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TEXT-BOOK OF PHYSIOLOGY. 



normal condition with the exception of a few millimeters at its peripheral 

 end. The degenerative process begins simultaneously throughout the entire 

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

 myelin 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 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 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 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 in 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 ventral root alone be divided 

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



a 



FIG. 49. DEGENERATION OF SPINAL NERVES AND NERVE-ROOTS AFTER SECTION A, 

 Section of nerve-trunk beyond the ganglion B, Section of ventral root C, Section of dorsal. 

 D. Excision of ganglion, a. Ventral root. p. Dorsal root. g. Ganglion. (Dalton.} 



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

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

 of the nerve. (See Fig. 49.) If the root be divided between the ganglion 

 and the cord, degeneration 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 degen- 

 erative in character, and the process is usually spoken of, after its discoverer, 

 as the Wallerian degeneration. 



When the nerve-cells from which the nerve-fibers arise, whether efferent 

 or afferent, undergo degeneration from any cause whatever, the nerve-fiber 

 becomes involved in the degenerative process and when it 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, apparently, 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. 



