NATURE OF THE NERVE IMPULSE. 125 



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* (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 affected by the interruption of their normal 

 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 

 strand of protoplasm with many nuclei, and the fiber thus produced, 

 which has no resemblance in structure to a normal nerve fiber, 

 is described by some authors as an "embryonic fiber"; by others 

 as a "band fiber" (Fig. 57). In the adult animal the process of 

 regeneration stops at this point unless an anatomical connection 

 is established with the central stump, and, indeed, such a connection 

 is usually established unless special means are taken to prevent it. 

 The central and peripheral stumps find each other in a way that 

 is often remarkable, the union being guided doubtless by intervening 

 connective tissue. 



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

 Mott and Halliburton, " Proceedings Royal Society," 1906, B. Ixxviii., 259, 

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

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



