1 68 



PHYSIOLOGY OF NERVE. 



[CH. XV. 



the axis cylinder process) cut off from the nucleus of the cell 

 degenerates and dies. 



If a nerve is simply cut, and allowed to heal, regeneration of 

 function in time occurs. This is hastened by the surgeon 

 suturing the cut ends of the nerve together. It must not, how- 



-P 



b-\\ 



D TT 

 /' t" 't' 



Fig. 179. Degeneration and regeneration of nerve-fibres. A, nerve-fibre, fifty hours after 

 operation, my, medullary sheath breaking up into myelin drops, p, granular proto- 

 plasm replacing myelin. n, nucleus, g, primitive sheath. B, nerve-fibre after four 

 days, cy, axis cylinder partly broken up and enclosed in portions of myelin. c.amore 

 advanced stage in -which the medullary sheath has almost disappeared. Numerous 

 nuclei, n" are seen. D, commencing regeneration ; several fibres (f, t!') have sprouted 

 from the somewhat bulbous cut end (6) of the nerve, a, an axis cylinder which has 

 not yet acquired its medullary sheath, s, s' primitive sheath of the original fibre.. 

 (Ranvier.) 



ever, be supposed that this is due to a restoration of the structure 

 of the fibres in the peripheral portion of the cut nerve. It is 

 due to new nerve-fibres sprouting out from the central end of the 

 cut nerve, and growing distalwards in the old sheaths. This is 

 illustrated in D, fig. 179. Regeneration of cut fibres never 

 occurs in the brain or spinal cord. 



When regeneration does not take place, the central ends of the 



