NATURE OF THE NERVE IMPULSE. 125 



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, 

 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 segments of myelin, each containing a piece of 

 the axis cylinder, and these segments in turn fragment very irregu- 

 larly into smaller pieces which eventually are absorbed f (Fig. 56). 

 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 ends of the fibers remain sub- 

 stantially intact, it is interesting to find that the nerve cells from 

 which they originate undergo distinct changes, which show that 

 they are profoundly afi'ected by the interruption of their normaJ 

 connections (see p. 127). In the peripheral end the process of 

 regeneration begins almost simultaneously with the degenerative 

 changes, the two proceeding, as it were, hand in hand. The regen- 

 eration is due to the activity of the nuclei of the neurilemmal sheath. 

 These nuclei begin to multiply and to form around them a layer of 

 protoplasm, so that as the fragments of the old fiber disappear 

 their place . is taken by numerous nuclei and their surrounding 

 cytoplasm. Eventually there is formed in this way a continuous 



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



tSee Howell and Huber, "Journal of Physiology," 13, 335, 1892; also 

 Mott and Halliburton, "Proceedings Royal Society," 1908, B. Ixxviii., 259, 

 and Cajal, "Trabajos del laboratorio de investigaciones biologicas (Univ. of 

 Madrid)," vol. 4, 119, 1906. 



