434 



THE CENTRAL NERVOUS SYSTEM. 



H. THE NEUROGLIA. 



The neuroglia tissue is an especially differentiated supporting 

 tissue found in the central nervous system, the optic chiasm, optic 

 nerve and retina and for some distance, at least, in the olfactory 

 nerve. Its relation to other tissues has long been a matter of con- 

 troversy, but modern observers have shown quite conclusively that 

 neuroglia tissue is of ectodermal origin. It should not be under- 

 stood, however, that the neuroglia tissue forms the only supporting 

 tissue of the central nervous system. In all parts of the central 

 nervous system, more especially, however, in the spinal cord, there 

 is found true connective tissue of mesoblastic origin, more especially 

 in connection with the blood-vessels. 



At an early stage of embryonic development there are seen in 

 the spinal cord, and also in the brain, elements radially disposed 

 around the neural canal, which upon closer observation appear 

 to be processes emanating from the epithelial cells lining the neural 

 canal. These processes may undergo repeated dichotomous divi- 

 sion, ending finally in a swelling 

 near the periphery of the cord. 

 These cells are known as epen- 

 dymal cells, and are differenti- 

 ated from ectodermal cells, called 

 spongioblasts. In later stages 

 the radial arrangement is still 

 preserved, but the cell-bodies no 

 longer all border upon the cen- 

 tral canal, many being found at 

 varying distances from the latter. 

 At this stage in the development 

 of the spinal cord, the elements 

 retaining their original charac- 

 teristics are situated only in the 

 region of the ventral and dorsal 

 fissures of the spinal cord, and 

 during further development in- 

 crease in number. 



These observations would 

 seem to indicate that at least a 



portion of the neurogliar cells, which develop from the ependymal 

 cells previously mentioned, originate from the epithelium of the 

 central canal, and that from here they are gradually pushed toward 

 the periphery of the cord. This assumption is still further strength- 

 ened by the fact that later the epithelial cells of the central canal 

 still continue to divide. Later observations (Schaper, 97) show, how- 

 ever, that neurogliar cells develop also from certain undifferentiated 

 germinal cells of the neural canal, of ectodermal origin,, which 



Fig. 349. Neurogliar cells : <?, From 

 spinal cord of embryo cat ; 6, from brain of 

 adult cat ; stained in chrome-silver. 



