Chap, i.] THE SPINAL CORD. 679 



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

 nerve-cells in the ganglion appear to be more sensitive to cer- 

 tain poisons than are the nerve-fibres of the nerve-trunk. But 

 beyond this, our knowledge concerning 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 (§ 78) said, when a mixed nerve-trunk is divided 

 the peripheral portion degenerates from the point of section 

 downwards towards the periphery. The central portion does 

 not so degenerate, and if the length of nerve removed be not 

 too great, the central portion may grow downwards along the 

 course of the degenerating peripheral portion, and thus regen- 

 erate 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 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 nutri- 

 tion 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 recognized 

 in a mixed nerve tract. When the anterior root is divided 



