354 THE NERVOUS SYSTEM. 



or motor, extends throughout their entire length beyond the point of 

 division to their peripheral terminations. Vulpian* found that in 

 dogs, six weeks after division of the sciatic nerve, no unaltered nerve 

 fibres could be discovered in the muscles of the corresponding foot. 

 According to the same observer, the alteration is simultaneous, or 

 nearly so, in all parts of the nerve beyond its division; being no 

 further advanced near the point of section than toward the periphery, 

 If there be any difference in this respect, the degeneration appears to 

 be more rapid at the terminal extremity of the nerve ; since, in the 

 experiments of Ranvier, on the rabbit, forty-eight hours after division 

 of the sciatic nerve, its terminal fibres in the muscles of the leg con- 

 tained only separate masses of myeline in the form of oily drops. 



The degeneration of divided nerve fibres involves the loss of their 

 physiological properties. Immediately after the division of a motor 

 nerve, the resulting paralysis is due only to its local discontinuity at 

 the point of section, which arrests the passage of a nervous stimulus 

 coming from the brain ; and a galvanic current applied to the nerve 

 below its division will still produce contraction in the muscles to which 

 it is distributed. So long as this can be done, it shows that the nerve, 

 though separated from the central parts, still retains its irritability, 

 and is capable of responding to a stimulus by muscular action. But 

 after a time this property disappears. In the rabbit, the irritability 

 of a divided nerve is lost in forty-eight hours, in the pigeon at the end 

 of three days, and in the dog at the end of four days ; while in the 

 frog it persists more or less for thirty days. These variations corre- 

 spond with the rapidity of degeneration in the nerve fibres ; and by 

 comparative observations on different animals, Ranvier has shown that 

 in all cases the disappearance of irritability of the nerve corresponds 

 in time with the loss of continuity in the axis cylinder. This cor- 

 roborates a conclusion derived from other sources, namely, that the 

 axis cylinder is the essential element of the nerve fibre, through which 

 its physiological action is transmitted. 



A nerve, accordingly, some days after its division, has lost both its 

 physiological properties and its anatomical structure. It can no longer 

 convey sensitive impressions from the integument, and it is incapable 

 of exciting muscular contraction. But this loss of function is not 

 permanent. After a time the divided nerve may reunite, and its 

 power of communication may be restored. This is shown, not only 

 by the consolidation of its divided extremities and the reappearance 

 of its normal physical aspect, but also by the reestablishment of its 

 functions. The portions of integument which had lost their sensi- 

 bility again become sensitive to external impressions, and the power 

 of voluntary motion returns in the paralyzed muscles. This takes 

 place by a regeneration of nerve fibres in the affected nerve beyond 

 the point of division. All observers are now agreed that the nerve 



* Le9ons sur la Physiologic du Systeme Nerveux. Paris, 1866, p. 243. 



