ii2 TEXT-BOOK OF PHYSIOLOGY. 



of the central nervous system and only subsequently b^ome con- 

 nected with it.. (See Fig. 47.) 'At the time of the closure of the 

 medullary tube a band or ridge of epithelial tissue develops near the 

 dorsal surface, which, becoming segmented, moves outward and forms 

 the rudimentary spinal ganglia. The cells in this situation develop 

 two axons, one from each end of the cell, which pass in opposite 

 directions, one toward the spinal cord, the other toward the per- 

 iphery. In the adult condition the two axons shift their position, 

 unite, and form a T-shaped process, after which a division into two 

 branches again takes place. In the ganglia of all the sensori-cranial 

 and sensori- spinal nerves the cells have this histologic peculiarity. 



The efferent fibers are therefore to be regarded as outgrowths 

 from the nerve-cells in the ventral horns of the gray matter, and serve 

 to bring the cells into anatomic and physiologic relationship directly 

 with the skeletal muscles and indirectly, through the intermediation 

 of ganglia (see sympathetic nervous system), with visceral muscles 

 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 special- 

 ized centers in the central nervous system. 



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 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. \unasmuch 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, v 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 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 anterior 

 root alone be divided, the degenerative process is confined to the 

 peripheral portion, the central portion remaining normal. If the 



