NATURE OF THE NERVE IMPULSE. 119 



ends of the fibers remain substantially intact, it is interesting to 

 find that the nerve cells from which they originate undergo dis- 

 tinct changes, which show that they are profoundly affected by 

 the interruption of their normal connections (see p. 121). In the 

 peripheral end the process of regeneration begins almost simul- 

 taneously with the degenerative changes, the two proceeding, as 

 it were, hand in hand. The regeneration 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. 52). 

 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* 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 influence 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 

 suture or grow together in any way, then, under the influence of the 

 central end, the " band fiber " gradually becomes transformed into 

 a normal nerve fiber, with myelin sheath and axis cylinder (Fig. 53). 

 It is usually believed that the axis cylinders are formed as out- 

 growths from those of the fibers of the central stump. These latter 

 penetrate the " band fibers" and grow throughout their length. 



From a practical standpoint it is interesting to note that this influence 

 of the central stump may be exerted months or even years after the injury 

 to the nerve. The peripheral stump after reaching the stage of " band fibers " 

 is ready, as it were, for the influence of the central end, and cases are on record 

 in which a secondary suture was made a long time after the original injury, 

 with the result that functional activity was restored to the nerve. 



Bethef has thrown some doubt upon this view, for he has 

 shown apparently that in young mammals (eight days to eight 

 * Forsmanns, " Ziegler's Beitrage." 27, 216, 1902. 

 f Bethe, " Allgemeine Anat. u. Physiologie des Nervensystems," 1903. 



