118 THE PHYSIOLOGY OF MUSCLE AND NERVE. 



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. The older physiolo- 

 gists thought that if the severed 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 de- 

 generation is inevitable once the living continuity of the fibers has 

 been interrupted in any way. Any functional union that may 

 occur is a slow process involving an act of regeneration of the 

 fibers in the peripheral stump. The time required for the degen- 

 eration differs somewhat for the different kinds of fibers found in 

 the animal body. In the dog and in other mammalia the degenera- 

 tion begins in a few (four) days, while in the frog it may require 

 from thirty to one hundred and forty days, depending upon the 

 season of the year. In the dog it proceeds so quickly that the 

 process seems to be simultaneous throughout the whole peripheral 

 stump, while in the frog, and, according to Bethe, in the rabbit it 

 can be seen clearly that the degenerative changes begin at the wound 

 and progress peripherally. The fibers break up into ellipsoidal seg- 

 ments of myelin, each containing a piece of the axis cylinder, and 

 these segments in turn fragment very irregularly into smaller pieces 

 which eventually are absorbed * (Fig. 51) . The central stump whose 

 fibers are still connected with the nerve cells undergoes a similar 

 degeneration in the area immediately contiguous to the wound, but 

 the degenerative processes extend for only a short distance over 

 an area covering a few internodal segments. Although the central 



* See Howell and Huber, " Journal of Physiology," 13, 335, 1892. 



