The lentiform nucleus (nucleus lentiformis) occupies the cone-like cavity of the internal capsule, by whose laminw it is separated from the ventricle (Fig. 39). It is shorter fore and aft than the caudate nucleus. It resembles a biconvex lens with a somewhat thickened anterior border, when viewed in hori zontal section (Fig. 38). In transverse vertical section through its center, it is triangular in shape. The hypotenuse and base are formed, respectively, by the superior and inferior laming of the internal capsule. The external capsule forms the per pendicular and separates the lentiform nucleus from the claustrum. The latter is a thin sheet of isolated gray matter, found just medial to the island (of Reil). In extent and posi tion, fore and aft, the island and lentiform nucleus coincide. The lentiform nucleus is subdivided by two white laminm, parallel with its external surface, into three zones. (Fig. 37). The outer zone, called the putamen, is deeply pigmented, and, like the caudate nucleus, is of a reddish-gray color; but the two inner zones, having less pigment, are of a pale yellowish tint. They form the globus pallidus. Anterior to the capsular laminm the lentiform nucleus fuses, in front, with the caudate nucleus and, below, with the nucleus arnygdalx.
The nucleus caudatus (the tailed nucleus) is a pear-shaped body of reddish-gray color, situated on the perimeter of the internal capsule (Figs. 37, 41 and 50). It is the intraventricular part of the striated body and forms a strip of the ventricular floor along the outer wall. The head (caput) of the caudate nucleus is directed forward. It is seen in the anterior horn of the lateral ventricle. From the head the nucleus tapers as it proceeds backward through the central part of the ventricle. Its tail (cauda) turns downward in the roof of the inferior horn, and ends in the nucleus amygdalce, while the head of the caudate nucleus fuses with the lentiform nucleus (Fig. 41). The caudate nucleus is covered on its ventricular surface by epen dyma. The opposite surface, resting against the fibers of the internal capsule, is irregular and serrated.
Along the ventricular floor the sulcus intermedius prosen cephali separates the caudate nucleus from the thalamus.
The nucleus amygdalae is the most inferior part of the striate body. It forms that part of the uncus anterior to the pars transversa of the dentate fascia and behind the rudimen tary gyri ciicumambiens and semilunaris. Superiorly it merges with the lentiform nucleus; posteriorly it joins the tail of the caudate nucleus and receives the stria terminalis, an olfactory fasciculus from the anterior perforated substance and septum pellucidum. The amygdalate nucleus constitutes a reflex center in which olfactory impulses excite the mechanism regulat ing movement (S. A. K. Wilson).
The stria terminalis (tenia semicircularis, Figs. 37 and 47) lies just medial to the nucleus caudatus in the sulcus intermedius. It is a band of white fibers traversing the floor of the central part of the ventricle and the roof of its inferior horn, but covered by the terminal vein and by the ependyma. The terminal vein is easily seen, but the stria terminalis is of microscopic size. The terminal vein and terminal stria are so called because they follow the boundary between the end-brain and the inter brain. The stria terminalis rises in the anterior perforated substance and the septum pellucidum; it partially decussates in the anterior commissure and ends in the nucleus amygdalm. It is a bundle of olfactory projection fibers. The vena ter minalis joins the chorioid vein and the vein of the septum pellucidum near the interventricular foramen; thus, the vena cerebri interne is formed.
The lamina chorioidea epithelialis is a single layer of epen dymal cells, derived from the roof-plate of the telencephalon; it stretches between the stria terminalis, in the sulcus inter medius, and the lateral border of the body and crus of the fornix. Like the roof-plate elsewhere it develops no neurones. It invests the lateral area of the superior surface of the thalamus for a distance of 4-6 mm. and the part attached to the thalamus is called the lamina affixa. The medial edge of the affixed portion constitutes the tania chorioidea. Between the tomia chorioidea and the fornix the epithelial lamina is invaginated into the lateral ventricle, producing the chorioidal fissure. A fold of pia mater, rich in blood-vessels, dips into that fissure and forms the chorioid plexus of the lateral ventricle. The chorioid epithelial lamina is seen to be made up of two parts: a lateral part affixed to the thalamus and resembling ependyma elsewhere and a medial part investing the chorioid plexus and possessing large cubical and cylindrical cells of specific function.
Thalamus (Figs. 37, 38, 47 and 5o).—A fusiform part of this ganglion of the inter-brain is visible in the floor of the lateral ventricle, between the stria terminalis and the chorioid plexus. It extends throughout the central part of the ventricle from the interventricular foramen to the inferior horn. A transparent layer of epithelium, the lamina chorioidea epithelialis, extending from the fornix to the stria terminalis and representing the hemi sphere wall, covers it; and the part called lamnia affixa is attached to it. The thalamus will be described with the third ventricle and inter-brain.