760 THE NERVOUS SYSTEM. 



caudatus. It is bounded externally by a lamina of white matter called the exter- 

 nal capsule, which is covered on its outer surface by a thin layer of gray matter 

 termed the claustrum. The claustrum presents ridges and furrows on its outer 

 surface, corresponding to the convolutions and sulci of the island of Reil, with 

 the white matter of which it is in immediate relation. 



Antero-inferiorly the ends of the two nuclei of the corpus striatum are united 

 by a thin gray lamina which appears at the base of the brain in the anterior 

 perforated space. The caudate nucleus terminates, after running downward and 

 forward in the roof of the descending cornu, in the nucleus amygdalae, a collec- 

 tion of gray matter in the apex of the temporal lobe. The base of the claustrum 

 is also in connection with this nucleus. 



The gray matter (Fig. 449) of the corpus striatum is permeated by tracts of 

 medullated nerve-fibres, some of which probably originate in it. The nerve-cells 

 are multipolar, both large and small, the larger being principally found in the 

 lenticular nucleus. 



On section, the substance of the corpus striatum appears of reddish-gray 

 color. On a transverse vertical section, the lenticular nucleus shows two laminae 

 of white matter parallel with its outer surface, forming three areas of gray matter, 

 the two inner of which are knoAvn as the globus pallidus, the outer as the putamen 

 (Fig. 447). The fibres of the nucleus enter and leave it, the former chiefly derived 

 from the ansa lenticularis (see page 747), the latter proceeding into the internal 

 capsule and corona radiata, which last is made up of the radiating diverging 

 fibres of the upward prolongation of the internal capsule which extend to the 

 cortex. 



The internal capsule is formed by fibres of the crusta of the crus cerebri, sup- 

 plemented by fibres derived from the optic thalamus and corpus striatum on each 

 side. In horizontal section it is seen to be somewhat abruptly curved, with its 

 convexity inward ; the prominence of the curve is called the genu, and projects 

 between the intraventricular portion of the corpus striatum and the optic thal- 

 amus (Figs. 447, 448, 450). In front of the genu the internal capsule separates 

 the two portions of the corpus striatum ; behind, it lies between the optic thal- 

 amus and lenticular nucleus. The portions of the internal capsule, anterior and 

 posterior to the genu, are known, respectively, as the anterior and posterior 

 segments. The fibres of the former proceed to the prefrontal region of the 

 cortex ; of the latter, to the occipito-temporal region ; while those of the mid- 

 dle third go to the Rolandic region (motor) of the cortex. Other fibres, in 

 the internal capsule, than those of the crusta are derived from the nuclei of the 

 corpus striatum, the optic thalamus, the subthalamic tegmerital region, and from 

 the cortex of the opposite side through the corpus callosurn (see also page 785). 



The taenia semicircularis (Figs. 445, 447, and 461) is a narrow, whitish band 

 of medullary substance situated in the depression between the nucleus caudatus 

 and optic thalamus. Anteriorly, it descends, between the head of the caudate 

 nucleus and the anterior extremity of the optic thalamus, to join the anterior 

 pillar of the fornix, below the level of the foramen of Monro, where most of the 

 fibres continue with those of the pillar, while the remainder pass over the anterior 

 commissure and terminate in the gray matter of the anterior perforated space. 

 Behind, it is continued into the roof of the middle or descending horn of the 

 lateral ventricle, lying parallel with the caudate nucleus, to enter, with it, the 

 nucleus amygdalae. Beneath it is a large vein (vena corporis striati], which 

 receives numerous small veins from the surface of the corpus striatum and optic 

 thalamus, and joins the venae Galeni. On transverse vertical section the tcenia 

 is seen to lie upon a projection from the internal capsule. 



The fornix (Figs. 442, 447, 451) is a longitudinal band of white matter situ- 

 ated beneath the corpus callosum, with which it is continuous behind, but sep- 

 arated from it in front by the septum lucidum. It may be described as consisting 

 of two symmetrical halves, one for either hemisphere. These two portions are 

 joined together in the middle line (along which is attached the lower edge of the 



