to give rise to conscious sensations is at least very doubtful, and is at any rate not proved.' But when the excitement reaches the brain, it gives rise not only to feelings, but also to very definite conceptions as to the nature of the excite- ment, its cause, and the locality at which it acts. It is true that sometimes this effect fails and the irritant does not reach consciousness, as, for example, when the attention is strongly attracted in some other direction, • The dispute about the so-called ' mind in the spinal marrow ' (^Ruckenmarh!>scelc),t\ic question, that is, whether more or less clear conscious conceptions can occur in the nerve-cells of the spinal cord, was long and hotlj' debated, but is now at rest. It appears to me that the whole form of the question is unscientific, for the question can simply not be solved with the means for research which we can command. Our own consciousness informs us as to our own sensa- tions and conceptions, and we learn those of others from their lips. Where this fails, opinion is always untrustworthy, as, for example, where we try to infer the feelings of men from their behaviour. It is, however, yet more hazardous to attach importance to the movements of a brainless animal, and it is therefore not surprising that two observers should draw quite different conclusions from the same facts, one explaining them as simple reflections, the other being of opinion that such behaviour under such circumstances is only ex- plicable as the result of conscious sensations and conceptions. The lower the animal is in the scale, the more i;ntrust worthy, naturally, is the decision. 278 PHYSIOLOGY OF MUSCLES AND NERVES. or as in sleep. The irritant can then elicit a reflex action, though there is no consciousness of this. That the origin of conscious conceptions is also an activity of the nerves is certain, and it is the cells of the grey matter of the brain which possess this activity. On the other hand, we are entirely unable even to indicate how this consciousness comes into being. It may be dae to molecular processes in the nerve-cells which result from the received excitement ; but mole- cular processes are but movements of the molecules, and though we can understand how such movements cause other movements, we are entirely unaware how these can be translated into consciousness.^ The excitements transmitted by the various sensory fibres do not all act in the same way on the brain, and the sensations to which they give rise differ. Accord- ingly, we may distinguish the various sensations of the various senses, and even within one and the same sense various sub-species, as the colours in the sphere of optical sensations, the various pitches in the sphere of auditory sensations. But as all the nerve-fibres which accomplish the various sensations differ in no way from each other, we are forced to look in the nerve-cells for the reason of the difference in sensations. Just as we assumed that motor nerve-cells differ from sensory, so we must further assume that among sensory nerve-cells, the excitement of which always elicits the conception of light, others again the excite- ' E. du Bois-Reymond has entered further into this question in his address to the assembly of naturalists at Leipzig ( Ueier die Grenxen des Katurcrliennens, Leipzig, 1872). Some of the younger natural philosophers seem inclined to avoid the difficulty by ascrib- ing, as does Schopenhauer, sensation and consciousness to all mole- cules, but this does not seem to me to be any real gain. SENSATION AND CONSCIOUSNESS. 279 ment of which always elicits the conception of sound, others again the excitement of which always results in the conception of taste, and so on. In entire accord- ance with this assumption is the fact that it does not matter what external cause effects the excitement of any one nerve-fibre, but that every excitement of a given nerve-fibre is always followed by a given sensation. Thus, the nerve of sight may be mechanically or elec- trically irritated, with the result of producing a sensa- tion of light ; mechanical or electric irritation of the auditory nerve effects a sound sensation ; electric irri- tation of the nerve of taste efi"ects just such a sensa- tion of taste as does the influence of a tasted substance. It even happens that the exciting cause is situated in the brain itself and directly excites the nerve-cells, and the sensations which are thus elicited are indis- tinguishable from those which are effected through the nerves. To this are due the subjective sensations, hallucinations and so on, which depend on an altera- tion in the character of the blood, or on an increase in the sensitiveness in the nerve-cells. Wherever the excitement occurs, whether in the nerve-cells themselves or anywhere in the course of the nerves leading to the cells, consciousness always refers the sensation to the presence of some external cause of excitement. If the nerve of sight is pressed, the patient believes that he sees a light external to his body-^ if a nerve of touch is irritated at any point in its course (e.g. the elbow-nerve at the furcation of the elbow-bones), the patient feels something in' the nerves distributed in the skin (in our example in the two last fingers, and in the outer edge of the palm of the hand). Our power of conception therefore always 280 PHYSIOLOGY OF MUSCLES AND NERVES. projects every sensation which reaches the conscious- ness ontward, that is, to where the cause of the excite- ment is normally. This so-called laiv of eccentric sensations finds an easy explanation in the supposi- tion that the conception of the locality of the efficient cause is gained from experience.^ It will easily be understood that this necessarily follows from the cha- racters which we have ascribed to the nerve-cells. When the nerve-cell is irritated, the same sensation and the same conception must always result. Just as it makes no difi"erence in the case of a muscle whether the excitement conveyed to it by a motor nerve starts from a higher or from a lower point on the nerve, or whether the nerve has been irritated mechanically, electrically, or by the will, so the process in the nerve-cell does not depend on the locality or the nature of the excite- ment. When the circumstances which give rise to the irritation are abnormal, the result is an illusion of the senses, that is, a false cause is assigned to a perfectly clear and true sensation. 8. The nature of the last of the capabilities which we have attributed to the nerve-cells, the retardation of a motion, is still very obscure. The fact of retarda- tion is as yet principally known in the case of auto- matic motion, though retardation of reflex action also occurs, as may be inferred even from the fact that the rise of reflex actions is hindered by the activity of the nerves, especially when this originates from the brain. The respiratory movements being of all automatic move- ' Details of this matter, into whicli we cannot enter further here, will be found in Bernstein's The Five Senses of Man (Inter- national Scientific Series, vol. xxi.), and in Hiixlej-'s Elementary Physiology. RETAEDATION. 