118 THE PHYSIOLOGY OF MUSCLE AND NERVE. 



conduction of nerve impulses, or there may be some kind of an 

 actual transferal of material. This latter idea is supported by the 

 interesting fact, which we owe to Meyer, that tetanus and diph- 

 theria toxins may be transmitted to the central nervous system 

 by way of the axis cylinders of the nerve fibers. By means of his 

 method Waller investigated the location of the nutritive centers 

 for the motor and sensory fibers of the spinal nerves. If an anterior 

 root is cut the peripheral ends of the motor fibers degenerate 

 throughout the length of the nerve, while the fibers in the stump 

 attached to the cord remain intact; hence the nutritive centers 

 for the motor fibers must lie in the cord itself. Subsequent histo- 

 logical work has corroborated this conclusion and shown that the 

 motor fibers of the spinal nerves take their origin from nerve cells 

 lying in the anterior horn of gray matter in the cord, the so-called 

 motor or anterior root cells. If the posterior root is cut between 

 the ganglion and the cord, the stump attached to the cord degener- 

 ates; that attached to the ganglion remains intact, and there is no 

 degeneration in the nerve peripheral to the ganglion (Fig. 50). If, 

 however, this root is severed peripherally to the ganglion degenera- 

 tion takes place only in the spinal nerve beyond the ganglion. The 

 nutritive center, therefore, for the sensory fibers must lie in the pos- 

 terior root ganglion, and not in the cord. This conclusion has also 

 been abundantly corroborated by histological work. It is known 

 that the sensory fibers arise from the nerve cells in these ganglia. 

 By the same means it has been shown that the motor fibers in the 

 cranial nerves arise from nerve cells (nuclei of origin) situated 

 in the brain, while the sensory fibers of the same nerves, with the 

 exception of the olfactory and optic nerves which form special 

 cases, arise from sensory ganglia lying outside the nervous axis, 

 such, for instance, as the spiral ganglia of the cochlear nerve, or 

 the ganglion semilunare (Gasserian ganglion) of the fifth cranial 

 nerve. 



Nerve Degeneration and Regeneration. When a nerve 

 trunk is cut or is killed at any point by crushing, heating, or other 

 means all the fibers peripheral to the point of injury undergo de- 

 generation. This is an incontestable fact, and it is important to 

 bear in mind the fact that the definite changes of degeneration are 

 exhibited only by living fibers. A dead nerve or the nerves in a 

 dead animal show no such changes.* The older physiologists 

 thought that if the several ends of the nerves were brought together 

 by sutures they might unite by first intention without degeneration 

 in the peripheral end. We know now that this degeneration is 

 inevitable once the living continuity of the fibers has been inter- 

 rupted in any way. Any functional union that may occur is a slow 

 * See Van Gehuchten, "Le Nevraxe," 1905, vii., 203, 



