9 8 



TEXT-BOOK OF PHYSIOLOGY 



muscles and indirectly, through the intermediation of ganglia (see sym- 

 pathetic nerve system), with visceral muscles, blood-vessels, and glands. 



The afferent fibers are to be regarded as outgrowths from the cells of the 

 dorsal nerve ganglia, and serve to bring the skin, mucous membrane, and 

 certain visceral structures into relation with specialized centers in the central 

 nerve system. 



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

 vided 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 degenera- 

 tion is applied. The degenerative process begins simultaneously through- 

 out the entire course of the nerve, and consists in a disintegration and reduc- 

 tion 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. From these facts 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. 



\ 



FIG. 48. 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. (Datton.) 



It has for a long time been stated that the portion of the divided nerve in 

 connection with the brain or spinal cord retains its normal condition with the 

 exception of a few millimeters at its peripheral end. This statement has 

 been disproved by recent observations which show that the cell body and 

 its attached axon also in the course of time undergo an atrophy which may 

 become permanent; also that the chromatin material of the cell, Nissl's gran- 

 ules, undergo dissolution and no longer stain with the usual dyes, a condition 

 which may last for two weeks or more. In some instances the granules 

 may be restored. These degenerative changes in the nerve cell are probably 

 the result of the cessation of its customary function. 



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 immediate degenerative process is confined to the peripheral portion, the 

 central portion remaining for a variable period 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. 48.) If the root be 