281 ments the best known, it is on these that the current views as to the retardatory nerves are based. It has been explained in § 5 that the respiratory movements result from the excitement of the nerve-cells of the re- spiratory centre. These movements may be accelerated or retarded, though all the other conditions remain unchanged, if certain nerve-fibres which pass from the mucous membrane of the air-passage to this respira- tory centre are irritated. These retardatory nerves are distinguished from those which pass to the heart by the fact that it is not known whether the latter pass to the muscles of the heart or to the nerve-cells situated in the heart, a doubt which is satisfied in the case of the former by their anatomical arrange- ment. Of the retardatory fibres of the heart it might be supposed that they in some way incapacitate the muscle from contracting ; in the case of the retar- datory nerves of the respiratory system such supposi- tion may be at once rejected, for they are in no way in contact with the respiratory muscles. The only pos- sible explanation is therefore, that the retardatory nerves act on the nerve-cells in which the excitement is generated, thus either preventing the excitement from even coming into existence, or preventing the excite- ment from passing from the nerve-cells in which it is generated to the appropriate motor nerve-cells. For various reasons the latter view has been preferred. It is supposed that the automatically acting ganglion-cells are not directly connected with the appropriate nerve- fibres, but that conducting intermediate apparatus are present between the two, and that these offer a great resistance. This explains both the occurrence of the rhythmic motions and the retardation. The latter, 282 PHYSIOLOGY OF MUSCLES AND NERVES. that is, is due to an increase in the resistance by which the motion is temporarily suspended.' Eetardatory nerves have been recognised in almost all automatic apparatus, and all are accounted for by the above explanation. The same explanation may also be applied at once to the retardation of reflex action ; for even in the passage of the excitement from the sensory to the motor nerves very great resistance has to be overcome, and an increase in this resistance must prevent the passage of the excitement and thus hinder reflex action. Our acquaintance with this sub- ject is, however, not yet by any means complete, and a final opinion on the matter is therefore for the time impossible. I will only mention further that the opposite effect, the facilitation of the passage of the excitement from the nerve-cells in which it originates, to the peripheric nerve-courses, appears to occur. Finally, it is sometimes observable that when those portions of the nerves which contain nerve-cells are continually and regularly irritated, a rhythmic or even an irregular movement results, instead of a regular tetanic contraction of the muscles concerned, — a cir- cumstance which is evidently to be explained in the same way as rhythmic automatic activity. The regu- lar excitement having to pass through nerve-cells is modified by the great resistance present in these, and is transformed into a rhythmic motion, while when the nerve and the muscle are directly connected, the latter responds to a continuous excitement of the nerve with a regular and continuous contraction. o > See my account of the automatic nerve-centres, to which refer- ence has already been made. SPECIFIC ENERGIES OF NERVE-CELLS. 283 9. From all these details it is very evident that the nerve-fibres are homogeneous the one with the other, and that the difference in their effects is to be referred to their connection with nerve-cells of varied form. This seems, however, to be opposed to the fact that the different sense-nerves are irritable by quite different influences, and each of them only by quite definite influences — the nerve of sight by light, the nerve of hearing by sound, and so on. It would, how- ever, be a mistake to infer from this that the nerve of sight is really different from the nerve of hearing. If the matter is examined more closely, it appears that the nerve of sight cannot be excited by light. The strongest sunlight may be allowed to fall on the nerve of sight without producing excitement. It is not the nerve, but a peculiar terminal apparatus in the retina of the eye with which the nerve of sight is connected, which is sensitive to light. The case of the other sense-nerves is similar ; each is provided at its peri- pheric end with a peculiar receptive apparatus, which can be excited by definite influences, and which then transmits these influences to the nerves. On the difference in the structure of these terminal apparatus depend which influences have the power of exciting them. When the excitement has once entered the nerve it is always the same. That it afterward elicits different sensations in us, depends again on the character of the nerve-cells in which the nerve-fibres end. Sup- posing that the nerves of hearing and of sight of a man were cut, and the peripheric end of the former were perfectly united with the central end of the latter, and contrariwise that the peripheric end of the nerve of sight were perfectly imited with the central 284 PHYSIOLOGY OF MUSCLES AND NERVES. end of the nerve of hearing, then the sound of an orchestra would elicit in us the sensation of light and colour, and the sight of a highly coloured picture would eHcit in us impressions of sound. The sensa- tions which we receive from outward impressions are therefore not dependent on the nature of these im- pressions, but on the nature of our nerve-cells. We feel not that which acts on our bodies, but only that which goes on in our brain. Under these circumstances it may appear strange that our sensations and the outward processes by which they are evoked are so entirely in agreement ; that light elicits sensations of light, sound sensations of sound, and so on. But this agreement does not really exist ; its apparent existence is only due to the use of the same name to express two processes which have nothing in common. The process of the sensation of light bears no likeness to the physical process of the ether vibrations which elicit it; and this is evident even in the fact that the same vibrations of ether meeting the skin elicit an entirely different sensation, namely, that of warmth. The vibrations of a tuning-fork are capable of exciting the nerves of the human skin, and then they are felt ; they may excite our auditory nerves, and then they are heard; and under certain circumstances they may be seen. The vibrations of the tuning-fork are always the same, and they have nothing in common with the sensations which they elicit. Though the physical processes of the vibrations of ether are called, sometimes light, and at another time heat, a more accurate study of physics shows that the process is the same. The usual classification of physical processes into those of sound, light, warmth, and so on. SPECIFIC ENERGIES OF ^•ER\■E-CELLS. 285 is irrational, because in these processes it gives pro- minence to an accidental circumstance, that is, to the way in which they affect human beings, who are endowed with various sensations, while in other, such as mag- netic and electric processes, it is based on quite different marks of classification. Scientific study of the phy- sical processes on the one hand, and of the physio- logical processes of sensation on the other, exposes this error, which penetrates further owing to the fact that language uses the same words for the different pro- cesses, thus making their distinction harder. Language is, however, but the expression of the human conception of things, and the conception of the innate identity of light and the sensations of light, of sound and of the sensation of sound, and so on, was regarded till quite recently as incontrovertibly true. Goethe ' gave expression to this in the lines — - Wiir' nicht das Auge sonnenhaft, Die Sonne konnt' es nie erblicken ; Lag' nicht in uns des Gottes eigne Kraft, Wie konnt' uns Gottliches entziicken ! Plato expresses himself in the same way in the ' Timseus.' On the other hand, Aristotle held correct conceptions on the subject. But it is only since the researches of Johannes Miiller laid new ways open to science that these conceptions have gained a scientific foundation, and have been brought in all points into harmony with the facts, so that they have now become the basis of the physiology of the senses and the psychology of the present day. One expression of the erroneous views once pre- valent is to be foimd in the theory of so-called ade- ' Zahme Xeiiien, iii. 70. 