24 PHYSIOLOGY [NERVOUS Remak," are found chiefly in branches of the sympathetic nerve. They have no double contour, and they often give off branches which anastomose, a condition never found in the course of niedul- lated nerves. If the axis-cylinder of a nerve-fibre be traced in wards to a nerve-centre it is found to end in the pole or process of a nerve-cell, whilst in the periphery it ends in a terminal organ, in muscle, blood-vessel, gland, skin, or electric organ. It may there fore be presumed that the axis-rod is the conducting and so far the essential constituent of the nerve-fibre. Chemi- Chemical Constitution. Little is known of the chemical con- cal con- stitution of the nerve-fibre. The medullary matter contains fatty stitution. substances such as lecithin (C^H^NPO,,). The axis-rod appears to contain albuminous substances. Cholesterin (C.jgH^O + HgO) and creatin (C 4 H 9 X.,0 2 ) have been obtained from nerves along with a small amount of gelatin and a horny matter, neurokeratin (Her mann). Nerves contain about 30 per cent, of water. Their reaction to test-paper is neutral. Nothing is known regarding the chemical changes occurring in a nerve-fibre during a state of activity, nor of the chemical products formed at that time. It has been stated that a nerve becomes acid after death (Funke). As to the micro-chemistry of the nervous elements, it may be stated that the axis-cylinder contains an albuminous substance different from myosin ; it gives a red colour on boiling with Milton s reagent (an acid nitrate of mercury) ; it is dissolved in weak solutions of ammonia, caustic potash, and chloride of sodium; it is hardened by solutions of chromic acid, bichromate of potash, and corrosive sublimate ; it reduces the chloride of gold ; and it shows transverse markings on the action of nitrate of silver. The white substance of Schwann is blackened by perosmic acid and is soluble in alcohol, ether, and oil of turpentine, thns showing its fatty character. T^he primitive sheath is a substance of a collagenous nature, that is, it yields gelatin on boiling. According to some, digestion in gastric or pancreatic juice leaves an insoluble matter like that found after the digestion of corneal or epidermic tissues, and hence called "neurokeratin." Excita- Excitability. The special property of a nerve -fibre is termed bility. " excitability." Every kind of living protoplasmic matter is irrit able, that is, it responds in some way to a stimulus. Thus, if a morsel of protoplasm, such as an amoeba or a cartilage-cell or a white blood-corpuscle, be stimulated mechanically or by shocks of electricity, it will contract or change its form. Again, if living muscular fibre be thus stimulated it also will contract. This pro perty of responding to a stimulus is termed "irritability" ; and in the structures mentioned the property is manifested by movement as the obvious phenomenon, but it is well known that more obscure phenomena follow the application of the stimulus. In the case of living nerve there is irritability also, that is, the nerve responds to a stimulus ; but in a portion of isolated nerve no change is visible. If, however, the nerve be connected at one end with its appropriate central or terminal organ we may have evidence of something having been transmitted along the nerve. Thus there may be sensation or movement, or both. Nerves are more irritable than contractile matter, and the term "excitability" is applied to the special irritability of nerve. The same strength of stimulus will act more powerfully on a nerve than on a muscle (Rosenthal). The use of the term "property" in physiology does not imply the idea of any kind of inherent force or entity, but simply that, in the case of muscle and nerve, irritation is followed by certain phenomena seen only in living matter. (1. ) The excitability of nerves is affected by certain conditions. Injury to the nerve, the application of caustics, and drying quickly destroy it. When a nerve is divided the excitability is increased for a short time, then rapidly diminishes, and finally disappears near the point of section. The end of the nerve still connected with central organs undergoes these changes in excitability more quickly than the portion cut off. As shown by Augustus Waller, when a nerve is separated from its central organ, such as the grey matter of the spinal cord for the motor roots, and the ganglia on the posterior roots for the sensory roots, the end of the nerve sepa rated from the centre undergoes fatty degeneration. If, however, the cut ends of the nerve be brought into accurate contact, union soon takes place. Surgeons have frequently observed a return of sensibility to a part within a few days after the sensory nerve had been divided and the cut ends again brought into contact. Con tinued or excessive activity of a nerve soon lowers and may abolish excitability, thus producing exhaustion. On the other hand, a lengthened period of absolute repose lowers excitability, and if the nerve be inactive beyond a certain time it wastes, becomes thinner, and fatty degeneration occurs in its substance. Heat increases, whilst cold diminishes, excitability. In the case of frogs nerves temperatures above 45 C. destroy excitability the more rapidly as they approach 70, at which point it is almost instantaneously destroyed. Below 45 a rise of temperature first increases and then diminishes excitability, and it has been observed that whilst increasing its intensity it diminishes its duration (Afanasieff, Hermann). Finally, a diminished supply of blood quickly causes a fall of excitability. Stimuli (2.) Nerves may be excited by various kinds of stimuli: (a) of nerves, mechanically, as by intermittent pressure, beating, section, prick ing, &c. ; (b) thermally, by variation of temperature ; (c) chemically, by