120 THE PHYSIOLOGY OF MUSCLE AND NERVE. 



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 

 weeks) the regeneration of the fibers in the peripheral stump does 

 not stop at the stage of "band .fibers/' but progresses until per- 

 fectly 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 connected 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 definitely 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 at 

 present uncertain. Some authors have confirmed his results, others deny 

 their accuracy. (For discussion with references, see Barker, The Neurons in 

 the Harvey Lectures, 1905-06, Philadelphia, 1906 ; also Journal of the 

 American Medical Assoc., 1906, and the Neuron Theory, Stefanowska, 

 Journal de Neurologic, 1906, Nos. 16-19.) 



The power of regeneration in the older animals is more limited 

 and carries the fiber only to the stage of the " band fiber." If under 

 the influence of the central stump an axis cylinder and myelin sheath 

 are now formed in this band fiber it is possible that this result 

 is due to local processes in the band fiber stimulated by nutritive 

 influences of some kind from the central stump, or more probably 

 there is an actual downgrowth of the axis cylinders from the central 

 ends. 



*Bethe, " Allgemeine Anat. u. Physiologic des Nervensystems," 1903. 



