THALAMENCEPHALON.
The thalamus opticus (Figs. 56 and 57) presents an ovoid mass of gray sub stance, with the thicker end behind. Its dorsal and medial surfaces are free, while its lateral and ventral ones are fused with the neighboring structures. The dorsal surface is slightly convex and covered by a thin layer of white fibres, the stratum zonale. The outer limit is formed by the stria terminalis, lodged within the sulcus intermedius ; the medial boundary is a white stripe, the stria medullaris, which indicates the boundary between the dorsal and medial thalamic surfaces. A furrow, the sulcus chorioieleus, runs from before backward and outward and lodges the plexus chorioideus of the lateral ventricle (Fig. 57). At its front end, the dorsal surface exhibits a small round elevation, the tuberculum anterius thalami ; behind is a similar projection, the pulvinar. The stria medullaris, the medial boundary, widens behind into a triangular field, the trigo num habenulae. From the latter proceeds medially a white fibre-strand, the haben ula, which in front joins with the habenula of the opposite side to form the commissura habenularum, while behind it passes into a flattened structure, the corpus pineale. Medially the stria medullaris is continuous with the lamina chorioidea epithelialis, over which spreads out the tela chorioidea. On removal of the latter, the epithelial layer is sepa rated from the stria medullaris. There remains, however, along the line of transition a delicate border, the taenia thalami, which behind adheres to the dorsal surface of the habenula and the pineal body and is continuous with the taenia of the opposite side.
The medial surface of the thalamus is vertical and contributes the lateral wall of the third ventricle. Its lower limit is indicated by the sulcus hypothalamicus or sulcus Monroi, that leads from the foramen interventriculare to the entrance of the aquaeductus cerebri. The median surfaces of the two thalami are united, about the middle, by the masses inkrmedea, often called the middle commissure. The ventral surface of the thalamus borders on the hypothalamus, the lateral surface on the capsula interna (Fig. 58).
Behind the commissura habenularum lies the corpus pineale, so called on account of its resemblance to a pine-cone. It extends from an outpouching of the dorsal brain-wall, the most posterior part of the roof of the third ventricle, and is a small unpaired body, whose base is directed forward and the apex backward. In its anterior part, at the base and between the upper and lower lies the small evagination from the third ventricle, termed the recessus pinealis. The upper lamella is continuous on each side with the habenula, the commissura habenularum forming the dorsal wall of the recess. The lower lamella is prolonged into the posterior commissure and the quadrigeminal plate. Since the lamina chorioidea epithelialis is attached to the dorsal surface of the pineal body, a considerable pocket is left between this surface and the lamina chorioidea of the third ventricle; this is the recessus supra,t5inealis. Sand-like granules, the brain-sand or acervulus, are usually present within the interior of the pineal body.
The posterior commissura, commissura cerebri posterior, is a bundle of trans versely coursing fibres which projects into the ventricle and ventrally bounds the entrance of the recessus pinealis. Its ventral surface defines the aditus ad aquae ductum cerebri. The commissure is best seen when the posterior wall of the third ventricle is viewed from in front (Fig. 59).
:Turning to the region behind the thalamus, two small protuberances, the corpora geniculata, are to be noted as additional parts belonging to the thalamencephalon. On following the tractus opticus in its course backward around the cerebral peduncle, two protuberances are encountered—the elongated oval corpus geniculatum mediale and the corpus geniculatum laterale. The latter is a small elongated elevation at the hind and lower end of the thalamus, lateral to the pulvinar. The medial body is separated from the lateral body and the pulvinar by a deep furrow.