70 HUMAN PHYSIOLOGY. 



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 nor- 

 mal condition. The degenerative 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. Coin- 

 cident with these structural changes there is a progressive alteration 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 histo- 

 logically 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 strengthened 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 the parent cell, the fiber appears to be incapable of itself of maintaining 

 its nutrition. 



The relation of the nerve-cells to the nerve-fibers, in reference to their 

 nutrition, is demonstrated by the results which follow section of the ventral 

 and dorsal roots of the spinal nerves. If the anterior root alone be divided, 

 the degenerative process is confined to the peripheral portion, the central 

 portion remaining normal. If the posterior root be divided on the peripheral 

 side of the ganglion, degeneration takes place only in the peripheral portion of 

 the nerve. If the root be divided between the ganglion and the cord, degener- 

 ation takes place only in the central portion of the root. From these facts it 

 is evident that the trophic centers for the ventral and dorsal roots lie in the 

 spinal cord and spinal nerve ganglia, respectively, or, in other words, in the 

 cells of which they are an integral part. The structural changes which nerves 

 undergo after separation from their centers are degenerative in character, 

 and the process is usually spoken of, after its discoverer, as the Wallerian 

 degeneration. 



When the degeneration of the efferent nerves is completed, the structures 

 to which they are distributed, especially the muscles, undergo an atrophic or 

 fatty degeneration, with a change or loss of their irritability. This is, ap- 

 parently, not to be attributed merely to inactivity, but rather to a loss of 

 nerve influences, inasmuch as inactivity merely leads to atrophy and not to 

 degeneration. 



Reactions of Degeneration. In consequence of the degeneration and 

 changes in irritability which occur in nerves when separated from their 

 centers and in muscles when separated from their related nerves, either 



