78 HUMAN PHYSIOLOGY. 



again takes place. In the ganglia of all the sensoricranial and sensori- 

 spinal nerves the cells have this histologic peculiarity. 



Posterior 

 Jbot 



FIG 9. DIAGRAM SHOWING THE MODE OF ORIGIN OF THE VENTRAL AND 

 DORSAL ROOTS. 



Nerve Degeneration. If any one of the cranial or spinal nerves 

 be divided in any portion of its course, the part in connection with 

 the periphery in a short time exhibits certain structural changes, to 

 which the term degeneration is applied. The portion in connection 

 with the brain or cord retains its normal condition. The degenera- 

 tive process begins simultaneously throughout the entire course of 

 the nerve, and consists in a disintegration and reduction of the 

 medulla and axis-cylinder into nuclei, drops of myelin, and fat, which 

 in time disappear through absorption, leaving the neurilemma intact. 

 Coincident with these structural changes there is a progressive altera- 

 tion and diminution in the excitability of the nerve. Inasmuch as 

 the central portion of the nerve, which retains its connection with 

 the nerve-cell, remains histologically normal, it has been assumed 

 that the nerve-cells exert over the entire course of the nerve-fibers 

 a nutritive or a trophic influence. This idea has been greatly strength- 

 ened since the discovery that the axis-cylinder, or the axon, has its 

 origin in and is a direct outgrowth of the cell. When separated from 



