46 ESSENTIALS OF PHYSIOLOGY. 



fibres is only available during the first three or four weeks after the 

 fibres have been cut off from the nerve cells. 



When the nerve fibres are completely degenerated, they can be 

 distinguished from normal fibres by a special method of staining with 

 hsematoxylin called the Weigert-Pal method. The hsematoxylin stains 

 the myelin bluish black, and fibres from which the myelin has dis- 

 appeared are left unstained. 



When a nerve is divided the portion still connected with the cell 

 does not degenerate, though changes take place in the nerve cell. 

 Within one or two days the cell swells and the Nissl granules disappear ; 

 and after a time the cell shrinks. Later, regeneration usually occurs, 

 and within three months the cell may return to a normal condition ; in 

 other cases complete atrophy of the cell takes place. 



Regeneration. After a time regeneration takes place and the nerve 

 may be restored to a normal condition ; this occurs more rapidly if the 

 cut ends of a divided nerve are sutured together. Regeneration is 

 brought about solely by the outgrowth of the part of the axon which 

 is still in connection with the nerve cell, though the process is 

 hastened by the presence of the neurolemma of the degenerated fibre, 

 which seems to assist the outgrowth of the new fibre. The part played 

 by the nerve cell is shown by the fact that extirpation of the nerve cells 

 in a portion of the spinal cord prevents regeneration in motor fibres 

 arising from that part of the spinal cord. Further, if the cut end of 

 the peripheral portion of a divided nerve is covered by a rubber cap so as 

 to prevent the growth of new fibres into it, no regeneration takes place. 



Histological observations show that in the course of regeneration 

 small fibrils with bulbous ends extend from the axis cylinders of the 

 central portion of the divided nerve, and pass into the neurolemmal 

 sheaths of the degenerated distal portion. The mode of growth seems 

 to be similar to that which has been observed to occur in the embryonic 

 nerve tissue of the frog. If fragments of the primitive nerve tube of a 

 frog embryo are kept in lymph, the fibres can be seen under the micro- 

 scope to grow out from the nerve cell. 



It has been held by some observers that regeneration of the peri- 

 pheral part of a divided nerve can occur apart from any outgrowth 

 from the central end of the nerve, the process being called autogenetic 

 regeneration. The experiments which have just been described show 

 that this is not the case. 



The time taken for a nerve to regenerate and to become functionally 

 connected with the motor or sensory structures to which it was formerly 

 attached varies with the distance to be traversed by the outgrowing 

 fibres, and may be several months.- ^^fc 



