THE NUCLEI OF THE DIENCEPHALON.
The Thalamus. The thalamus consists of three chief nuclei, the nucleus anterior, the nucleus medialis and the nucleus lateralis, which are imperfectly separated from one another by white medullary stripes, the laminae medullares.
The nucleus anterior includes the front and dorsal portion of the thalamus ; it is, therefore, also known as the dorsal nucleus. It penetrates wedge-like between the medial and lateral nuclei, is covered dorsally by the stratum zonale, and rests ventrally upon a bifurcation of the lamina medullaris interns. The thickened front end produces the protuberance on the dorsal surface of the thalamus known as the tuberculum anterius or corpus album subrotundum.
The nucleus medialis is bounded laterally by the lamina medullaris interna and medially by the central gray substance, a sheet of gray matter which invests the floor of the third ventricle and the medial surface of the hypothalamus and also forms the mas.sa intermedia or middle commissure. Anteriorly the medial nucleus is closely con nected with the nucleus anterior, although it does not reach the front end of the thalamus ; hence, in a series of vertical sections carried through the brain, from before backward, the medial nucleus first appears after the anterior nucleus begins to diminish. Behind, the medial nucleus passes into the pulvinar.
The nucleus lateralis, the largest of the thalamic nuclei, includes the upper and lateral portion of the thalamus and surrounds, in large part, the anterior and medial nuclei. Its medial boundary is formed by the lamina medullaris interna ; laterally it is bounded by the posterior limit of the internal capsule, from which it is separated by the lamina medullaris externa and the stratum reticulare. The dorsal surface of the nucleus is covered by the stratum zonate and assists in forming the dorsal surface of the thalamus. The lateral part of this last-named surface is clothed, by the eiiendyma of the lateral ventricle and contributes that portion of the floor of the ventricle known as the lamina affixa ; the medial part of the same surface belongs to the external surface of the diencephalon and is covered by the ventral sheet of the tela chorioidea. The ventral surface of the nucleus lateralis rests upon the regio hypothalamica. In front, the lateral nucleus aids the anterior one in defining the foramen interventriculare ; behind it passes into the pulvinar.
The lamina medullaris externa covers the entire outer surface of the lateral nucleus and in the region of the pulvinar broadens into a triangular medullary area, known as Wernicke' s field (Fig. 207).
The stratum reticulare, or the lattice layer, forms the real outer limit of the thalamus and constitutes a thin lamella of gray substance that invests the entire outer surface of the lateral nucleus and of the pulvinar, separating the latter from the internal capsule.
As special nuclei of the thalamus are to be noted the centrum medianum and the nucleus semilunaris, the latter being also known as the corpus fiatellare.
The centrum medianum (Luys) belongs to the nucleus medialis and presents a rounded mass of gray substance that is lodged between the medial and lateral nuclei and the pulvinar. Laterally it is bounded by the lamina medullaris interna, medially
it blends with the nucleus medialis (Fig. 207).
The nucleus semilunaris (Flechsig) belongs to the nucleus lateralis, in whose ventral part it lies, and leans against the centrum medianum in the form of a crescent.
Additional special nuclei of the diencephalon are : the nucleus habenulae or ganglion habenulae, within the trigonum habenulae, and the nucleus corporis geniculati medialis and lateralis, within the corresponding geniculate bodies.
Ventral to the thalamus, the regio subthalamica or the hypothalamus spreads out between the internal capsule and the central gray substance of the third ventricle.
Within each corpus mamillare lie two nuclei, a larger round nucleus medialis and a smaller nucleus lateralis, which arches around the medial nucleus and includes the front and outer part of the mammillary body (Fig. 201). Close to these two nuclei, at the lateral and ventral side of the nucleus lateralis, is found a small nucleus accessorius. The nucleus hypothalamus, or corpus subthalamicum (Luys), lies within the hind part of the hypothalamus. This lentiform nucleus lies beneath the nucleus lateralis thalami and medially to the globus pallidus of the lenticular nucleus (Fig. 64). The Capsula Interna. Let us turn once more to the internal capsule. It lies between the nucleus lenticularis, on the one side, and the nucleus caudatus and the thalamus on the other. In frontal sections, it appears as a lamella of white substance that runs obliquely from above downward and inward, bounded externally by the lenticular nucleus and medially by the caudate nucleus, the thalamus and the subthalamic region (Fig. 67). An upfier and a lower region may be distinguished in the internal capsule. The upper region, between the lenticular nucleus on the one side and the caudate nucleus and thalamus on the other, is known as the regio Ihalamica capsulae interne. The lower region lies between the nucleus lenticularis and the hypothalamus and is the regio subthalamica capsulae interne.
In horizontal sections (Fig. 68), the internal capsule forms, in the region of thalamus, an outwardly opening angle with a shorter anterior limb, pars fronlalis, lodged between the lenticular and caudate nuclei, and a longer posterior limb, pars occzitalis, between the lenticular nucleus and the thalamus. The two limbs come together at the knee, genu capsulae inlernae. The anterior limb of the capsule is also called the pars lenticulo-caudata, the posterior one the pars lenticulo-thalamica. The hind limb extends some millimeters beyond the nucleus lenticularis, this part constituting the pars retrolenticularis.
The relations are different in horizontal sections passing through the subthalamic region. Here, the posterior limb and the pars retrolenticularis of the internal capsule alone are seen, the anterior limb having disappeared. These relations are readily under stood, when we recall that in the front part of this region the nucleus lenticularis is continuous with the head of the nucleus caudatus, from which it follows, that in the subthalamic region the anterior limb must disappear between the lenticular and caudate nuclei (Fig. 48).