146 



THE TISSUES 



followed by disintegralion and disappearance of the chromophilic bodies begin- 

 ning near the centre of the cell, and a displacement of the nucleus toward the 

 periphery. This reaction on the part of the cell body to injury to its axone — 

 " central chromatolysis " and " nuclear eccentricity " — is sufficiently characteristic 

 to have led to the designation "axonal degeneration" (Fig. 89). 



If the neurone body survive the injury regeneration takes place. This con- 

 sists in a reappearance of the chromophOic substance, beginning near the nucleus, 

 a return of the nucleus to its normal position, and a slow subsidence of the tur- 

 gescence. While this is going on the axones of the central stump grow across 

 the scar to the protoplasmic bands beyond formed by the neurilemma cells of the 

 peripheral stump, and find their way along or in them to their former termina- 

 tions. According to some authorities the neurilemma cells can do more than 

 this and form, in young animals at least, new nerve fibres which, however, do not 

 persist unless they form connections with the central stump. The outgrowth 

 (centrogenetic) theory seems more probable than the latter (autogenetic) theory. 



A B 



Fig. 90. — A, Neuroglia Cell — Spider Type — Human Cerebrum. 



— Mossy Type — Human Cerebrum. 



B, Neuroglia Cell 



The rapidity of the above degenerative and regenerative changes varies 

 according to the metabolic activity of the animal, i.e., they are much more rapid 

 in warm-blooded than in cold-blooded animals, and in summer than during 

 hibernation. They are also influenced by the age of the- animal. 



The importance of these degenerations from the standpoint of anatomy lies 

 in the fact that, by using such methods as Nissl, Weigert, and Marchi, one is 

 enabled to trace the connections between cell bodies and nerve fibres throughout 

 the nervous system. 



Neuroglia 



This is a peculiar form of connective tissue found only in the 

 nervous system. Unlike the other connective tissues, neuroglia 

 is of ectodermic origin, being developed from the ectodermic cells 

 which line the embryonic neural canal. These cells, at first mor- 

 phologically identical, soon differentiate into neuroblasts or future 