286 PHYSIOLOGY OF MUSCLES AND NERVES. quale irritants, according to which there is such a sufficient irritant for each sense-nerve, that is, an irritant in its nature adapted to the nature of the sense-nerve, and that this was alone able to excite it. We know now that this is not true. Yet the expres- sion may be used to indicate the irritants which are especially able to act on the terminal organs of the nerves. In the same way we may look upon the idea of so-called specific energies of the sense-nerves, if by this it is intended to express any character of " the nerves, as disproved. But we must ascribe specific energies to the individual nerve-cells in which the sen- sations are originated. It is these alone which are able to produce in us different kinds of sensation. If all the nerve-cells of the sensations were alike, sensa- tions could indeed be elicited in us by the influence of the outer world on our sense organs ; but these would only be of one and the same kind, or at most it could only be in the strength of this one undefined sensation that differences would be perceptible. There may be animals which are only capable of such a single undefined sensation, their nerve-cells being all alike and not yet differentiated. Such animals would be able to form a conception of the outer world as distinguished from their own bodies, that is, they would be able to evolve self-consciousness ; but they would not be able to attain a knowledge of the pro- cesses in the outer world. The development of such knowledge in us is greatly assisted by a comparison of the different impressions brought about by the different organs of the senses. A body presents itself to our eye as occupying a certain space, being of a coxcLusiox. 287 certain colour, and so on. By tasting we may gain further conceptions of this body. If it is out of reach of our hands, by approaching it we may observe how the apparent size of the body, as the eye shows it to us, increases as we approach. These and many thousand other experiences which we have gained since our earliest youth have gradually put us in a position to form conceptions as to the nature of a body merely from a few sensations. In this act many com- plete inferences are unconsciously involved, so that that which we believe to have been directly perceived is really known by inference from many sensations and from a combination of former experiences. For instance, we think that we see a man at a certain dis- tance ; really, however, we only feel a picture of a certain size of the man on our retina. We know the average size of a man, and we know that the apparent size decreases with the distance ; moreover, we feel the degree of contraction of the muscles of our eye which is necessary to direct the axis of our eye to the object and for the adjustment of our eye to the neces- sary distance. From all these circumstances, the opinion, which we erroneously regard as a direct sensa- tion, is formed. 10. We have already (chap. iv. § 2 ; chap. vii. § 3) made acquaintance with the methods by which Helmholtz measured the details of the time occupied by the contraction of the muscle and the propagation of the excitement in the motor nerves. By the same, or very similar methods, Helmholtz, and others after him, determined the propagation of the excitement in sensory nerves, and found that it was about 30 m. per second, and therefore, at nearly the same rate as in the 288 PHYSIOLOGY OF MUSCLES A^B NERVES. motor nerves of men. More than this has been done: the time has been measured which is requisite for an irritant conducted to the brain to be transmuted into consciousness. Such determinations, in addition to their theoretical value, are of practical interest to observing astronomers. In observing the passage of stars on the meridian and comparing the passage seen through the telescope with the audible beats of a second-pendulum, the observer always admits a slight error, dependent on the time which the impressions on the two senses require to reach the state of conscious- ness. In two different observers this error is not of exactly the same value ; and in order to render the observations of different astronomers comparable with each other, it is necessary to know the difference between the two cases, the so-called personal equation. In order to refer the observations made by each indi- vidual to the correct time, it is necessary to determine the error which is made by each individual. Let us suppose that an observer sitting in complete darkness suddenly sees a spark, and thereupon gives a signal. By a suitable apparatus, both the time at which the spark really appeared and that at which the signal was given are recorded. The difference between the two can be measured, and it is called the physio- logical time for the sense of sight ; the physiological time for the sense of hearing and for that of touch may be determined in the same way. Thus Professor Hirsch, of Neufchatel, found — In the case of the sense of sight 0*1974 to 0-2083 sec. „ „ hearing 0*194 „ „ „ touch 0"1733 „ "WTien the impression which was to be recorded was CONCLUsio:^. 289 not unexpected, but was known beforehand, the physio- logical time proved to be much shorter ; in the case of the sense of sight it was only from 0*07 to O'll of a second. From this it follows that, in the case of excitement the advent of which is expected, the brain fulfils its work much more quickly. Certain experiments made by Donders are yet more interesting. A person was instructed to make a signal, sometimes with the right hand, sometimes with the left, according as a gentle irritant applied to the skin was felt in one place or the other. If the place was known, the signal succeeded the irritant after an in- terval of 0"205 of a second, but if the place was not known, only after an interval of 0*272 of a second. The psychological act of reflection, as to where the irritant occurred, and that of the corresponding choice of the hand occupied, therefore, a period of 0*067 of a second. The physiological time in the case of the sense of sight was somewhat dependent on colour ; white light was always noticed somewhat sooner than red. If the observer knew the colour which he was to see, he gave the siofnal sooner than when this was not the case and he had first to reflect as to what he had seen before he gave the signal. In such experiments, the observer always forms a preconception of the coloiur which he , expects to see. If the colour when it becomes obser- vable coiTesponds with that which he expected, the reaction in the observer takes place sooner than when this is not the case. Similar observations were made in the case of the sense of hearing : the recognition of any sound heard follows sooner when it is known beforehand what sound is to be heard than when this is not the case. 290 PHYSIOLOGY OF MUSCLES AND NERVES. This sluggishness of the consciousness, if we may so call it, is exhibited in another way in certain experi- ments instituted by Helmholtz. The eye sees a figure, which is immediately followed by a bright light : the more powerful the latter is, the longer must the iirst have been seen, if it is to be recognised at all ; more- over, complex figures require more time than simpler. If letters are seen lighted up on a bright ground for a very short time, no other light following, a shorter time is necessary for the recognition, the larger are the letters and the brighter the illumination. It is true that it is only very simple brain activities the origin of which can be in any way made clearer by such experiments as these ; but yet these are the rudi- ments of all mental activity — sensation, conception, re- flection, and will; and even the most elaborate deduction of a speculative philosopher can only be a chain of such simple processes as those which we have been observ- ing. These measurements, therefore, represent the beginnings of an experimental physiological psychology, the development of which is to be expected in the future. It seems to me that remunerative study of the processes in nerve-cells must start from the very simplest phenomena. Eesults are, therefore, to be first looked for in the study of the .processes of reflection ; possibly these will prepare the ground on which at some future time a mechanism of the nervous processes may be built. ' In truth,' says D. F. Strauss, in ' The Old and the New Faith,' ' he who shall explain the grasp of the polyp after the prey which it has perceived, or the contraction of the insect larva when pierced, will indeed be yet far from having in this comprehended human thought, but he will be on the way to do so, and CONCLUSION. 