GENERAL, STRUCTURE OF THE NERVOUS SYSTEM. 353 



regards their size ; and in the essential details of their structure, 

 namely, the tubular sheath, the medullary layer, and the axis cylinder, 

 they are to all appearance completely identical. 



Degeneration and Regeneration of Divided Nerves. The imme- 

 diate effect of dividing nerve fibres is to suspend their function. The 

 communication between their extremities being cut off, the sensitive 

 fibres can no longer transmit an impression from the skin to the 

 nervous centre, and the motor fibres can no longer convey a stimulus 

 of motion from the nervous centre to the muscles. This paralysis of 

 motion and sensibility follows instantaneously upon the division of 

 the nerve fibres. 



But in addition to this result there also takes place, in the separated 

 portion of the nerve, a structural degeneration of its fibres. The first 

 indication of this change is visible in the medullary layer. It divides, 

 in the course of each interannular segment, into two, three, or four 

 distinct masses, the intervals between which are occupied, according 

 to Ranvier,* by a new growth of nearly transparent, finely granular 

 albuminous matter from the inner surface of the sheath of Schwann ; 

 which is already sufficient, in the rabbit, at the end of forty-eight 

 hours, to fill at certain points the whole calibre of the sheath. The 

 division of the medullary layer goes on until it is entirely broken up 

 into globular masses of varying size, scattered irregularly through the 

 substance of the fibre, and completely obscuring its normal structure. 

 By this process, the continuity of the medullary layer is destroyed, its 

 myeline being- reduced to the condition of isolated oily-looking drops, 

 and gradually transformed into a diffused granular mixture. Finally, 

 the granules themselves disappear, and the tubular sheath, partially 

 emptied by the atrophy of the medullary layer, becomes collapsed and 

 wrinkled. Owing to the disappearance of the myeline, the nerve loses 

 its white glistening aspect and assumes a grayish hue. According to 

 the testimony of all recent observers, degeneration goes on at the 

 same time in the axis cylinder. This portion of the fibre is enveloped 

 and encroached upon by the growth of new matter, its continuity is 

 broken at various points, its separated fragments are bent or folded 

 upon themselves, and at last can no longer be made visible by the 

 staining action of a carmine solution. Thus all the structural elements 

 of the nerve fibre, excepting, the sheath of Schwann, undergo a degen- 

 eration which results in complete atrophy. 



The rapidity with which this change takes place varies with the 

 species and age of the animal. It is less rapid in the cold-blooded, 

 more so in the warm-blooded species. It goes on more quickly in the 

 young, more slowly in full-grown animals. According to Yulpian, in 

 young dogs, as a rule, the disappearance of the medullary layer is 

 complete in six weeks or two months from the date of the injury. 



The degeneration of the fibres of a divided nerve, whether sensitive 



* Histologie du Systems Nerveux. Paris, 1878, tome i., p. 315. 



X 



