126 THE PHYSIOLOGY OF MUSCLE AND NERVE. 



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* 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 wliich 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 ''embryonic fiber" gradually becomes transformed 

 into a normal nerve fiber, with myelin sheath and axis cylinder 

 (Fig. 58). It is possible that this result is due to local processes in 

 the embryonic fiber stimulated by nutritive influences of some kind 

 from the central stump, but more probably there is an actual 

 downgrowth of the axis cylinders from the central ends. In 

 support of this latter view, it may be said that the outgrowth of the 

 new axis cylinders from the old ones present in the fibers of the 

 central stump has been followed more or less successfuUy by a 

 number of histologists. 



Bethef has thrown some doubt upon this view, for he has shown appar- 

 ently that in young mammals (eight days to eight weeks) the regeneration ot 

 the fibers in the peripheral stump does not stop at the stage of "band fibers," 

 but progresses until perfectly normal nerve fibers are produced, even though 

 no connection is made with the central stump. It should be added, however, 

 that the fibers so formed do not persist indefinitely unless they become con- 

 nected with the central stump. If this connection fails to take place, the 

 newly formed fibers will degenerate after an interval of some months. Still, 

 the fact, if true, that in the young fiber the regeneration is complete seems to 

 indicate that the axis cylinder may arise independently of the fibers in the 

 central stump. 



Whether or not Bethe's observations upon the autoregeneration of the 

 axis cylinders in the severed nerves of young animals can be accepted is 

 doubtful, the balance of evidence at present seems to indicate that what he 

 took for autoregenerated fibers were really fibers which grew into the de- 

 generated trunk from the surrounding tissue. 



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



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