291 may attain his end without requiring the help of a single new principle.' Whether this end will ever be attained is another matter. But we can always gain fuller knowledge of the conditions under which it may come to pass, and of the mechanical processes which form its first principles. Such is the lofty aim after which the science of the General Physiology of Muscles and Nerves strives — an aim worthy of the labour of the noblest. NOTES AND ADDITIONS 1. Graphical Represextatiox. Idea of Mathematical Function (p. 49), The method employed in fig. 16 of representing by a sign the dimensions of the expansion relatively to the amount of the expanding -weights, admits of such a vai-iety of appli- cations, and will be used so frequently, that a brief explana- tion of it may not be out of place here. "When two series of values bear such a relation the one to the other that each value of one series corresponds with a definite value in the other, mathematicians speak of the one value as the function of the other. This relation may always be exhibited in tabular form, as in the following example : — 1234 5 6 7 8 9 10 2 4 6 8 10 12 14 16 18 20 The relation which prevails in this case is very simple. Each number in the uj^per series corresponds with a number in the lower, and the latter is always double the value of the former. Representing the numbers in the upper series by X, those in the lower by y, the relation between the two series of numbers may be expressed in the formula : y=2x This formula expresses the same and even more than the 294 PHYSIOLOGY OF MUSCLES AND NERVES. table. Substituting for tbe unknown x, wMch may repre- sent any number, the number 4, then the table expresses that the value of the corresponding y is 8. If x=5, then the table expresses that 3/= 10. But when the value of x is intermediate between 4 and 5, e.g. 4-2371, the table does not help us; but by the use of the formula the value of the corresponding y may easily be found ; it is = 8'4742. The formula may be reversed, and written thus : that is to say, for any given value of y we may calculate the corresponding value of x. It is exactly the same in the case of the similar formula : y^-3x, ■ which may also be written thus : x=^y. In this case, therefore, with each given value of x corresponds a certain value of y, the latter beiug three times the value of the former. In the two corresponding formulae , 1 y = ax and a;=— y, a is a somewhat wider expression to this kind of relation ; in this case x and y are again the signs of the two correspond- ing series of numbers, a expresses a definite figure which is to be i-egarded as unchangeable within each particular case. In our first example a=2, in our second example a=:3, and similarly in any other instance a may have any other value. Lookinjf now at the followins: table : 1 2 3 4 5 6 etc. 1 4 9 16 25 36 etc. we see that any number in the lower series is found by multiplying the corresponding number in the upper series by itself, as may be expressed iu the formula y^x X or y=X' NOTES AND ADDITIONS. 295 This formula when reversed appears thus : Provided with a formula of this sort, which expresses the mutual relation of two corresponding seiies of values, it is always possible to draw out a table, though, on the contrary, the relation laid down in the table cannot always be ex- pressed in a formula, for the relations are not always as simple as in our examples. Generally the values which are treated in the table are such as have been found by observa- tions, as for instance in our case, the expansion of the muscle caused by various weights. With each weight an expansion corresponds, and this is found by experiment and may be expressed in tabular form, thus : Weight : 50 100 1.50 200 250 300 grm. Expansion : 3-2 6 8 9-5 10 10-5 mmt. A A' A" A'" r b \ c c r" c" \ I" ol" fl'" d \^^ ^\^ b iT^ "~~-j^^ ~^~~.^_^^ // '' ^ — ^- ^^---^ Fig. 69. GRArmcvL repkesentatiox of muscle-expansiox. All that is shown by the table is that the expansion does not increase proportionately with the Aveight (as would be the case in inorganic bodies), but increase in a continually decreasing proportion. But any required function-character, whether it is expressed by a comparison or in a table drawn up on the basis of observations, may be diagrammatically 296 PHYSIOLOGY OF MUSCLES AND NERVES. shown by a inetliod first employed by Descartes, whicli it is our present object to explain. The amounts treated may be of tbe most varied kinds : numbers, weights, degrees of warmth, tbe number of births or deaths, and so on. In all cases the amount may be diagram- matically shown by the length of a line. If a line of a cer- tain length represents any given amount, then double this amount is represented by a line twice the length of the former. It does not matter what is the standard selected ; but when once selected it must not be varied in the same representation. Two Hnes are drawn at right angles to each other ; from the point of section B (fig. 69) the lengths which are to represent the values of one series (in our case, the weights attached to the muscle) are measured off on the e ^ f A Fig. 70. Diagrah of positive axd negative values. horizontal Hne. From each of the points thus obtained, d', h", d", d'", a line is drawn at right angles to the first, care being taken to make its length express the expansion corresponding with each weight respsctively. This gives the lines d' B', h" B", d" B'", d'" B^", By connecting these points we obtain the cirrve BB' B" B'" B'"' x", which at a glance shows the relation between the weight and the expansion. In exactly the same way the curve h V h" h'" B'' y is pro- jected, and this represents the expansion of the active muscle by the corresponding weights. In many cases it is required to represent values of oppo- site kinds. If, for example (fig. 70), the wire a 6 is tra- versed by an electric current, then one half assumes positive tension, the other negative tension. To express this, the NOTES A2s'D ADDITIONS. 297 lines which, are to represent positive tension are drawn upward, those which are to represent negative tension down- ■ward, from the basal line. The figure then shows that the tension in the middle of the wire = 0, and that toward the left the positive, toward the right the negative, tensions increase regularly. In order to find the amount of the tension, prevailing at any particular point, e.g. at e, a per- pendicular line is erected at that point; and the length of this, e f, accurately represents the tension there pre- vailing. 