Page:Encyclopædia Britannica, Ninth Edition, v. 19.djvu/36

Rh 26 PHYSIOLOGY [NBBVOUS to the muscle A, the muscle stimulated contracts, aud draws by means of the stylet, on the smoked surface of the glass, the curve seen in the lower part of it at A. This leaves the horizontal line (which would be drawn by I lie stylet were the muscle at rest) at A. Arrangements are then made for Knottier experiment, in which the nerve will be stimulated at a distance from the muscle, at the point B in the upper part of the diagram. This is done by again placing the smoked-glass plate in proper position, closing the primary circuit by the brass arm at the binding screws, as already described, and re versing the commutator so as to send the shock along the wires to B. Tin- muscle again contracts when the primary circuit is opened, and this time it describes on the smoked surface the curve B, seen to the left of the curve A. It will be perceived that this curve leaves the horizontal line at B, that is, a little later than when the nerve was stimulated close to the muscle. It follows, therefore, that the distance on the horizontal line from A to B represents the time occupied by the transmission of the nervous impulse from B to A of the nerve. With suitable arrangements, the rate of movement of the glass plate can be measured by bringing into contact with it a marker on one of the prongs of a vibrating tuning-fork. The waves thus recorded enable the experimenter to measure with accuracy the rate of movement of the glass plate, and conse quently the minute interval of time between A and B. In the diagram it will 1* observed that there are 2J waves between A and B ; each represents ,J n th of a second ; therefore the 5 ith of a second is the time represented by the distance A, B ; or, in other words, the ^ O th of a second was occupied by the nerve-current in passing along the portion of nerve from B to A. (2.) Measurement of I elocity in Sensory Xerves. Suppose a sensory nerve to be excited in the hand ; the theory of nervous conduction is that a change is pro pagated along the nerve to the brain, and that in the brain the molecular changes occur which result in a sensation. The individual having the sensa tion may feel it and make no sign by which any one else might l&amp;gt;e made aware that he has felt it, or the subject of the sensation might, by a muscular move ment, such as the motion of an arm, let any one else see that he has felt the sensation. We have no means of knowing whether or not an individual has felt a sensation except by the individual making some kind of gesture or muscular movement. Xow it is clear that, if we regard the brain as the seat of the changes resulting in sensation, the nearer any stimulated portion of skin is to the brain the sooner will the brain feel and respond to the stimulus. Thus, if the skin on the big toe of the right foot be stimulated, the effect of the stimulus passes to the brain and there calls forth a sensation, but if the stimulus be applied to the skin at the top of the thigh it is evident the effect has to pass along a shorter length of nerve and that the sensation in the brain will be aroused sooner. If we suppose that in each case the individual who is the subject of the experiment indicates the moment he feels the sensation, and that the instant the stimulus is applied successively to the skin on the, toe and on the thigh is also accurately recorded, it is clear that he will signal the sensation of stimulation of the toe a little later than when he signals stimu lation of the skin on the thigh, and that the difference will indicate the time required by the change in the nerve to pass along the length of nerve from the toe to the thigh. In the observation it is assumed that the time required for the changes in the brain resulting in sensation and volition, for the trans mission along the motor nerve, and for the muscular contraction required to signal is the same in each experiment. Thus, supposing the total time between the moment of stimulating to the moment when the signal that the sensation has been felt and responded to is x, it is clear that this time is composed of a, the time required for the passage of the nerve-current in the first experiment from the toe to the brain, of b, the time required for the changes in the brain involved in sensation and volition, and of c, the time required for the trans mission along the motor nerves and for the muscular contraction to move the signal, that is, x=a+b+c. But, if the time between the moment of stimu lating the thigh to the moment of signalling be shorter, and supposing that b and c are constant, then varies according to the length of the nerve. Suppose the difference of time between the registration of stimulating at the toe and at the thigh to be y, then in the second experiment x=a-y+b+c, that is, ?/ = the time occupied by the passage of the nerve-current from the toe to the thigh. This method has also been used to measure the time required for signalling a nervous impression in various circumstances, or what is usually called the &quot;reaction period.&quot; The most convenient apparatus for the purpose is a chronograph made by Konig of Paris, the instrument being fully described in M Kendrick s Outlines of Physiology, pp. 538-542. The general result of measurements made by these methods is that the nerve-current travels slowly compared with the velocity of electricity or of light. In the motor nerves of the frog the velocity is about 87 feet (26 to 27 metres) per second, and in man aud warm-blooded animals somewhat faster, 115 to 130 feet (35 to 40 metres) per second. The results as to velocity in sensory nerves vary from 50 to 100 metres per second. Cold retards, heat accel erates, the velocity. As already stated, the velocity is also retarded in a nerve in an auelectrotonic, and accelerated in a katelectrotonic state. The remarkable point is that the transmission of the nerve- current is slow, and that events appearing to our consciousness instantaneous require a considerable time for their occurrence. It may be laid down as a general truth that all kinds of nervous actions, even those considered as purely psychical, require time. Heat- Production of ffeat by Nerves. It is extremely doubtful whether produc- the production of heat by a nerve in action has been detected, tion by although theoretically one would expect heat to be so produced, nerves. Schiff observed an increase of