412 GENERAL STRUCTURE AND FUNCTIONS 



portion of the nerve has been cut out, there may be a reproduction of 

 the lost parts, and the nerve may finally regain its natural continuity. 

 The fibres of new formation, thus produced, are at first of small diameter 

 and of grayish aspect. They gradually increase in size, become pro- 

 vided with a medullary layer, and at last present all the anatomical 

 characters of the healthy nerve fibre. Schiff, Yulpian, and Philippeaux 

 have found that it is possible for the continuity of a nerve to be re- 

 established, after the excision of portions of its trunk equal to five or 

 even six centimetres in length. According to Yulpian, in very young 

 animals, a loss of nerve substance from one to two centimetres in length 

 may be restored at the end of six weeks ; and the same observer has 

 seen, in young rats, a portion of the sciatic nerve, six millimetres long, 

 reproduced in the course of seventeen days. 



At the same time, the degenerated portion of the nerve, situated 

 beyond the point of its division, becomes restored. There is a reproduc- 

 tion of the medullary layer, which had become atrophied by the de- 

 generative process, and the entire nerve again exhibits its normal 

 anatomical character. The time required, for the complete regeneration 

 of the peripheral portion of a divided nerve, is in general from three to 

 twelve months, according to the age and species of the animal upon 

 which the experiment is performed. 



The complete regeneration of a divided or exsected nerve is indicated 

 by the restoration of its normal function. If it be a sensitive nerve, the 

 power of sensation, which was at first lost, returns in that portion of 

 the integument to which its fibres are distributed ; if it be a motor 

 nerve, the power of voluntary motion is regained in the corresponding 

 muscles. The observations of Yulpian have shown that, after the ex- 

 cision of the central extremity of the hypoglossal nerve in dogs, its 

 peripheral portion may become capable of exciting contraction in the 

 muscles of the tongue at the end of four months j 1 and according to 

 those of Schiff upon young dogs and cats, sensibility may reappear in 

 the tongue and lip in fourteen days after the excision of portions of the 

 lingual and infra-orbital nerves, from two to two and a half centimetres 

 in length. 



In the human subject, at least in adult life, the restoration of divided 

 nerves is much less rapid ; and, according to L'Etievant 2 and Weil- 

 Mitchell, 3 often either does not take place at all, when the injured nerves 

 are of considerable size, or does so very imperfectly. 



The smaller nervous branches supplying the skin are frequently 

 divided by accidental incisions, causing a local anaesthesia, or loss of 

 tactile sensibility in the immediate neighborhood. This anaesthesia 

 persists usually for weeks, or even months, after the healing of the 

 wound ; but it almost invariably disappears at last, and the skin re- 



1 Le<jons sur la Physiologie du SystSme Nerveux. Paris, 1866, p. 272. 



2 Trait6 des Sections Nerveuses. Paris, 1873. 



3 Injuries of Nerves, and their Consequences. Philadelphia, 1874, p. 84. 



