GRAY MASSES AND NUCLEI.
In addition to the gray cortex, the substantia corticalis, other definite gray masses, known as the nuclei or ganglia of the end-brain, are found within the interior of the hemispheres. They are the nucleus caudatus, the nucleus lentiformis, the claustrum and the nucleus amygdalae, and are constituent parts of the stem of the telencephalon.
The nucleus caudatus forms the part of the corpus striatum that has been mentioned in connection with the lateral ventricle. The corpus striatum is divided by a traversing fibre-mass into two portions, a dorsal and medial one, the nucleus caudatus, and a lateral one, the nucleus lentiformis. The separating fibre-mass is the cafisula interna. The thickened- front end of the corpus striatum, that projects into the anterior horn of the lateral ventricle, and the narrow band, that extends backward through the pars centralis and into the inferior horn, belong to the nucleus caudatus. These are, therefore, more appropriately called respectively the caput and the cauda nuclei caudati, than the head and tail of the corpus striatum. The lateral edge of the dorsal surface of the caudate nucleus reaches the lateral margin of the lateral ventricle, its medial edge touches the stria terminalis, and its lateral surface lies against the internal capsule (Figs. 49 to 52).
The nucleus lentiformis, or nucleus lenlicularis, constitutes a mass, whose base is directed outward and the apex inward. It lies lateral and, at the same time, ventral to the nucleus caudatus and thalamus, separated from the latter by the internal capsule. In front and ventrally, the lenticular nucleus is directly continuous with the head of the nucleus caudatus. Dorsally, delicate gray stripes connect the two nuclei ; hence the designation " corpus striatum " applied to the nuclei conjointly. The nucleus lentiformis bounds the internal capsule laterally with its downward and inward sloping medial surface. Its slightly convex lateral surface is vertical and borders the
capsula extern, a thin white medullary lamella which is limited externally by a narrow scroll-like sheet of gray substance known as the claustrum. The ventral surface of the lenticular nucleus is horizontal and, in the middle part, continuous with the cortex of the substantia perforata anterior. Two thin medullary sheets, more or less parallel with the lateral surface, subdivide the lenticular nucleus into three segments. The outer one, the ,t5utamen, exceeds the others both in intensity of color and size. The inner segments are of paler color, smaller, and together form the gloaus pallidus.
In the internal capsule, which extends between' the nucleus caudatus and the thalamus on the medial side and the nucleus lentiformis on the lateral (Fig. 53), two limbs are distinguished, an anterior pars frontalis capsulae interne, between the caudate and lenticular nuclei, and a posterior limb, pars occipitalis capsulae interne, between the lenticular nucleus and the thalamus. The two limbs meet in a laterally opening angle known as the knee, the genu capsulae interne.
The claustrum constitutes a broad flattened nucleus, a narrow plate of gray sub stance, which ventrally is somewhat thickened and, more medially, joins the substantia perforata anterior. Its medial surface is smooth and bounds the thin capsula externa. The lateral surface presents small projections and borders a white medullary sheet, the capsula extrema, between the claustrum and the cortex of the island.
- The nucleus amygdalae lies beneath the lenticular nucleus in the extreme anterior segment of the temporal lobe. It is continuous with the cortex of the gyrus hippo campi and of the substantia perforata anterior.