THE LENTICULAR NUCLEUS 879 



of white substance which contains embedded within it certain masses of grey sub- 

 stance, the chief of which are known as the caudate and lenticular nuclei of the 

 corpus striatum. In addition, the hemispheres of the telencephalon overlie and 

 are in functional connection with the structures of the diencephalon below, the 

 chief of which are the thalamencephalon and the bases of 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 

 substance 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 [substantia corticalis] is distributed over the entire surface 

 of each hemisphere except the peduncular region of the base and the region of the 

 corpus callosum and fornix of the medial surface. Numerous measurements 

 have been made to determine its average thickness. These have shown that the 

 mantle 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 hemisphere, somsesthetic area, than at the poles; (3) 

 that in the average normal specimen 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 thickness 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 ante- 

 rior and posterior central gyri, in the somsesthetic area, while it attains its mimi- 

 mum 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 practically non- 

 nervous floor of the third ventricle and that of the chorioid fissure are very much 

 thinner but are not considered in these measurements. 



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

 type (fig. 604), 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 shghtly internal to this. They are known as the inner and outer stripes of Baillarger. In 

 addition, a thin, superficial or tangential layer of fibres may often be distinguished lying in the 

 surface 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 

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

 the cortex, have already been mentioned. 



The corpus striatum is so called on account of the appearance in section of 

 its component parts, 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. 691), and in addition they are connected by 

 numerous small bands of grey substance which pass from one to the other through 

 the internal capsule, especially its anterior part. Also each nucleus contributes 

 numerous fibres to, and receives fibres from, the internal capsule. These bundles 

 of fibres both arising and terminating within the nuclei, together with the grey 

 substance among the fibres of the capsule, produce the ribbed and striped appear- 

 ance suggesting the name, corpus striatum. The caudate nucleus — the intra- 

 ventricular part of the corpus striatum — hes with its thicker anterior part (head) 

 closely related to the internal capsule, but its tail passes posteriorly around the 

 posterior border of the capsule and carves downward and anteriorly into the roof 

 of the inferior cornu of the lateral ventricle. 



The lenticular nucleus [nucleus lentiformis] — the extraventricular part of 

 the corpus striatum — is embedded in the white substance of the cerebral hemi- 

 sphere. It is somewhat pyriform in shape, not being so long as the caudate 

 nucleus, and neither having a tail nor extending so far anteriorly. Its lower sur- 

 face is separated from the inferior cornu of the lateral ventricle by the white sub- 

 stance of the roof of that cornu, and by the tail of the caudate nucleus, and, fur- 



