364 THE NERVOUS SYSTEM. 



muscles of the leg, contraction takes place. The muscular irrita- 

 bility survives that of the nerves, and is therefore essentially dis- 

 tinct from it. 



The independence of muscular and nervous irritability is also indicated 

 by the effects following degeneration of divided nerve fibres (page 35.3). 

 When a motor nerve is divided, the separated portion after a few days 

 loses its irritability, so that no stimulus applied to it will excite con- 

 traction in the corresponding muscles. But if a galvanic current be 

 applied to the muscles themselves, they contract. Longet* has demon- 

 strated this fact upon the dog, from five days to twelve weeks after 

 section of the facial nerve ; and a similar result has been found by 

 Vulpianf in the rabbit thirty days after section of the same nerve. 

 The contraction in these cases cannot be attributed to the irritability 

 of small nerve branches included in the muscular tissue, since the 

 degeneration of a divided nerve takes place throughout its peripheral 

 portion ; and according to RanvierJ, from forty-eight hours to five days 

 after division of the sciatic nerve in the rabbit, the terminal nerve fibres 

 in the muscles of the leg are as fully degenerated as those in the 

 trunk of the nerve near its point of section. The irritability of the 

 muscles must therefore be regarded as a property belonging to their 

 own tissue, but capable of responding to a stimulus communicated by 

 the nerves. 



Identity of Action in Sensitive and Motor Fibres. 



The results of nervous action are different in the two kinds of 

 nerve fibres. The stimulation of sensitive fibres produces a sensation, 

 or sensitive impression in the nervous centre ; that of motor fibres 

 causes muscular contraction at the periphery. Moreover, if a sensitive 

 nerve be divided, stimulus applied to its central extremity still excites 

 a sensation, while the same stimulus, applied to its peripheral portion, 

 produces no apparent result. On the other hand, if a motor nerve 

 be divided, irritation of its. attached extremity, which is still in con- 

 nection with the nervous centre, has no effect ; but irritation of its 

 peripheral portion causes muscular contraction as before. In other 

 words, the nervous force, in a sensitive nerve, appears to move in a 

 centripetal direction, that is from without inward; and in a motor nerve, 

 in a centrifugal direction, or from within outward. The excitement of 

 a sensitive nerve, furthermore, never produces any other immediate 

 effect than a sensation; that of a motor nerve only gives rise to the 

 phenomena of movement. 



The above facts suggest the idea that the two kinds of nerve fibres 

 may be distinct in their properties and modes of action ; that the sensi- 

 tive fibres may be capable of acting only in a centripetal direction and 

 of exciting sensibility ; and that the motor fibres can only act from 



* Traite de Physiologic. Paris, 1850, tome ii., p. 51. 



f Le9ons sur la Physiologic due Systeme Nerveux. Paris, 1866, p. 245. 



J Le9ons sur PHistologie du Systeme Nerveux. Paris, 1878, tome ii., p. .349. 



