854 SPINAL NERVES. [BOOK m 



to close up to the ganglion. When the posterior 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 carefully 

 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 

 place in opposite directions, and starts from different nutritive 

 or ' trophic ' centres. The afferent fibres grow away from the 

 ganglion either towards the periphery, or towards the spinal cord. 

 The efferent fibres grow outwards from the spinal cord towards 

 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. Con- 

 versely, when the anterior roots are cut, the efferent fibres alone 

 degenerate, and can be similarly recognized in a mixed nerve tract. 

 When the anterior root 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 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 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 degeneration, called the "degeneration method," or 

 sometimes from the name of the physiologist who introduced 



