200 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



tutes an "embryonic fibre." The embryonic fibres lying on one side of the 

 cut, unite with those on the other, union taking place in the intervening 

 cicatricial tissue. Next the myelin appears in isolated drops, usually near 

 the nuclei, and these subsequently unite to form a continuous tube, the for- 

 mation of the myelin proceeding centrifugally from the wound. Then follows 

 the outgrowth of the new axis-cylinder slightly behind the organization of 

 the myelin into the tubular form. 



It must not be forgotten that the last act, the formation of the axis- 

 cylinder, is the important event ; and while the whole process of repair may 

 require many months, the rate at which the axis-cylinder, when started, 

 grows out from the central end may he comparatively rapid. As a rule, 

 regeneration does not occur in the central system, 1 and thus the method of 

 experimentally causing degeneration has been one used for the study of the 

 architecture of both the brain and cord. 



That the regeneration is i]\w to an outgrowth from the central stump has 

 been clearly shown by Huber, 2 who inserted a bone tube between the cut 

 ends of the sciatic nerve of the don', and obtained regeneration of the nerve 

 with a return of function, although the initial interval between the two parts 

 of the nerve was more than three centimeters. The rate of growth from the 

 central end has been specially studied by Vanlair. 3 In the facial nerve of 

 the rabbit, function was restored in eight months after section, and in the 

 pneumogastric and ischiadic nerves of the dog in about eleven months. In 

 the latter case, this gives an average rate of growth of about 1 millimeter a 

 day. In the scar-tissue between the two parts of the nerve the rate is not 

 more than ()."2"> millimeter a day, and hence the return of function tends to 

 be delayed by any increase in the distance between the cut ends of the nerve. It 

 appears also that the return of the cutaneous sensibility is more rapid than 

 the return of motion (Howell and Iluber), from which we infer that the 

 afferent fibres (from the skin) regenerate more rapidly than the efferent fibres 

 to the muscles. 



Vanlair found that when the regenerated sciatic nerve of a dog was cut 

 a second time, it not only again regenerated, but did so more rapidly than in 

 the first case. 



Much interest has always attached to the exact course taken by the re- 

 generating fibres. They appear in a general way to be guided by the old 

 sheaths of the peripheral portion. Hut the peripheral nerves contain both 

 afferent and efferent fibres, and it would appear most probable that in the 

 process of reformation the new fibres should undergo much rearrangement. 

 Since the peripheral portion of the nerve acts as a guide to the growing fibres, 

 the experiment has been tried of cross-suturing two mixed nerves. This has 

 been done with the median and ulnar nerves in doers. Reunion of the crossed 



'Worcester: Journal of Experimental Medicine, 1S98, vol. iii. p. 597, describes a case of 

 apparent regeneration of a fibre-bundle in the mid-brain, and cites the literature. 

 1 Journal of Morphology, 1895, vol. xi. 

 s Archives de Physiologie normale et pathologique, 1894. 



