528 THE SPINAL CORD. 



seen, these are in reality processes of the nerve-cells ; thus the nerve-cells in 

 the ganglion appear to be more sensitive to certain poisons than are the 

 nerve-fibres of the nerve-trunk. But beyond this, our knowledge concern- 

 ing the function of the ganglion is almost limited to the fact that it is in 

 some way intimately connected with the nutrition of the nerve. As we have 

 already ( 81) said, when a mixed nerve-trunk is divided the peripheral 

 portion degenerates from the point of section downward toward the periph- 

 ery. The central portion does not so degenerate, and if the length of nerve 

 removed be not too great, the central portion may grow downward along the 

 course of the degenerating peripheral portion, and thus regenerate the nerve. 

 This degeneration is observed when the mixed trunk is divided in any part 

 of its course from the periphery to close up to the ganglion. When the pos- 

 terior root is divided between the ganglion and the spinal cord, the portion 

 attached to the spinal cord degenerates, but that attached to the ganglion 

 remains intact. When the anterior root is divided, the proximal portion in 

 connection with the spinal cord remains intact, but the distal portion between 

 the section and the junction with the other root degenerates ; and in the 

 mixed nerve-trunk many degenerated fibres are seen, which, if they be care- 

 fully traced out, are found to be motor (efferent) fibres. If the* posterior 

 root be divided carefully between the ganglion and the junction with the 

 anterior root, the small portion of the posterior root left attached to the 

 peripheral side of the ganglion above the section remains intact, as does also 

 the rest of the root from the ganglion to the spinal cord, but in the mixed 

 nerve-trunk are seen numerous degenerated fibres, which when examined are 

 found to have the distribution of sensory (afferent) fibres. Lastly, if the 

 posterior ganglion be excised, the whole posterior root degenerates, as do also 

 the sensory (afferent) fibres of the mixed nerve-trunk. Putting all these 

 facts together, it would seem that the growth of the efferent and afferent 

 fibres takes places in opposition directions, and starts from different nutri- 

 tive or " trophic " centres. The afferent fibres grow away from the ganglion 

 either toward the periphery or toward the spinal cord. The efferent fibres 

 grow outward from the spinal cord toward the periphery. This difference 

 in their mode of nutrition is frequently of great help in investigating the 

 relative distribution of efferent and afferent fibres. When a posterior root 

 is cut beyond the ganglion, or the ganglion excised, all the afferent nerves 

 degenerate, and in the mixed nerve-branches these afferent fibres, by their 

 altered condition, can readily be traced. Conversely, when the anterior 

 roots are cut, the efferent fibres alone degenerate, and can be similarly recog- 

 nized in a mixed nerve-tract. When the anterior is divided 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 pos- 

 terior 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 posterior 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 de- 

 generation, 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 