temperature on tetanization in the nerves of warm-blooded animals that had been artificially cooled ; on the other hand, Helmholtz and Heideuhain s experiments yielded only negative results. Electrical Pfanomena of Nerve. When a piece of nerve is pro perly brought into contact with the terminals of a sensitive galvano meter, a current flows through the galvanometer from the surface of the nerve to its transverse section (see fig. 4). If metallic conductors, composed (say) of zinc, from the galvanometer were brought into connexion with a piece of nerve removed from an animal newly killed, little or no current would be obtained, and even if there were a current it might be due to contact of the metallic conductors with the living tissue exciting electrolytic decomposition. Hence it is necessary to have a fluid in terposed between the metal and the animal tissue, say, for example, the zinc wire or plate forming the terminals of the galvanometer is immersed in a saturated solution of sulphate of zinc. But as sulphate of zinc solution would have the effect of irritating the living muscle it is necessary to have an inactive substance between the tissue and the sulphate of zinc solution. All these conditions are fulfilled by the non - polarizably electrodes of L)u Bois- Roymoml, of which there are various forms. Two zinc troughs, mounted on insulating plates of vulcanite, have the inner surfaces carefully amalgamated. These are filled with a saturated solution of sulphate of zinc, and in each trough is placed a small cushion of clean blotting or filter paper, which quickly becomes permeated with the solution. Finally, a small plate of sculptor s clay, or kaolin, moistened with a half per cent, solution of common salt, or, Fio. 4. Diagram of apparatus of Du Bois-Reymond for experiments on elec trical condition of muscle and nerve, a, zinc troughs, mounted on pieces of vulcanite b ; c, paper pads ; d, e, small pieces of moist clay ; /, /, binding screws for attaching terminals of galvanometer g. A small piece of paper connects d and e, and thus completes the galvanometer circuit. (Wundt.) still better, with saliva, is laid on each paper pad. These clay pads are for guarding the tissue from the irritant action of the sulphate of zinc. Wires are carried from the troughs to the galvanometer, and a key is interposed in the circuit. The object of these careful arrangements is to secure that no current is formed by the apparatus itself. If now a small piece of nerve be so placed on the clay pads that the transverse section touches one pad and the longitudinal surface the other, and the key is opened, a current passes through the galvano meter, as indicated by the swing of the needle. Suppose that the needle is allowed to come to rest, the amount of deflexion of course indicating the strength of the current, and the nerve is now irritated so as to call forth its physiological activity, then the needle swings back towards zero. This back ward swing is called the negative variation of the nerve-current. The electro motive force of the current obtainable from a frog s sciatic nerve is about 022 of a volt, and somewhat more from the sciatic nerve of a rabbit. This is some what less than the electromotive force of a frog s muscle, which varies from 035 to 075 of a volt. According to the views of Hermann, the negative variation- current is a true current indicating, and indeed preceding, the physiological activity of the nerve. He denies that the currents pre-exist in nerve or muscle, and states that the first current observed when the nerve is laid on the pads is simply due to the lower potential of the transverse section, caused by the rapid death of the nerve-substance. The nerve-current excited by a series of irrita tions, say feeble induction-currents, maybe regarded as composed of a wave- like series of momentary currents, each of which is preceded by a negative variation-current. Thus the electrical phenomena of nerve are similar in kind to those manifested by living muscle. Nutrition of Nerves. Probably nerves are nourished by the Nutri- plasma reaching the axis-cylinder at the nodes of Ranvier ; but it tion of would appear from the researches of &quot;Waller that the nutrition of nerves. the nerve-fibre is influenced by the nerve-cell with which it is con nected. The so-called &quot;law of Waller&quot; is well illustrated in the case of division of the roots of the spinal nerves. Each of these nerves has two roots, a posterior, sensory, on which there is a ganglion ; and an anterior, motor. If the anterior root be divided, in the course of a few days the end of the nerve cut off from the spinal cord is found to be undergoing degeneration, whilst the end attached to the cord is still normal. Again, if the posterior root be divided between the ganglion and the cord, the end remaining in connexion with the ganglion remains unaffected, whilst the other end undergoes degeneration. This degeneration, in the case of a motor nerve, affects the nerve to its very terminations. The axis- cylinder disintegrates into drops of fatty matter, and the medullated structure entirely disappears. It is well known that when a nerve is cut the ends may reunite so completely as to ensure a return of the normal function in from two to five weeks. According to Ranvier, the axis-cylinders in connexion with the central portion play an important part in this regeneration. They become larger, striated, and by and by form new axis-cylinders, which pass into the cicatricial tissue and come into contact with the other end of the divided nerve. This is a remarkable confirmation of the view of Waller that the nutritional activity of a nerve -fibre is in the direction of its physiological activity. Nature of Nerve-currents. The intrinsic nature of the change Nerve- in a nerve-fibre effected by a stimulus is quite unknown ; but it is current important to appreciate clearly the view that a nerve is both a receiver and a conductor of impressions. It can be stimulated in any part of its course, and from the stimulated point some kind of change is propagated along the nerve. This change is analogous to the passage of electricity along a conductor, or to the rapid !&amp;gt;assage onwards of a series of chemical decompositions, as when a ong thin band of gun-cotton properly prepared is seen to slowly burn from end to end, or to the quick transmission of isomeric changes ; but the analogy is not complete in any case. Whatever