124 THE PHYSIOLOGY OF MUSCLE AND NERVE. 



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. 55). 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 



Fig. 55. Diagram to show the direction of degeneration on section of the anterior 

 and the posterior root, respectively. The degenerated portion is represented in black. 



instance, as the spiral ganglion of the cochlear nerve, or the gan- 

 glion 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 included under 

 the term 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 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 degeneration is inevitable once the 

 living continuity of the fibers has been interrupted in 

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



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



