The thalamus (Figs. 5o, 87 and 88) should be restudied on p. 136. It was pointed out by Burdach that the thalamus is composed of two great parts, a medial and a lateral; and this fact is further emphasized by the recent studies of Ernest Sachs and others. The medial part includes the anterior, medial and habenular nuclei; the two former are closely linked to the cau date nucleus and all three are connected with the sense of smell. The lateral part of the thalamus embraces the lateral, arcuate and central nuclei the nucleus of the pulvinar; it receives the common sensory and taste paths and a part of the visual path, and is connected with the globus pallidus and cerebral cortex (Brain, Vol. 32). According to Head and Holmes the thalamus is more than a relay in the common and special sensory Baths. It is also an organ of consciousness for impulses of pain, temperature and state of being, and a part of the mechanism through which the cerebral cortex exercises an inhibitory con trol over afferent impulses. Hence, cortical lesions never abolish sensations of pain and temperature and cortical isolation of the thalamus is accompanied by greatly exaggerated response to painful stimuli (Brain, Vol. 34).
The thalamus is made up of six definite nuclei, of the internal medullary lamina separating them, the stratum zonale forming its superior and medial surfaces, and another sheet of medullated fibers, the zona lateralis, forming its lateral surface. The nuclei are the anterior, medial, habenular, arcuate, central (centrum medianum), the lateral and the nucleus of the pulvinar, which is a posterior prolongation of the lateral nucleus. The internal medullary lamina, which intervenes between the nuclei, is a curved sagittal sheet of fibers, regular and convex on its lateral surface; it separates the lateral nucleus from the other nuclei and, by three lamellar branches extending toward the median plane, it separates the remaining nuclei from each other. The lamella; vary in different regions. There are two main lamellae, a superior and an inferior, the inferior divides into two in its posterior part. The undivided vertical internal lamina sepa rates the anterior and lateral nuclei in front. Farther back ward, at the transverse plane cutting the anterior edge of the mammillary bodies, the medial nucleus appears just below the anterior nucleus and the superior lamella separates them. The lamina here has a Y-form. The inferior lamella is first seen between the medial and arcuate nuclei in the frontal plane cutting the anterior part of the red nuclei; this lamella trends ventro-medially. Before the section cutting the largest part of the red nucleus is reached, this inferior lamella delaminates to inclose the central nucleus (centrum medianum). Now,
four nuclei appear one above the other separated by three lamella': the arcuate, central, medial and a very thin zone of the anterior nucleus at the top. As the sectioning proceeds backward, the anterior and arcuate nuclei first disappear, then the medial nucleus and, finally, the central and red nuclei drop out together in sections through the pulvinar. The nucleus of the pulvinar is directly continuous with the lateral nucleus; but, as it has a special function, it is convenient to give it a separate name.
The several nuclei contain small and medium-sized multi polar neurones, the larger neurone-bodies being in the lateral nucleus. From the lateral nucleus some thalamo-cortical and thalamo-fugal axones originate which are of large caliber; all other thalamic axones are small or medium in size.
1. The nucleus of the anterior tubercle (Fig. 56) receives the fasciculus mammillo-thalamicus (Vicq d'Azyri) from the corpus mammillare and is thus connected with the columna of the for nix and it also receives olfactory fibers from the anterior per forated substance (Fig. 52). The anterior nucleus contains two sets of neurones, the thalamo-caudate and the internuncial, whose axones are very delicate. There are very many thalamo caudate axones; they end in the caudate nucleus, losing their medullary sheaths as they enter it. The internuncial axones are few in number; they connect the anterior nucleus with the outer part of the medial nucleus. As no fibers are found having any other destination, the anterior nucleus is intermediate be tween the mammillary body and the caudate nucleus.
2. The medial nucleus is joined to the opposite medial nu cleus by the massa intermedia and is continuous with the hypothalamic gray matter in the wall and floor of the third ventricle; but the internal medullary lamina with its superior and inferior lamellw separates it from the other nuclei of the same thalamus. The medial nucleus possesses thalamo-caudate and internuncial neurones. The cell-bodies are small and the axones are fine. The thalamo-caudate fibers ascend in the lamina medullaris interna and break up into fine branches which end in the caudate nucleus. Some internuncial fibers pursue the same course and terminate in the dorsal part of the lateral nu cleus; a few run toward the massa intermedia, but not through it; and a number run ventro-laterally to the central nucleus. The medial nucleus receives fibers from both brachia conjunctiva.