2. Direction of the Muscle-Fibres, Height of Elevatiox, AND the Accomplishment of Work (p. 93). Because of the extreme rarity of long parallel-fibred muscles, it is interesting to examine more closely the in- fluence wliich oblique arrangement of the „ fibres exercises on their force, height of ele- vation, and on the work which they accom- plish. AVhen a muscle-fibre is so arranged that it is incapable of efiecting a movement in the direction of its own contraction, only a part of the force of tension which is generated in it by its contraction comes into play, and this part may be easily found by the law of the parallelogi-am of forces. This is the case in all simply and doubly psnniform muscles. Supposing that the muscle-fibre A B (fig. 71) contracts to the extent B h, but that motion of the point B, on account of the attachment of the muscle to the bone, and of the nature of the sockets of the latter, can only occur in -^ _ the direction B C ; in that case the muscle- okobi.iquemus- fibre, in contracting, undergoes a change in ci-e-fibkes. direction from its fixed point of origin A, and thus assumes the position A h' ; the elevation which is really effected is, 14 298 PHYSIOLOGY OF MUSCLES AND NERVES. therefore, B V. The small triangle BhV may be regarded as a right-angled triangle. This gives sm p The force with which the muscle-fibre strives to contract in the direction A B being called h, only part of this force, the component k' lying in the direction B C, finds expression. According to the law of the parallelogram of forces, this com- ponent is k'= k sin j). This force may be regarded as proportionate to the weight which the muscle-fibre is able to raise to the given height of elevation. If we then calculate the woi'k which the muscle can accomplish, we find, if the motion can take place in the direction A B, A=Bbk; but if motion can only occur in the direction B C, A = Bh' k' = -^^ k si7ii3=Bb k. sm J3 The value in the two cases is therefore exactly the same, or, in other words, the amount of work accomplished by the muscle is quite independent of the direction in which its action takes place. This is, naturally, true of every other muscle-fibre, and, consequently, of the whole muscle. The statements which we have made of parallel-fibred muscles are therefore also true of those of which the fibres are irre- gular. The possible height of elevation is always greater the longer the fibres are, and the force proportionate to the diameter or to the number of the fibres. In oblique-fibred muscles the fibres are generally very short, but very nume- rous ; these must, therefore, whatever their accidental form, be regarded as short and thick muscles, possessed of small elevation and great force. NOTES AND ADDITIONS. 299 3. Excitability and Strength of Ireitant. CoMBixATioy OF Irritants (p. 119). When the coils of a sliding inductive apparatus ai'o brought nearer together, the strength of the inductive current does not increase in exact proportion with the decreasing distance betweeen the two, but in a complex way, -which must b3 provided for in each apparatus separately. Pick, Kronecker, and others have shown methods by which this calibration of the apparatus may be accomplished. If the real strength of the irritating current is compared with the height of the pulsation which it elicits, it appears that when the current is very weak no action is observable; action first appears, in the form of a slight, just a- isible pulsa- tion, when the current has reached a certain strength, greater or less according to the condition of excitability of the nei've. As the cui-rents increase further in strength, the lieights of elevation increase in exact proportion to the strength of the currents, till a certain maximum has been reached. If the strength of the current becomes yet greater, the pulsations remain constant for a time; but then they again increase and reach a second maximum, above which they do not pass. These so-called ' over maximum ' pulsations are due to a combination of two ii*ritants. An inductive shock is, as we have seen, a very brief current, in which the commence- ment and the end succeed each other very rapidly. For reasons which will be further explained in ISTote 7, the com- mencement of an inductive ciu-rent is a more powerful irritant than its end. As long, therefore, as the current does not pass a certain strength, only the commencement of the current irritates ; but in the case of very powerful cur- rents the end may be suflficiently effective : this gives two initations following each other in rapid succession, and these 300 PHYSIOLOGY OF MUSCLES AND NERVES. together effect a greater pulsation than does a single irrita- tion. If more than two irritants follow each other in rapid succession, tetanus results, as we know. In this case also the height of elevation is always greater than that which can be attained by a single pulsation. For the muscle has the power of being again ii-ritated even when it is ah-eady in the act of contraction, a more powerful contraction being thus induced in it. The bearing of these facts on the case of nerve is that the separate excitements effected in it by these rapidly successive irritations do not mutually disturb each other, but are transmitted one after the other, in the sequence in which they originate, to the muscle on which they act. But when the number of the irritants becomes too great, the nerve-molecules are no longer able to keep pace with the rapidly succeeding shocks, and the nerve is vmexcited. The limit at which this intervenes has, how- ever, not yet been determined with any certainty. It appears to lie at between SOO to 1000 irritants per second. 4. Curve of Excitability. Eesistance to Transmissiost (p. 123). The increased excitability at the upper parts of the un- injured sciatic nerve, when not severed from the body, which, on the strength of our earlier experiments, we have assumed in the text, has recently been again defended by Tiegel against various objections. For reasons explained in the text it is inadmissible to infer an avalanche-like increase in the irritation merely from this higher excitability of the upper parts. Beside the experiments of Munk alluded to on page 116, there are other experimeqts from which a resistance to transmission in the nerve may be inferi'ed. Such a resistance, weakening the iiTitant during its propa- gation, and an avalanche-like increase in the irritant, are irreconcilable contradictions which mutually exclude each NOTES AND ADDITIONS. 301 otlier. If resistance to transmission can be shown, then the irritation cannot increase in strength during its propagation through the nerve. I will, therefore, here briefly mention the reasons which induce me to declare in favour of one, and against the other, of these assumptions. As is mentioned on p. 141, transmission becomes con- sidei'ably harder when the nerve is in an anelectrotonic condition, and in strong anelectrotonus it is even rendered altogether impossible. It is natural to regard this greatrr difiiculty as an increase of a resistance already present, A more impoi-tant reason is however to be found in the phe- nomena which occur in reflex actions. If a sensory nerve is irritated, the excitement can be ti-ansmitted to the dorsal marrow and the brain, where it may be transferred to a motor nerve (cf. p. 274). This transference always occupies a considerable time, which I call reflex-time. If a sensoiy nerve is initated sufficiently to cause a poweif ul reflex action (called a 'suflicient ii-ritant '), if the reflex-time in this case is determined, and if irritants of continually increasing strength are then allowed to act on the same point in the nerve, then the reflex-time is found to become continually shorter. If, however, a point in the nerve lying very near the dorsal marrow is irritated, then even in the case