680 SPINAL NERVES. [Boon in. 



some few fibres in it do not, like the rest, degenerate, and 

 when the posterior root is divided, a few fibres in the anterior 

 root are seen to degenerate like those of the posterior root; 

 these appear to be the fibres which give to the anterior root its 

 " recurrent sensibility ." In the case of certain spinal nerves 

 at all events, it has also been ascertained that when the pos- 

 terior root is divided, while most of the fibres in the part of 

 the root thus cut off from the ganglion but left attached to the 

 cord degenerate, some few do not. These few appear to have 

 their trophic centre not in the ganglion, but in some part of 

 the spinal cord itself ; we shall refer to these later on. 



This method of distinguishing nerve fibres by the features 

 of their degeneration, called the "degeneration method," or 

 sometimes from the name of the physiologist who introduced 

 it, the " Wallerian method," has proved of great utility. Thus 

 in the vagus nerve which is composed not only of fibres which 

 spring from the real vagus root but also of fibres proceeding 

 from the spinal accessory roots, the two may be distinguished 

 by section of the vagus and spinal accessory roots respectively. 

 We shall presently see that this method may be applied to the 

 differentiation of tracts of fibres in the brain and spinal cord. 



