THE CENTRAL NERVOUS SYSTEM. 319 



Since the so-called fifth ventricle is really a cut-off portion of the 

 great longitudinal fissure, those surfaces directed towards the cleft 

 correspond to the free surface of the hemispheres ; the rudimentary 

 layer of gray matter, therefore, forms the immediate lateral boun- 

 daries of this space, and is homologous with the cortex of other 

 regions, while the thin white stratum next the lateral ventricles 

 represents the medulla. 



The mesially-placed gray cortex of the septum lucidum con- 

 tains a thin superficial stratum of medullated nerve-fibres next 

 the interseptal cleft ; following this lies a layer of gray matter con- 

 taining many small pyramidal cells (16-18 p.\ the apical processes 

 of which are directed towards the surface homologous with the pe- 

 riphery of the hemispheres ; the deeper zone of the gray matter 

 exhibits spindle-cells. The white substance of the hemisphere is 

 represented by the thin stratum of medullated fibres interposed 

 between the gray layer and the ependyma of the lateral ventricle. 



The blood-vessels supplying the cerebral cortex, after a short 

 course within the pia almost parallel to the free surface, enter the 

 nervous tissue vertically ; the larger arteries pierce the gray matter 

 and enter the medulla, while those of smaller size break up 

 within the gray cortex into capillary net-works. The law, appli- 

 cable to all parts of the nervous system, that regions rich in large 

 nerve-cells are plentifully supplied with blood-vessels, is illustrated 

 by the distribution of the capillaries within the cortex, where a rich 

 capillary net-work exists within the layer of large pyramidal cells, 

 while the outer cortical zones, on the contrary, possess only a meagre 

 capillary supply : the net-works within the deepest layers of the 

 cortex are intermediate in the closeness of their meshes. The blood- 

 vessels are surrounded by perivascular lymph-spaces, the pial 

 tissue accompanying the vessels as a delicate sheath attached to the 

 adventitia and enclosing a prolongation of the subarachnoidean 

 space. 



The corpus striatum consists of the special masses of gray matter, 

 the nucleus caudatus and the nucleus lenticularis, and their 

 associated tracts of nerve-fibres. 



The ventricular surface of the nucleus caudatus is covered 

 by a well-developed layer of ependyma, beneath which lies the 

 zone of gray substance, containing nerve-cells of two kinds : large 

 multipolar cells (25-30 ^), and much more numerous smaller 

 ganglionic elements, whose size is about half that of the former. 

 The outer surface of the caudate nucleus, directed towards the 

 internal capsule, is broken up by numerous bundles of fibres, which 

 penetrate deeply into the gray mass and produce the characteristic 

 white striae exhibited on section. 