the application of such substances as acids, alkalis, or metallic salts ; (d) electrically, by continuous or induced currents ; and (e) normally, by changes in the central or terminal organs. Mechanical irritation is applied during life when the trunk of a nerve is pressed upon. Radiant heat acts on the nerves of the skin, or heat may be applied by conduction from a hot body in contact with the surface. Little is known as to the specific effects of heat on the nerves of the human being. In the frog it has been found that a temperature of from 34 to 45 C. stimulates the motor nerves ; about 40 C. sudden alterations of temperature may cause twitching of the connected muscles (Hermann). Many chemical substances in sufficient concentration will quickly destroy a nerve; but if they are in weak solutions the result may be stimu lation. Thus, concentrated solutions of the mineral acids, alkalis, alkaline salts, concentrated lactic acid, and concentrated glycerin may act as strong stimulants (Kiihne). (3.) The influence of electrical stimulation of nerves demands more elaborate description. The effects, as already indicated, can be observed only when the nerve is connected with a muscle or with a central organ. In the first case electrical stimulation is followed by contraction of the muscle, in the second by a sensation if the central organ is the brain. Consequently we have to consider the phenomena following electrical stimulation (a) of a motor nerve and (b) of a sensory nerve. (.) Electrical Stimulation of a Motor Nerve. A perfectly con- Elec- stant current of electricity, of moderate quantity and intensity, trical flowing through a portion of nerve produces no evident effect on stimula the muscle, but any variation in the intensity or density of the tion of current causes irritation, and the muscle gives a twitch. The motor effect is most apparent when the current is allowed to flow into nerve, the nerve and when it is suddenly cut ofT, or, in other words, at the moment of opening and of closing the circuit. The rapidity with which the variation in the density of the current is effected also has an important influence. Thus the shocks of factional electricity stimulate strongly, because, although the amount of electricity is small, the currents are extremely rapid in appearing and disappearing. In like manner the quick shocks from induction- coils produced by rapidly opening and closing the primary circuit are strongly stimulating. Again, a very powerful current may pass through a nerve without exciting it, if it pass gradually. Occasionally a very weak current sent through a portion of nerve will cause a contraction, whilst a very strong current may fail to do so. In fact, the phenomenon of contraction of a muscle is influenced (a) by the direction and Q3) by the strength of the current sent through the nerve. When the current is transmitted from the muscle in the direction of the spinal cord it is called an "upward" or "centripetal" current, when from the cord in the direction of the muscle it is called a " downward " or " centrifugal " current. Its strength is graduated by employing small Grove s cells, one cell giving a weak current, two or three giving a medium current, and four to six or seven a strong current. To graduate its amount more precisely resistance-coils may be introduced into the circuit, or we may employ a rheochord, by which a portion of the current is shunted back to the battery, whilst the remainder is allowed to pass to the nerve. In ths circuit a key or interrupter is interposed, and so arranged that when the key is opened the current is broken or interrupted, and when the key is closed the circuit is completed and the current passes to the nerve. With these arrangements, and employing the sciatic nerve of a frog attached to the limb, the following results are readily obtained. Current Strength. Key. Weak. Weak. Medium. Medium. Strong. Strong. Close. Open. Close. Open. Close. Open. Upward Current. Contraction. Rest. Strong contraction. Strong contraction. Rest. Very strong contraction. Dmrnirard Current. Contraction. Rest. (Strong contraction. Strong contraction. Rest. Contraction. That is to say, on beginning with a very feeble current neither opening nor closing causes a contraction, but on strengthening it up to a certain point contraction appears first on closing, whilst opening produces no effect. By increasing the strength of the current a contraction is obtained both on opening and on closing the key, and by and by, when a certain strength of current is reached, the closing contraction becomes weaker and finally dis appears, leaving only a contraction on opening the key. Thus the effects of a strong current are usually the reverse of those caused by a weak current. These facts, usually included under the term "Pfliiger s Law of Contraction," have been specially investigated by Pfliiger, and the following is the explanation offered by him and generally accepted by physiologists. Suppose that the sciatic nerve of a frog connected with the isolated limb is stretched over two wires passing from the positive and negative poles of a com bination of Grove s elements, with the distance of an inch and a half between the wires. If a key be interposed in the circuit a current will thus pass along one and a half inches of nerve when the key is closed, and be cut oft when the key is opened. By having also a commutator or reverser in the circuit we can send the current up or down the nerve at pleasure. Arrangements can also be made for irritating the nerve by another couple of wires coming from an induction-machine, either near the negative or near the positive pole of the current coining from the Grove s elements. It will then be found that near
the negative pole the excitability of the nerve is increased, whilst near the