of a ' sufiicient irritant ' the reflex-time is short. It is evident that the duration of the reflex-time depends on the strength of the iri'itant when it reaches the dorsal marrow. The irritant which comes from the point in the nerve adjacent to the dorsal marrow is but slightly affected ; but that coming from a more remote point is weakened ; so that a much stronger irritant must be applied to these more remote point«!, if an equally short reflex-time is to be attained. It is true that these observations have been made with sensory nerves. But owing to the entirely similar character exhibited by all kinds of nerve-flbres in all points, where comparison is possible, we are justified in applying the views thus gained to the motor-nerves. It is, at all events, im- 302 PHYSIOLOGY OF MUSCLES AND NERVES. probable that in one nerve-fibre a resistance to transmission exists, and in another an avalanche-like increase. All the facts are more easily and simply explained by assuming that there is a resistance to transmission in all nerves, allowance being at the same time made for the difference in the ex- citability of different points in the nerve. Moreover the curve of excitability in the case of the sciatic nerve is not a simple ascending line from the muscle to the dorsal marrow. This nerve is found, as is shown in fig. 72, by the union of several roots; it then, at various Fig. 72. The sciatic xerve and calf-muscle of a frog. points, gives off branches which enter the muscles of the upper leg, and then separate into two branches, one of which provides for the calf-muscle {gastrocnemius), the other for the flexor muscle of the lower leg. If various points of this nerve are irritated in the living animal, the nerve having been merely exposed and isolated from the surrounding parts, but not separated from the dorsal marrow, it is very evident that the excitability at the upper jDoints is generally greater than at the lower ; but points are also fou.nd in the course of the nerve at which a gi^eater excitability exists than at the points above and below, as also, on the contrary, a less ex- citability than at the adjacent points. Such irregularities are most abundantly exhibited at the points where nerve- branches separate from the main trunk, especially when these branches have been cut away. This is partly due to elec- trotonic influences {cf. p. 125 et seq. ; p. 21-5 et seq., Note 13). The nerve- fibres which are cut generate a current which NOTES AND ADDITIONS. 303 passes through those which are not cut off, those the excita- bility of which is tested, and alters their excitability. This influence changes in the whole mass, as the cut nerves die, thus giving rise to irregularitiss the further nature of which we need not trace. 5. Ikfluence of the Length of the Portio:: of the Nerve excited (p. 138). If the irritant reroains the same, the longer is the portion of the nerve irritated, the stronger is the action on the muscle. If the excitability of a portion of the nerve is found by the method of minimum iriitants, that is, if the weakest irritant capable of effecting an observable pulsation is looked for, and if various degrees of excitability prevail in the por- tions of the nerve simultaneously exposed to the irritant, action may result, even if only a part of the portion of nerve is really excited ; in reality, therefore, it is but the excita- bility of the most excitable part of the whole nerve-portion which is tested. In a fresh nerve this is generally the upper part of the nerve-portion. But when there is no great dif- ference in excitability within the nerve-portion, then every part of the poi'tion will be excited by an irz'itant of a certain strength in an approximately like manner, and the action observed in the muscle will therefore be the combined effect of the excitement of the separate parts of the nerve-portion. But if, as we have assumed, the loss of excitability in each part follows the highest excitability very suddenly, the effect must be that the portion actually in-itated continually be- comes shorter; tlie parts which ai'c irritated are however still in the highest state of excitability, and therefore exhibit the third stage of pulsatioji (the testing current having been so chosen that, in the fresh nerve, it originally produced the first stage). The form in which the third stage exhibits itself — pulsation on the closing of a descending curi-ent and on the opening of nn ascending current — must therefore remain 304 PHYSIOLOGY OF MUSCLES AND NERVES. tinclianged, but the pulsations must gradually decrease in strength, and all effect must finally disappear, just when tlie maximum of excitability, and the death which follows this, pass the lower limit of the excited port'on. 6. Difference between Closing and Opening Induc- tive Currents. Helmholtz's Arrangement (p. 151). When an electric current is suddenly closed in a spiral, this not only acts inductively on a neighbouring spiral, but the individual coils of the 2)rimary spii'al act inductiA'ely on each other ; an analogous effect wonld occur on the opening, but that the sudden interruption of transmission prevents the development of this opening inductive current in the primary coil. The inductive current which originates on the closing of the current being in an opposite direction to the closed I current itself, the former must weaken the latter ; the cur- rent can therefore attain full strength, not at once, but only gradually ; but on the opening the current suddenly ceases. This difference in the duration of the closing and opening of the j)rimary current corresponds with differences in the currents induced by them in the secondary spiral, which are used for the irritation of the nerve. Figui'e 73 exhibits these characters. The upper part of the figure represents the tem- poral course of the main current in the primary spiral of an inductive apparatus ; the lower part repres3nts the temporal course of the induced currents in the secondary spiral. The line . . .0 . . .t represents the duiation. The piimary current is closed at the moment o. Were the retardatoiy influence which has been mentioned not present in the primary spiral, the current would at once attain its full strength J ; but owing to that influence it attains this strength only gradually, somewhat as shown by the crooked line .3. With this gradu- ally occurring cuiTont corresponds a closing inductive curi-ent in the secondary spiral, as is represented by the curve 4 ; NOTES AXD ADDITIONS. 305 the curve is drawn dov/uward from the time-line o . . .o . . . t, to indicate that the direction of this induced current is opposed to the direction of the primary current. If the primary current is interrupted, it suddenly falls from the ■^ / • /0| >< si / 1 / N 1 ^ strength J, as inuicated by the straight line 1. With this fall corresponds an inductive current, which suddenly rises very abruptly and again falls somewhat less abruptly, as shown in curve 2. From this it is evident that the latter must be physiologically much more elleetive than the former. 306 PHYSIOLOGY OF MUSCLES AND NERVES. Occasionally it is desirable to remove this difference, and to provide two inductive currents whicli flow and act nearly in the same way. This may ba managed, if, instead of closing and interrupting the current of the primary coil, an additional closing wire offering small resistance is provided, and the interruption is effected in this. If this additional apparatus is pi-esent, only a very small part of the current passes through the primary coil. The strength of this part is indicated by J, J,. When the closing in the additional ap- paratus is interruj)ted, the primary current slowly increases in strength from , and can at its commence- ment and cessation (closing and opening) excite the nerve h, and cause pulsation in the muscle of the nerve. This is spoken of as secondary pulsation from the nerve. By rapidly repeated closings and openings of the circuit, tetanus may be elicited. But this secondary pulsation is caused only by electrotonus and not by negative variation, so that it can be more easily brought about by constant currents than by inductive currents. It is thus distinguished from the secon- dary pulsation effected by muscle, which was described on p. 209. The negative variation of the nerve-current is too weak to cause any noticeable efiect in a second nerve. NOTES AXD ADDITIONS. 