856 THE NERVOUS SYSTEM 



terminalis of the thalamus (tsenia semicircularis). The end of its tail extends for- 

 wards below to the level of the anterior horn of the ventricle above. Owing to its 

 much curved shape, both horizontal and vertical sections of the hemisphere passing 

 through the inferior horn may contain the nucleus cut at two places (see fig. 640). 

 The caudate nucleus is the intra ventricular of the two masses of grey substance which 

 together are sometimes referred to as the basal </ lu/lia . The extra ventricular of these 

 masses is the lenticular 'nucleus, which is buried in the substance of the hemisphere, 

 laterally and below the caudate nucleus. The two masses are separated by the inter- 

 nal capsule, a thick band of nerve-fibres continuous into the cerebral peduncles, and 

 connecting the grey cortex of the hemisphere with the structures below it. Anteriorly 

 and below, the two nuclei become continuous and the white substance of the internal 

 capsule, in separating them posteriorly, contributes to their striated appearance in 

 sections, whence they are known collectively as the corpus striatum (figs. 638 and 641). 

 The corpus striatum as such is described below. 



Internal structure of the prosencephalon. From the above examinations of 

 their external and ventricular surfaces, it is apparent that the cerebral hemi- 

 spheres consist of a folded, external coating of grey substance, the cortex cerebri, 

 spread more or less evenly over an internal mass of white substance which contains 

 embedded within it certain masses of grey substance, the chief of which are known 

 as the basal ganglia or the caudate and lenticular nuclei of the corpus striatum. In 

 addition, the hemispheres of the telencephalon overlie and are in functional connec- 

 tion with the structures of the diencephalon below, the chief of which are the thala- 

 mencephalon and the cerebral peduncles. 



The grey substance of the telencephalon. The grey substance is in intimate 

 relation with the white substance, and in fact its cells give origin to the greater 

 part of the fibres composing the white substance. The accumulations of grey sub- 

 stance to be considered are the cerebral cortex, with its variations in thickness and 

 arrangement, the corpus striatum, the claustrum, and the amygdaloid nucleus. 



The cerebral cortex is distributed over the entire surface of each hemisphere ex- 

 cept the peduncular region of the base and the region of the corpus callosum and for- 

 nix of the medial surface. Numerous measurements have been made to determine 

 its average thickness. These have shown that it is not uniformly distributed: 



(1) that it is thicker on the convex surface than on the basal and medial surfaces; 



(2) that on the convex surface it is thicker on the central region of the hemi- 

 sphere, somsesthetic area, than at the poles; (3) that in the average normal speci- 

 men it averages somewhat thicker on the left than on the right hemisphere; (4) that 

 its average thickness varies greatly in different individuals, and that the thick- 

 ness decreases with old age ; (5) that it is probably somewhat thicker in males than in 

 females, and (6) that in a given specimen it averages thicker on the summits of the 

 gyri than in the floor of the corresponding sulci. In the normal adult conditions it 

 averages about 4 mm. thick on the anterior and posterior central gyri, in the somKS- 

 thetic area, while it attains its mimimum thickness of about 2'5 mm. on the basal 

 surface of the occipital and frontal lobes. Its total average thickness is about 

 2'9 mm. 



The cortex consists of layers of the cell-bodies of neurones, chiefly of the pyramidal type 

 (fig. 555), which receive impulses from the structures below and from other regions of the cortex 

 by way of fibres reaching them through the internal mass of white substance, and which in turn 

 contribute fibres to the white substance. Certain fibres of shorter course and numerous collateral 

 branches of fibres passing out of the cortex are devoted to the association of the region of their 

 origin with the cortex of the immediate vicinity of their origin, and most of these course within 

 the grey cortex itself. In certain gyri, such as the anterior central gyri and those of the medial 

 surface of the occipital lobe, these short association fibres accumulate into strata, and in vertical 

 sections give the cortex a stratified appearance. Two such strata of white substance may be 

 noted in the above localities, one lying about midway in the thickness of the cortex and one 

 slightly internal to this. They are known as the inner and outer stripes of Baillarger. In addi- 

 tion, a thin, superficial or tangential layer of fibres may often lie distinguished lying in the outer- 

 most region of the cortex. Transverse sections through the anterior end of the hippocampus 

 show a coiled arrangement of the layers of white substance, to which has been given the name 

 cornu ammonw. The peculiar structure and appearance of the olfactory bulb and tract, parts of 

 the cortex, have already been mentioned. 

 



The corpus striatum is the name given to the appearance in section of the 

 caudate and lenticular nuclei (basal ganglia) and the internal capsule between them. 

 The two nuclei are directly continuous with each other at their anterior ends (fig. 637), 

 and in addition they are connected by numerous small bands of grey substance which 



