GENERAL STRUCTURE OF THE NERVOUS SYSTEM. 355 



fibres thus produced are fibres of new formation. The old fibres have 

 completely disappeared throughout the peripheral ramifications of the 

 nerve, and their place is taken by others of subsequent growth. 



The details of this regeneration are not fully known ; but its essen- 

 tial characters, so far as they have been ascertained, are as follows : 

 The new fibres begin to show themselves in the divided nerve before 

 the complete disappearance of the old medullary granules. They are 

 always smaller than the average size, and their interannular segments 

 are shorter than in the fully developed condition ; but in other respects 

 their structure is normal, and even when very slender they exhibit 

 annular constrictions, and a distinct medullary layer, capable of being 

 stained by perosmic acid. They gradually increase in diameter, and 

 in the thickness of their medullary layer ; and when the process of 

 regeneration is complete, the nerve again presents its normal whiteness 

 and opacity. 



There is some uncertainty as to the direction in which the growth 

 of new fibres takes place. By several histologists it is maintained that 

 the regenerated axis cylinders are offshoots from those in the central 

 undegenerated extremity of the nerve ; their growth extending thence 

 into the peripheral portions. But this opinion is based only on analogy, 

 from a similar growth of embryonic nerve fibres in the tail-membrane 

 of the tadpole, and does not rest on any certain results of direct obser- 

 vation. It is possible that the new fibres may grow simultaneously 

 throughout the separated portion of the nerve, increasing everywhere 

 in development until their normal structure is attained. From numer- 

 ous observations on this subject, it was the conclusion of Yulpian* 

 that the regeneration of the fibres at any given time is the same at all 

 points in the separated portion of a divided nerve. Tulpiau and Philip- 

 peaux have also found that if the hypoglossal or the lingual nerve be 

 divided, and the central portion extracted, so that no communication 

 can be reestablished with the nervous centres, the peripheral portion 

 may be regenerated in the usual manner, notwithstanding its perma- 

 nent separation from the central extremity. This would show that the 

 power of regeneration resides in the nerve itself, the materials being 

 supplied by the nutritive plasma of its own tissues. 



The rapidity of regeneration in the fibres of a divided nerve, and 

 the length of an excised portion which may be restored, vary with the 

 age and species of the animal. According to Ranvier, after simple 

 division of a nerve, in the rabbit, regeneration is in full progress at the 

 end of nine or ten weeks, though many of the new fibres are of less 

 than the average diameter. Yulpian found, in very young animals, a 

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

 at the end of six weeks ; and in young rats a portion of the sciatic 

 nerve six millimetres long was reproduced in seventeen days. In 

 adult animals, and especially in man, the restoration of divided nerves 



* Le9ons sur la Physiologic du Systeme Nerveux. Paris, 1866, p. 258. 



