OF THE NERVOUS SYSTEM. 411 



As the process goes on, the continuity of the medullary layer is entirely 

 destroyed, and this substance is reduced to the condition of isolated 

 oily-looking drops, scattered through the interior of the tubular sheath, 

 which become gradually transformed into a diffused mixture of minute 

 granules. Finally, the granules themselves disappear, and the tubular 

 sheath, partially emptied by the atrophy of the medullary layer, becomes 

 collapsed and wrinkled. The nerve which has suffered these changes 

 has lost its white glistening color, and has assumed a grayish hue. The 

 axis cylinder either does not participate in the above alterations, or its 

 changes are not so manifest to the eye ; since, according to some ob- 

 servers, it remains visible after the medullary layer has disappeared. 



According to various observers (Waller, Krause, Yulpian), the de- 

 generation of divided nerve fibres, both in the sensitive and motor 

 nerves, may be propagated throughout their peripheral extremities, 

 extending even to the sensitive papillae of the tongue and the tactile 

 corpuscles of the skin. Yulpian 1 has found that in dogs, six weeks 

 after the division of the sciatic nerve, no nerve fibres could be dis- 

 covered in the muscles of the foot which had not undergone the same 

 alteration. 



The rapidity with which degeneration takes place in the fibres of a 

 divided nerve varies with the species and age of the animal to which it 

 belongs. The change is less rapid in the cold-blooded, more so in the 

 warm-blooded animals. In those of the same species, it goes on more 

 quickly in the young, more slowly in animals which are fully grown. 

 According to Yulpian, in young dogs, as a general rule, the disappear- 

 ance of the medullary layer is complete at the end of six weeks or two 

 months from the date of the injury. 



The degeneration of the peripheral portions of divided nerves has 

 often been utilized in order to determine the source of particular bun- 

 dles of nerve fibres. If a nerve, for example, receives roots or commu- 

 nicating branches from two different sources, and afterward supplies 

 by its ramifications several organs, it may be ascertained whether the 

 fibres coming from one source are or are not distributed to a particular 

 organ. For this purpose the root or communicating branch in question 

 is divided ; and when the subsequent process of degeneration is com- 

 plete, the atrophied nerve fibres derived from this source may be fol- 

 lowed by microscopic examination throughout their course, and recog- 

 nized in the organ to which they are distributed. 



Union and Regeneration of divided Nerves The loss of function 

 in a divided nerve is not permanent ; but, if the neighboring parts be 

 healthy and no other injury have been inflicted, the nerve fibres may 

 reunite, and their power of communication be restored. When the 

 division has been a simple one, the two extremities of the divided nerve 

 remaining in contact or in close proximity with each other, their union 

 takes place with comparative readiness ; but even when a considerable 



1 Leqons sur la Physiologie du Systdme Nerveux. Paris, 1866, p. 243. 



