THE TELENCEPHALON. 1169 



The Internal Nuclei of the Hemisphere. 



Embedded within the white matter of each hemisphere and, for the most part, 

 completely separated from the cerebral cortex, lie certain masses of gray matter to 

 which the name basal ganglia is often applied. These include : (i) the caudate 

 nucleus, (2) the lenticular ynicleus, (3) the claustrum and (4) the amygdaloid yiucleus. 

 The first two, the caudate and lenticular nuclei, are parts of the corpus striatum, 

 one of the three fundamental divisions of the end-brain or telencephalon. Although 

 almost completely separated by the intervening tract of white matter, the internal 

 capsule, the caudate and lenticular nuclei are continuous for a limited distance below 

 and in front (Fig. 1008), and together constitute a large mass composed chiefly of 

 gray matter, that extends from the lateral ventricle almost to the cortex of the 

 insula. Between the latter and the lenticular nucleus lies a thin tract of gray 

 matter, the claustrum, whilst within the temporal lobe, above and in front of the 

 anterior extremity of the inferior horn of the lateral ventricle, is situated the 

 amygdaloid nucleus. 



The Caudate Nucleus. — This mass (nucleus caudatus), the inner division of 

 the corpus striatum, is well seen from the lateral ventricle, where it appears as the 

 large and conspicuous elevation which contributes the infero-lateral wall of the anterior 

 horn, and the outer part of the floor of the body of the ventricle. The caudate 

 nucleus is an elongated pyriform or comet-shaped mass of gray matter, whose bulky 

 rounded anterior end or head (caput nuclei caudati) rapidly diminishes into the 

 attenuated and recurved tail (cauda nuclei caudati), which sweeps backward and then 

 downward and forward within the roof of the inferior horn to the tip of the temporal 

 lobe, where it ends in relation with the lower part of the amygdaloid nucleus. 



The relations of its two chief surfaces, the mesial and lateral, are best seen in 

 frontal sections. When sectioned through its head near the anterior pole (Fig. 1007), 

 the caudate nucleus appears as an ovoid area of gray matter which mesially bulges 

 strongly into the lateral ventricle, but from which it is separated by the ependyma, 

 and laterally is embedded within the white matter of the hemisphere. In sections 

 passing a few millimeters farther back (Fig. 1009), the form of the nucleus has 

 become somewhat changed, its inner convex surface being more extensive and its 

 outer one, now somewhat concave, being serrated by the invasion of obliquely hori- 

 zontal stripes of white matter due to the appearance of the anterior strands of the 

 internal capsule. In the plane under consideration, these strands are not continuous 

 but interspersed with stripes of gray matter, which below still connect the caudate 

 with the laterally situated lenticular nucleus and produce the coarse striation from 

 which the entire mass, the corpus striatum, derives its name. 



In sections passing through the body of the ventricle (Figs. loio, 1025), from 

 the plane of the foramina of Monro backward, the caudate nucleus is much reduced 

 in size, whilst, on the contrary, the lenticular nucleus, as well as the thalamus, become 

 more conspicuous. The internal capsule, being now well established, appears as a 

 large oblique tract of white matter, which completely separates the two parts of the 

 corpus striatum and lies to the outer side of the thalamus (Fig. 1008). By reason 

 of the recurved course of its attenuated tail, in horizontal sections, as well as in frontal 

 ones passing in front of the splenium, the caudate nucleus is twice cut, one cross- 

 section of the nucleus appearing above in the lateral wall of the body of the ventricle 

 and the other in the roof of the inferior horn (Fig. 967). 



The Lenticular Nucleus. — This division of the corpus striatum (nucleus len- 

 tiformis) is a wedge-shaped mass of gray matter, broken by laminae of white, that lies 

 bordered by the internal capsule mesially, and laterally is separated from the cortex 

 of the insula by a narrow tract of white matter containing a thin stratum of gray sub- 

 stance, the claustrum. The lenticular nucleus reaches neither as far forward nor as 

 high as the caudate nucleus, and lies lateral to both the latter and the thalamus, 

 separated from them respectively by the anterior and posterior limbs of the internal 

 capsule. Its dorso-mesial surface, when seen in frontal sections, is directed from 

 above downward and inward ; in transverse sections (Fig. loii) this surface is 

 replaced by an antero-mesial and a postero-mesial face in correspondence with the 

 limbs of the internal capsule. Its slightly convex lateral surface is approximately 



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