315 A special form of secondaiy pulsation effected through the nerve has been described by du Bois-Eeymond as paradoxical pulsation. If a constant cuiTent is passed through the bi-anch of the sciatic nerve to which allusion is made in Note 4, which passes to the flexor muscle of the lower leg, then the calf -muscle may also pulsate when the current is closed and Fig. 75. Secondary pulsation effected by nerve. opened. This is an apparent exception to the law of the isolated transmission of the excitement {cf. p. 117); but actually the excitement has not passed from the irritated fibres to the adjacent fibres, but the electrotonic current of the one fibre has flowed through the neighbouring fibies aud has independently irritated them. 14. Pai?electronomy (p. 237). The real causes of parelectronomy and the conditions under which it is more or less strongly developed, are as yet 316 PHYSIOLOGY OF MUSCLES AND ]S"ERVES. far from being understood. But at any rate it is impossible to conceive the matter, as though tbe currentless condition of the muscles — that is to say, the same tension on the longi- tudinal and transverse sections — were normal, and as if every negativeness on the transverse section were the result of injury. For all possible degrees of j)arelectronomy are to be found — even the reversed order, in which the cross-section is more positive than the longitudinal section — in uninjured muscles ; while in other cases the ordinary muscle-current is found powerfully developed in quite uninjured muscles. Moreover, as we have stated in the text, the question whether differences of electric tension occur in uninjured muscle has no bearing on the question whether electromotive forces are present within the muscle. We declare ourselves in favour of this hypothesis, because it most simply and easily explains all the phenomena. We also apply it to structures on the outer suiface of which it can be proved with certainty that no differences of tension are present, as in the electric plates of fishes. Por this assumption we have the same grounds on which physicists rely in claiming the existence of molecular magnets in every, even quite unmagnetic piece of iron. l^Hiatever, therefore, may be the true explanation of parelectronomy, it cannot essentially affect our well-founded conception of the electric forces of muscles. If, however, du Bois-Ile}Tnond's supposition is confirmed, that the pulsa- tions which occur during life leave behind them an after- effect on the muscle-ends, which makes the latter less nega- tive, some approacli woukl be made to an explanation of the phenomenon. 15. Discharge Hypothesis akd Isolated Teansmissiox IN THE Nerve-Fibre (p. 249). The explanation of the fact that the processes of ex- citement remain isolated in a nerve-fibre without passing into adjacent nerve-fibres, ajjpears the more inexplicable, if NOTES AND ADDITIONS. 317 we regard these processes as electric, in that tlie separate fibres are not electrically isolated from each other. But the explanation which we gave of the isolated excitement of but one muscle-fibre by a variation of the electric current in the appropriate nei-ve, also explains isolated transmission in the nerve-fibres. For if the electrically active parts are very small, comparatively powerful electric action can take place in them, and yet the current may be quite unobserv- able at a little distance. This is a consequence of the law of the distribution of currents in irregular conductors, explained in chapter x. § 2. We must, therefore, assume that the electrically active particles situated in the axis of a nerve-fibre aie small in comparison with the diameter of the fibre, aud that therefore their effect at the outer surface of the fibre is already so weak that it cannot act and cause irritation in an adjacent fibre. In Note 13 we have seen that no action takes place by negative variation from one fibre on an adjacent fibre. Our multipliers are much more sensitive than nerve-fibres, so that the separate negative variations during the tetanisation of the nerve can combine their action on the multiplier ; but this is impossible in the case of the excitement of nerve-fibres. INDEX. ABS A BSOLUTE force of muscles, ■^ 67, G8 Acid, formation of, in muscle, 73,87 Activity of muscle, 37, 202, 235 ; of nerve, 107, 216 Adamkicxewicz, 76 Adequate irritants, 285 Aeby, 100 Albuminous bodies, 73, SO Ammonia, 257 Amoehce, 6 Amoeboid movements, 7 Anelectrotonus, 129, 111 Animal, 5 Anode, 128, 220 Arches, diverting homogeneous, 177, 181 Akistotle, 155, 285 Ascending currents, 131: Attachment of muscles, 17 Automatic movement, 271 Avalanches, 250 Avalanche-like increase in the excitement of nerves, 122, 300 Axis-band, 104 Axis-cylinder, 104 IMCON as food, 85 ■^ Ball-sockets, l'.», 93 Beclard, 73 Beknard, 253 CHE Beexstein, 100, 219 Bert, 312 Blood, 78, 273 Blood-corpuscles, 7 Blood-vessels, 96, 272 DU Bois Eeymond, 25, 30, 35, 36, 53, 59, 73, 87, 111, 150, 156, 165, 181, 183, 186, 205, 208, 217, 230. 248, 278, 313, 315, 316 Bones, 17, IS, 93 Branched muscle fibres, 101 Branching of electric currents, 132, 150 Brownian movements, 3 Brijcke, 89 Burden, 23, 39, 64 BuEDOX- Sanderson, 223 rULF-MUSCLE. Sec Gastro- ^ cnemius Carbonic acid, formation of, in muscle, 42, 73, 81 Carrying-height (Traghche) 41 Cells, 9. Si^e also Nerve-cells I Central-organ of the nervous j system, ~i03, 117, 265 I Centrifugal and centripetal { nerves, 266 I Cerebrum, 277 ■ Chamois-hunters, 85 j Chemical composition of mus- ! clcs, 73 320 INDEX. Chemical irritants, 30, 109, 257 Chemical processes in miiscle, 42, 73 Ciliary cells, 10 Ciliary-movements, 10 Circuit, electric, 159, 165 Claudius Claudtanus, 155 Closing of a current, 32, 132, 30i Closing inductive current, 151, 299 Combination of tensions, 228 ; of irritants, 299 Compensation, 183 Compensator, round, 186 . Conception, 279, 286 Conine, 253 Conscious sensation, 277 Conservation of energv, 77 Constant currents, 34," 109, 126, 131 Correlative action, 270 Correlative sensation, 276 Creatin, 74 Cross-section of the muscle, 66, 190, 198, 203, 236, 256 ; of the nerves, 120, 216, 256 Curare, 253 Current-curves, 1 78 Current-planes, 179 Curve of excitability, 121, 300 TjARWIN, 224 -^ Death of the muscle, 86, 207 ; of the nerve, 120, 124 Death-stiffness-, 87 Degeneration of a cut nerve, 312 Descending currents, 134 Bio nee a vivscijnila, 149, 224 Discharge-hypothesis, 248, 316 Discs of muscle-fibres, 14 Disdiaclasts, 15, 102 Dislocation of the neck, 273 Diverting arches, 177 Diverting cylinders, 181 Diverting vessels, 166 Division of electric currents, 132, 170 DONDEES, 289 Dorsal marrow, 106, 277 FIB Double refraction, 15 Duplex transmission, 217, 312 Dynamite, 251 Dj'namometer, 69 ■pLASTICITT, 21 ; alteration of, ^ on contraction, 44, 70 ; co-ef- ficient of, 23 ; law of, 22 Electric current, 159 Electric eel, 156 Electric fishes, 154, 222, 227, 241 Electric irritation, 32, 109, 149 151 Electric organs, 158, 222 Electric plates, 158, 222, 227, 241 Electric ray, 156 Electric wheel, 33 Electrodes, 128 ; unpolarisable, 181 Electromotive force, 168, 232 ; of the muscles and nerves, 153, et seq. Electromotive surface, 179, 227 Electrotonus, 127, 139, 220, 238, 309, 314 Element. See Muscle-element and Nerve-element Elementary organisms, 8 Energy, U, 50, 64, 72, 77 ; spe- cific, 286 Engelmakn, 100 Equipoise, unstable, 250 Equator, electromotive, 190 Ermann, 45 ExcitabiUtA-, 119, 122, 126, 299, 300 Excitement, 126, 141, 150, 151, 313 Exhaustion, 79, 121 Extension, 21, 92, 295 ; gradual, 24 Extrapolar regions, 220 "PARADAT, 156 -*- Feet of the diverting arch, 176 Fibres. See Muscle-fibres and Nerve-fibres Fibre-cells, 96 INDEX. 