NATURE OF THE NERVE IMPULSE. 125 



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 

 process involving an act of regeneration of the fibers in the peripheral 

 stump. The time required for the degeneration differs somewhat 

 for the different kinds of fibers found in the animal body. In the 

 dog and in other mammalia the degeneration begins in a few (four) 

 days; in the frog it may require from thirty to one hundred and 

 forty days, depending upon the season of the year, although if the 

 frog is kept at a high temperature (30 C.) degeneration may 

 proceed as rapidly as in the mammal. 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, 



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



