NERVE 77 



corpuscles proliferate and absorb the remains of the white 

 sheath, so that nothing is left but the primitive sheath filled 

 by nucleated protoplasm. 



Into this, axons may grow downwards from the central 

 end of the nerve, and regeneration may occur. This 

 generally begins after about forty days and is Avell marked 

 after about one hundred davs. 



Some investigators have maintained that regeneration 

 occurs by the development of new fibrils in the degenerated 

 nerve itself, but the mass of evidence indicates that, when 

 an apparent peripheral regeneration has occurred, it has 

 been due to the ingrowth of axons from adjacent cut 

 nerves. 



The neurolemma with its nuclei appears to be essential for 

 regeneration. It is not present in the white fibres of 

 the central nervous system, and hence regeneration does not 

 occur there after the nerve fibres have been severed and 

 have degenerated. 



(6) The cell is dependent for its proper nutrition 

 upon the condition of the rest of the neuron. When the 

 axon is cut, the chromatin of the cell nucleus slowly 

 decreases, and the nucleus becomes displaced to one side, 

 and ultimately the whole cell may degenerate. This is 

 sometimes called Nissl's degeneration (see fig. 19, h). 



B. ACTION OF NEURONS IN SERIES. 



So far, the physiology of single neurons, or of neurons 

 running side by side in nerves, has been considered. But 

 in the nervous system, as already indicated, they are 

 arranged in chains or series, the activity of one set leading 

 to changes in other sets (fig. 17, p. 53). 



The apparently inextricable labyrinth of neurons running 

 throughout the nervous system may be arranged in three 

 groups or arcs, according to their distribution. 



Each arc consists of ingoing neurons, starting from 