321 Fibrillie, U FiCK, 41, 299, 309 Fish, electric, 154, 222, 227, 241 Flat-bones, 18 Flesh, 2, 11, 86 Force, electromotive, 168, 232 Force, muscular, 50, 67 Forms of muscles, 91 Form, changes of, in muscle during contraction, 45 Freeing of forces, 249 Function, 293 AALVAXOMETER, 160 ^ Ganglion-cells. See Nerve- cells Ganglion-balls. See Nerve-cells Gaatroenemius, 17, 67, 109, 199, 200, 203, 209, 302 Gauss, 58 Gerlach, 246, 247, 259 Gizzard, 96 Glands, 212, 227, 262 Glycerine, 257 Glycogen, 73, 80, 87 Goethe, 285 Graphical representation, 293 s'Geavesande, 23 Grey nerve-fibres, 104 Gunpowder, 250 Gijmnotus, 156 TJALLER, 252 -'-'■ Hallucination, 279 Harlesr, 253 Head of muscle, 1 3 Heart, the, 101, 210 Heidenhatn, 76, 146 Height of elevation, 37, 2;t7 Helmholtz, 50, 52, 59, 73, 75, 115, 228, 287, 290, 306 Hermann, 70, 100 Hinge-socket, 19, 93 HiRSCH, 288 Homogeneity of all nerve-fibres, 263 Hook, 23 Humboldt, 156 Hypotheses, 229, 234 15 mag TNCREASE in thickness of -^ muscle on contraction, 44 Induction, magnetic, 243 Inductive currents, 31, 110, 139, 304, 308 Induction coil, 31, 35, 119, 306 Inertia of consciousness, 290 Inosit, 74, 87 Internal work during tetanus, 41, 76,77 Intestine, 96, 272 ; of the tench, 101 Intrapolar regions, 129, 221 Involuntary movements, 271 Irregular movements, 272 Irritants. 30, 109 IiTitability, 30, 108 ; independent, 255 Isolated transmission in the nerve-fibre, 117, 315 Isoelectric curves. See Tension- lines r'ATELECTROTONUS,129,141 •^ Kathode, 128, 220 Kernel (nucleus) 5, 7, 11, 16, 96, 105 Key, tetanising, 36 Kleistian jar, 30 Kolliker, 253 Kronecker, 299 Kt'HNE, 89, 256, 257 T ABOUR accumulator, 41 ^ Lactic acids, 73, 80, 87, 258 Latent irritation, 56, 64 Law of eccentric sensation, 280 Law of pulsations, 13.5, 142 Leverage of bones, 93 Leyden jar, 30 Life centres, 272 Light, 15, 284 Long bones, IS M AGNET, compared to muscle and nerve, 147, 230, 260 322 INDEX. Malaptcrurus, 156 Mateucci, 229 Mechamcal irritants, 30, 109,146 Medullary sheath, 104, 245, 254, «63 Mimosa jJudica, 2, 224 Mirror, reading of small angles by means of, 57, 162 Moditication of excitability, 131, 143 Molecular hypothesis, 238 Molecular movement, 3 MoUusca, 102 3Io)-myrus, 159. Motor nerves, 261 Movement, 1 ; in jilants, 2, 8, 224 ; of the smallest organisms, 4 ; molecular, 3 ; protoplas- mic, 6 ; amoeboid, 6 ; ciliary, 9 ; muscular, 9 et seq. ; peri- staltic, 98, 272 ; voluntary and involuntary, 98, 275 ; au- tomatic, 273 ; rhythmic, 272 ; tonic, 272 MiJLLER, 285 Multiplier, 161 MuNK, 116, 224, 300 Muscle, 2, 11, 12 et seq., 189 et seq., 226 et seq. Muscle-current, 191, 202, 226 Muscle-element, 232, 239 Muscle-fibre, striated, 14, 45, 96, 245 ; smooth, 96, 101 ; zigzag arrangement of, 14 Muscle-fibre pouch. See Sarco- lemma Muscle-fluid, 88 Muscle-prism, 189, 230, 234 Muscle-rhombus, 193, 195, 230 Muscle-note, 43, 211 Muscle-telegraph, 30 Myograph, 26, 37, 52, 100, 111 Myosm, 74, 90 l^ASSE, 73 -'-' Negative variation, 203, 210, 214, 216, 226, 235, 313 Nerve-cells, 103, 266, 269 POL Nerve-centres. See Central Or- gans. Nerve-current, 215, 226, 236 Nerve-element, 237 et seq. Nerve-fibres, 103 et seq. ; termi- nation of, in muscles, 245 Nerve-net, 246 Nerve -processes, 107, 265 Nerve-sheath, 104, 111 Nerve, terminal plates of, 245 Nervous system, 103 Nettle, stinging, movements in hairs of, 8 Newilemma. See Nerve-sheath Neutral point, 129 Nicotin, 253 Nitroglycerine, 250, 251 Nucleolus, 105 Nutriment of labourers, 82 Nut-socket, 19, 93 APENING of a current, 32, 131, ^ 308 Opening induction-current, 150, 304 Opening-tetanus, 132, 143 Oppian, 155 Organs. See Central Organs and Electric Organs Over-bm-den, 65 Oxidation, process of, in muscle, 42 PARADOXICAL pulsation, 314 ^ Parelectronomy,208,236,315 Penniform muscles, 91, 199 Peripheric nerves, 103, 107 Peristaltic movement, 98, 272 Pflugee, 122, 140 Physiological time, 288 Plants, movements of, 2, 9, 224 ; electric action of, 153, 223 et seq. Plates, electric, 158, 222, 227, 241 Pliny, 155 poggendoef, 183 Polarised light, 15 I]S'DEX. 323 PEE Prevost and Dctmas, 45 Prism. See Musde Prism Propagation, of the pulsation ■within the mascle-fibre, 99; of the irritation within the nerve-fibre, 110, 114, 287; of the negative variation in the nerve-tibre, 129 Protoplasm, 5 Protoplasmic movement, 6 Protoplasmic processes, 106 Pulsation, 31. 56, 210 ; secondary, 210, 314; law of, 135, 142,299 pADIATIOX of sensations, 276 -'-'' Eate of excitement in the nerve-fibre, 98 ; of transmission within the nerve-fibre, 110, 1 1 4, 129, 287 Reaction in muscles, 87 Eeceptive apparatus of sensory- nerves, 283 Reflection, 290 - Reflex actions, 274, 290, 301 Respiratory movements, 272 Respiratory centre, 272 Rest of muscles, 37 Retardation (^Hemvuinri'^,, 80 Retardatory nerves, 263 Rheochord, 133, 149, 184 Rhombus. See Muscle-rhombus Rhythmic movements, 272, 281 Ritter's tetanus, 132, 143 OARCOLEMJ/A, 16, 101, 233 ^ Schwann, 70 Secretory nerves, 213, 262 Secondary pulsation, 210, 314 Secondary tetanus, 211 Semi-penniform muscles, 91 Sensation, 1, 262 Sensitive machines, 251 Sensitive plant, 2 Shaft of a bone, 19 Short bones, 1 8 Shortening of muscles 12. 28 Skeleton, muscles of, 13 Skin-currents, 207, 213 Sliding inductive apparatus, 35, 119' Smooth muscle-fibres, 12, 96, 206 Sockets, 19, 93 Source of muscle-force, 42 Specific energies, 286 Specific warmth, 76 Steam engine, comparison of, with muscle, 82 Stinging-nettles, movements in hairs of, 8 Steauss, 290 Striated musc^.e, 11 et seq. Sugar, 73, 80, 85 Surface, electromotive, 179, 227 TAIL of muscle, 13 Tangent galvanometer, 162, 310 Temperature, influence of, on muscles and nerves, 86, 124 Tench, 101 Tension, electric, 168, 171, 229, 311 Tension-curves, 179, 190 Tension, ditferences of, 182, 311 Tension-lines, 179, 190 Tension-surfaces, 179 Terminal apparatus of nerves, 262, 267 Tetanus, 34, 37, 41, 109, 300; secondarj-, 211, 314 Tliermic irritants, 109 Thermo-electricity, 74 TiEGEL, 300, 309 Time, measurement of, 51, 61, 98, 111, 115, 131,288 Tonic contraction, 272 Tm'pedo, 155, 158 Transmission, in the nerve-fibre, 110, 141, 287 ; isolated, 117, 316 : duplex, 217, 312 Transverse currents through the nerves, 309 Trunk of a muscle, 13 324 INDEX. UNI TJNIPOLAR-irritation, 309 Unpolarisable electrodes, 181 Urinary duct, 100 Urea, 80, 84 yASO-AIOTOR nerves, 261 ' Yolume of muscle, 45, 66 WAGNER'S hammer, 34, 306 '' Warmth equivalent, 76, 81 WOR Warmth, generation of, in muscle, 42, 73, 74 ; in nerve, 123 Weber, 45, 263 Weiss, 73 Whip-cell movement, 11 AVill, 270, 290 ; deflection of magnetic needle by, 205 Woodcutters, TjTolese, 85 Work accomplished by the muscle, 37, 38, 72, 76, 296 INTERNATIONAL SCIENTIFIC SERIES. NOW READY. In 12mo, and bound in cloth. 1. FORMS OF WATER. P,v Prof. John Ttxdall. $1..50. 2. PHYSICS AND POLITICS; or. The Applicition of the Principles of "Natural Selection" and '-Inheritance" to Political Societj'. Bv W. Bageuot. $1.50. 3. FOODS. By Edwikd Smith, M. D., LL. B., F. R. S. $1.7.5. 4. MIND AND BODY. Bv Alexander Bain, LL.D. $1.50. 5. THE STUDT OF SOCIOLOGY. Bv Herbert Spencer. $1.,?0. 6. THE NEW CHEMISTRY. By Prof. Josiaii P. Cooke, Jr., of Harvard Uni- versitv. $2.(i0. T. THE CONSERVATION OF ENERGY. 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