390 



THE NERVOUS SYSTEM 



cord. The fibres of the posterior root degenerate wherever they are 

 separated from connection with the ganglion ; and if the ganglion 

 be excised, they degenerate in both directions ; namely, inward to the 

 spinal cord and outward to the periphery (Fig. 105, D). 



FIG. 105. 



DEGENERATION OF SPINAL NERVES AND NERVE ROOTS AFTER SECTION. A. Section of Nerve 

 Trunk beyond the Ganglion. B. Section of Anterior Root. C. Section of Posterior Root. D. 

 Excision of Ganglion, a. Anterior Root ; p, Posterior Root ; g, Ganglion. 



These facts, first discovered by Waller,* have since been confirmed by 

 all observers. They show that the nutrition of the anterior and poste- 

 rior nerve roots is connected with different centres; since the fibres of 

 the anterior root degenerate when separated from the gray substance 

 of the anterior horn, while those of the posterior root degenerate when 

 separated from the spinal ganglion. Such points are designated as 

 "trophic centres," or centres of nutrition for the nerve fibres con- 

 nected with them; indicating that the fibres preserve their normal 

 structure so long as this connection is retained, and degenerate w r hen 

 it is cut off. 



The nature of this relation between nerve fibres and their centres 

 is unknown. We cannot assume that the nutrition of the fibres is 

 immediately derived from the cells of the gray substance ; since although 

 the fibres of a divided nerve degenerate in the part separated from its 

 centre, they are afterward regenerated by a process taking place, so 

 far as we know, in the nerve itself (page 355). But it is a relation of 

 great physiological importance, and extends to considerable tracts of 

 white substance in the brain and spinal cord. 



Motor and Sensitive Transmission in the Spinal Cord. The sim- 

 plest fact determined, in this respect, both by experimental research 

 and pathological observation, is that the spinal cord is the exclusive 

 organ of communication between the brain, on the one hand, and the 

 external organs of sensation and motion, on the other ; since if it be 

 divided by a transverse section, compressed by fractured bone, or dis- 

 organized by disease at any part of its length, the result is a complete 

 loss of sensibility and voluntary motion below the point of injury. The 



* Comptes Bendus de FAcade'mie des Sciences. Paris, 1851, tome xxxiiL, p. 606; 

 and 1852, tome xxxiv., p. 524. 



