126 THE PHYSIOLOGY OF MUSCLE AND NERVE. 



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. 



Forsmanns f has emphasized this peculiar attraction, as it were, be- 

 tween the peripheral and the central ends, giving some reason to believe that 

 it is a case of chemotaxis or chemotropism. When the ends of the nerves 

 were given very unusual positions by means of collodium tubes into which 

 they were inserted they managed to " find " each other. Moreover, he states 

 that a central stump, if given an equal opportunity to grow into two collo- 

 dium tubes, one containing liver and the other brain tissue, will chose the 

 latter, a fact which would indicate some underlying chemical attraction or 

 affinity in nerve tissue for nerve tissue. A directive influenc , of this kind 

 depending upon some property connected with chemical relationship is desig- 

 nated as " chemotaxis." 



If the central and peripheral stumps are brought together by 



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

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

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

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



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