XI FUNCTIONS OF NERVE ROOTS 475 



part of the nerve which is still connected with the cells of 

 the spinal cord ; this part does not degenerate in the 

 same way. Thus if the anterior eflFerent (motor) root 

 of one of the spinal nerves be cut at y (Fig. 152), all 

 the fibres of that root beyond y towards and along the 

 trunk of the nerve T degenerate, while the portion of the 

 root between y and the spinal c(n'd does not degenerate. 



If, now, we apply the same method of experiment to a 

 posterior root the following results are observed. When 

 the root is cut at w (Fig. 152), the fibres of that root 

 towards and along the trunk of the nerve T degenerate ; 

 the central parts connected with the ganglion do not. 

 If, on the other hand, the posterior root is cut at z, 

 then the part of the root which lies between z and the 

 spinal cord degenerates and the degeneration may be 

 traced as far as the cut axis-cylinders penetrate into the 

 central nervous system, whereas the portion still con- 

 nected with the ganglion does not. Evidently, then, the 

 life of the fibres in the posterior root is dependent upon 

 their continued connection with the ganglion of that root, 

 that is to say with the cells of that ganglion, of which the 

 fibres are processes, as we have previously explained. 

 These facts lead to the inevitable conclusion that the 

 ganglion on the posterior root is the structure upon which 

 the proper nutrition of the afferent filn-es depends, or, in 

 other words, the one clear and definitely ascertained 

 function of the ganglion is to provide for the nutrition of 

 these efi"ei-ent nerve fibres wliich originate from the 

 processes of the nerve cells in the ganglion. 



This method of determining and localising the 

 nutritional centres from wliich nerve-fibres grow is known 

 as the " degeneratiim method," ^ and has proved to be 

 most helpful in determining the various " tracts," or paths 

 in the spinal cord (and brain) along which nervous 

 impulses of various kinds pass ; with these we shall have 

 to deal later on (see p. 489). 



1 Also as the " Walleriau method," after the name of tlio physiologist 

 who first employed it. 



