NEUROGLIA. 



209 





covered apparently by a neurilemma which exhibits numerous 

 nuclei. The axis-cylinders are similar to those of medullated 

 fibres, except that they freely divide 

 and anastomose with one another, FlG ; 79 - 



forming plexuses. On their surface 

 at intervals rest nuclei ; these prob- 

 ably belong to a neurilemma, which, 

 however, is so thin and delicate as to 

 be difficult to distinguish. 



Nerve-centres. 



The nerve-centres are the cerebrum, 

 cerebellum, and spinal cord, making 

 up the cerebro-spinal structures ; and 

 the numerous ganglia in different parts 

 of the body. 



The characteristic elements making 

 up the central nervous structures are 

 nerve-cells and processes, nerve-fibres, 

 and a peculiar sustentacular tissue 

 called neuroglia. 



The pia mater also contributes a 

 certain amount of ordinary connec- 

 tive tissue to the sustentacular frame- 

 work. 



Through differences in the distri- 

 bution of these elements two parts 

 are distinguishable in the substance 

 of the cerebro-spinal structures, gray 

 matter and white matter. 



The gray matter consists of neu- 

 roglia, nerve-cells, ramifying den- 

 d rites, and scattered nerve-fibres, with and without medullary 

 sheaths. 



The white matter is an almost solid mass of parallel medul- 

 lated fibres, supported by neuroglia. 



Neuroglia, the chief sustentacular tissue of the cerebro- 

 spinal structure, "is a sort of connective tissue, but is of 

 epiblastic origin and derived from epithelial elements. It 

 14 Hist. 



Non-medullated nerve-fibres 

 (Key and Retzius). n, nu- 

 clei of neurilemma; m, a 

 fibre which has a partial 

 medullary sheath. 



