3. The nucleus of the habenula belongs to the epithalamus (Fig. 87). It lies beneath the trigonum habenula. It receives fibers from the rhinencephalon through the medullary stria of the thalamus, and originates a bundle of fibers, the fasciculus habenulo-peduncularis or retroflexus (Meynerti), which may be traced back through the tegmentum to the interpeduncular nucleus in the substantia nigra. Beyond this, connections are probably established with the motor nuclei of cerebral nerves.
4. The arcuate nucleus (semilunar nucleus) is a small crescent of gray substance in the ventral part of the thalamus, just medial to the main internal medullary lamina. The inferior lamella of that lamina separates it from the outer half of the inferior sur face of the medial nucleus, in a frontal section through the ante rior part of the red nucleus; but, farther backward, the central nucleus develops in that inferior lamella, delaminating it and further separating the arcuate from the medial nucleus. Pos teriorly, the arcuate nucleus ends slightly before the central and red nuclei.
5. The central nucleus (centrum medianum Luysi) lies be tween the arcuate and medial nuclei and is intermediate in size, smaller than the medial but larger than the arcuate nucleus. It extends backward beyond the limit of the medial nucleus, into the sections through the pulvinar.
The arcuate and central nuclei contain only internuncial neurones and yet the cell-bodies and axones are of medium size. The axones terminate almost wholly in the lateral nucleus; but the arcuate nucleus associates all thalamic nuclei except the anterior, and the central connects all except the anterior and medial nuclei. Rubro-thalamic fibers and fibers of the brach ium conjunctivum terminate in the central nucleus and, prob ably, other afferent fibers end in these nuclei.
6. The lateral nucleus is the largest. It extends from supe rior to inferior surface the entire length of the thalamus. It also fuses with the nucleus of the pulvinar. It forms the terminal nucleus for the larger part of the tegmental fibers, especially of the medial fillet, the spino-thalamic tract, the gustatory tract, a part of the medial longitudinal bundle, the rubro thalamic fibers* and fibers of the brachium conjunctivum of the cerebellum; and it constitutes the nucleus of origin for most of the fibers of the cortical fillet. The lateral nucleus also receives
cortico-thalamic fibers from nearly every projection area of the cerebral cortex. The neurones of the lateral nucleus are, the greater number, of medium size, though some are small and a few are of large size. They fall into three groups, the inter nuncial, the thalamo-cortical and the thalamo-spinal. Fine and medium internuncial fibers are contributed to the arcuate and central nuclei, from which and the medial nucleus the lateral nucleus receives internuncial fibers. The thalamo cortical fibers are numerous; they enter the cortical fillet and terminate in the posterior part of the frontal gyri, the central gyri, the fronto-parietal operculum, the paracentral gyrus and, probably, in the gyrus cinguli. By far the greater number end anterior to the central sulcus (Ernest Sachs); many of these anterior fibers are of medium size, though there are a few coarse and fine fibers interspersed. Many fine and a few medium fibers terminate behind the central sulcus. The thalamo spinal fibers are traced by Sachs from the extreme ventral part of the lateral nucleus (ventral nucleus) downward along the medial longitudinal bundle. He describes collaterals to cra nial nuclei (as the V, VI, X). Sachs found no other descending fibers; but J. S. Collier has traced a tract, in the cat, from the thalamus down the lateral column of the spinal cord. It descends with the rubro-spinal tract.
Five paths for common sensory impulses from thalamus to cerebral cortex (Head and Holmes). These paths carry: (r) Impulses of posture and passive movement (muscle sense) and tension impulses, enabling one to estimate lifted weights. (2) Impulses of light touch and pressure touch (tactile sensibility) making it possible to estimate weights on supported hands. (3) Impulses of tactile or spacial discrimination produced by two or more simultaneous contacts. These underlie recogni tion of size, shape and form in three dimensions. (4) Impulses localizing successive points of contact, tactile localization. (5) Thermal impulses discriminating between degrees of heat and cold.