384 BIOLOGY: GENERAL AND MEDICAL 



6. Muscular Tissues. It is improbable that the mus- 

 cular tissue of the mammals undergoes any effective 

 regeneration after injury. Wounds of the unstriated 

 muscle of the uterus and intestines repair through inter- 

 mediate fibre-connective tissue cicatrices. Wounds of 

 cardiac and voluntary muscles usually do the same, 

 though peculiar formations sometimes appear at the 

 injured ends of the voluntary muscle fibres which many 

 interpret to mean that regenerative attempts are in prog- 

 ress. However this may be, the attempts are abortive, 

 little new formation results, and such tissue of supposedly 

 new formation as may be found at the ends of the fibres is 

 distinctly atypical. 



7. The Nervous Tissues. It is not known that the nerve 

 cells can be replaced when destroyed, but the nerve 

 fibres regenerate quite well. The process is not perfectly 

 understood. When a medullated fibre is cut or torn the 

 proximal end degenerates to the next higher "node of 

 Ranvier," and that of the distal end appears to degener- 

 ate altogether. If there is no infection or other un- 

 favorable condition to prevent it, the regeneration of 

 the nerve begins within a few days by an outgrowth 

 from the proximal end. Such outgrowths from the axis 

 cylinders of the proximal ends grow down in the path 

 of the medullary sheaths, extend through whatever 

 cicatricial tissue may be in process of formation, and 

 on to the distal fragment. When these growing axis 

 cylinders are able, as the result of a neurotropic influence, 

 to find their way into or along the old sheaths, the prog- 

 ress toward the completion of the conducting tract is 

 rapid, otherwise it is slow and more complicated and 

 perhaps less perfect in the end. Some obser ers deny 

 that the distal fragment degenerates completely, but 

 think that, like the proximal end, it only degenerates a 

 short distance so that the growing proximal end need 

 not renew the entire path of conduction, but only so 

 much as has been destroyed. Others think that the 

 axis cylinder fibre is restored through the activity of 



