174 THE CEREBRUM. 



larger meshes of the connective tissue network. There are three 

 varieties of neuroglia cells, viz., (a) the short rayed cells, found 

 in the cortex of cerebellum and cerebrum, the processes of which 

 are strong and richly branched; (b) the long rayed cells, located 

 chiefly in the white substance, whose processes are long and fine 

 and but little branched; and (c) the arborescent cells, which lie 

 near the surface in the cortex. Upon reaching the surface the 

 tree-like branches form a limiting membrane of neuroglia. 



(B) The columnar ciliated ependymal cells which line the 

 ventricles are also epiblastic in origin and should be classed with 

 the neuroglia cells. They form the only sustentacular tissue 

 present in the neural tube when it is first formed, at which time 

 they extend from the ventricular to the exterior surface of the 

 tube and, in the peripheral zone, form a rich supporting network. 

 They appear to be of little importance in the adult condition. 



(2)' Connective Tissue Network. That is of mesoblastic origin 

 and is formed by branching processes from the inner surface of 

 the pia mater. It transmits the blood-vessels into the nervous 

 substance. 



The neurones constitute 53 per cent, of the brain and cord 

 (cell-bodies, 6 -per cent.) and the sUstentacular tissue 47 per cent. 

 (Donaldson). 



The white matter of the cerebro-spinal axis is made up chiefly 

 of bundles of medullated axones imbedded in neuroglia and 

 supported by connective tissue. The fibers possess no neuri- 

 lemma. 



The gray matter of the central nervous system is composed of 

 cell-bodies and dendrites, chiefly, but also contains axones. These 

 nerve elements are supported by connective tissue and blood-vessels 

 and are imbedded in a great abundance of neuroglia. The 

 nerve fibers in the gray matter are to a large extent non-medul- 

 lated and naked. 



The cerebral gray substance (substantia grisea cerebri) is con- 

 veniently divided into three groups or classes : 



I. Cortical. 

 II. Ganglionar. 



III. Central, or ventricular. 



