THE CEREBRUM. 



697 



Fig. 332. 



STRUCTURE OF THE CEREBRUM. 



The structure of the brain is certainly one of the most interesting points 

 in the study of the nervous centres; for on a perfect knowledge of it 

 depends the solution of the most difficult problems in the physiology of the 

 nervous system. Numerous attempts have been made to elucidate its 

 intimate organisation ; but we must here omit the multitude of secondary 

 details revealed by these researches, and limit ourselves to the essential 

 and fundamental facts. 



The two substances enter into the texture of the cerebral hemispheres, 

 and both are exactly disposed as in the cerebellum. 



The grey substance extends over the entire external surface of the brain, 

 and dips into the sulci ; thereby augmenting the extent of that surface, and 

 forming the cortical layer of the cerebral convolutions. This layer, it is 

 necessary to remark, though perfectly similar to that of the cerebellar 

 lobules, is not homogeneous throughout its thickness, but may be decom- 

 posed into several secondary stratified layers, between which are extremely 

 thin lamellae of white substance ; one of these lamellae nearly everywhere 

 forms the most superficial pellicle of the convolutions. 



According to Kolliker, there are six layers in the cortical substance of 

 the brain, and these are disposed as follows : 1, A superficial white layer ; 

 2, Grey layer; 3, First white streak; 4, Yellowish-red layer, external 

 portion ; 5, Second white streak ; 6, Yellowish-red layer internal portion. 

 In all these layers are nerve-cells, but in proportion as 

 they are pale, these cells are few and small ; the cells 

 themselves contain colouring matter in the reddish- 

 yellow layers. Everywhere they are furnished with 

 from one to five fine prolongations, which bring them into 

 communication with the very fine nerve-tubes of the 

 cerebral hemispheres. (Lockhart Clarke gives seven 

 layers for this cortical substance.) 



In the middle of each hemisphere, the white substance 

 constitutes a considerable nucleus, which, from its form, 

 is named the centrum ovale of Vieussens (Figs. 325, 1 ; 

 330, 1), and which is united to that of the opposite side 

 by the great cerebral commissure, or corpus callosurn, 

 sending a prolongation into each convolution ; thus ex- 

 hibiting the exact disposition of the lateral white masses 

 of the cerebellum, with which the nuclei of the hemi- 

 spheres have also another point of resemblance, in that 

 they are attached to the cerebral peduncles, as the first are 

 to the cerebellar. But the latter peculiarity is less evident CORTICAL SUBSTANCE 

 than the others which are at once obvious in horizontal or THE CEREBRAL 

 and transverse sections of the brain and can only be 

 clearly demonstrated by the manipulations necessary to a 

 unravel the intimate texture of the white substance. 



In studying this texture in brains hardened by nitric 

 acid, washed in pure water, and exposed to dry air for 

 a day or two, we perceive that the white cerebral sub- 

 stance is entirely composed of fine fibrous lamellae, 

 diverging in every direction, corresponding by their con- 

 centric extremity to the centre of the hemisphere, and abutting, by their 



HEMISPHERES. 



Medullary sub- 

 stance ; by Reddish- 

 grey layer ; c, Clear 

 white streak, com- 

 posed of horizontal 

 fibres -d, Grey layer ; 

 e, External white 

 layer. 



