THE SPINAL CORD 



663 



the periphery. The path of degeneration which follows section of 

 the posterior root depends upon the site of section. If it be between 

 the ganglion and the cord, then, since the nerve cells are in the ganglion, 

 degeneration will take place in the parts of the fibres which enter the 

 spinal cord; if, on the other hand, it be peripheral to the ganglion, 

 then the nerve-fibres will degenerate towards the periphery, where 

 they form connections with the various rereptor mechanisms. The 



Degeneration of efferent and of afferent 

 fibres below a section of entire nerve. 



Degeneration of efferent fibres belosv a 

 section of anterior root. 



Degeneration of afferent fibres below a 

 section of posterior root beyond the 

 ganglion. 



Degeneration of afferent fibres above 

 a section of posterior root above the 

 ganglion. 



FIG. 388. DIAGRAMS TO ILLUSTRATK WALLERIAN DEGENERATION OF NERVE-ROOTS. 



(Waller.) 



ingoing afferent fibre from the posterior root ganglion makes a 

 variety of connections. These are best considered after the structure 

 of the spinal cord has been described. 



The Structure of the Spinal Cord. The spinal cord is the long 

 strand of nervous tissue which passes down the vertebral canal from 

 the base of the brain to the level of the first lumbar vertebra a length 

 of about 18 inches. It develops in three zones from the neural tube 

 (Figs. 389, 390). From it are given off the anterior and posterior 

 roots of the spinal nerves. In the cervical and again in the lumbar 

 region there is an enlargement from which arise respectively the nerves 



