band of white fibers passes across the back part of the third ventricle and supports the posterior end of the roof epithelium. That band is the posterior commissure (com missura posterior, Figs. 34 and 5o). It crosses immediately in front of the corpora quadrigemina. Beneath it is the anterior orifice of the cerebral aqueduct. The pineal body is above and behind it, and the commissure fuses with the ventral pineal lamina. The posterior commissure stretches from the central gray substance of the mid-brain on one side, over the aqueduct, to the gray substance of the opposite side and also contains decussating fibers of the medial longitudinal bundle (Heald). The commissure is in need of further investigation.
The roof epithelium (Figs. 48 and 54) of the third ventricle stretches from the posterior commissure to the anterior corn missure and laterally is attached to the upper medial border of the thalamus. It is the superficial layer of the ependyma; but it is here the only adult representative of the roof of the diencephalon. The roof epithelium presents two longitudinal folds suspended in the ventricle. The lower layer of the chorioid tela of the third ventricle invests the roof epithelium superiorly; and, dipping down into the longitudinal folds, that inferior layer forms the chorioid plexuses of the third ventricle. At the back part in the middle line there is a pouch like evagination of the roof of the diencephalon in the embryo which develops into the pineal body; and there remains a slight pit called the pineal recess in the adult condition. A second evagination occurs just above the pineal recess which forms the epipineal recess. The epipineal evagination probably represents the anterior pineal body of reptiles.
Pineal Body (Corpus pineale, Figs. 5o, 55 and zoi).—It is a cone-shaped body, 6 mm. (0.25 in.) high and 4 mm. (0.17 in.) in diameter, joined to the roof of the third ventricle by a flattened stalk, the habenula. It is also called the epiphysis. The pineal body is situated in the floor of the transverse fissure of the cerebrum, directly below the splenium of the corpus callosum and rests between the superior colliculi of the quadri geminal bodies on the posterior surface of the mid-brain. It is closely invested by pia mater. The habenula splits into a dorsal and a ventral lamina, which are separated by the pineal recess. The ventral lamina fuses with the posterior commissure; but the dorsal stretches forward over the commissure in continuity with the roof epithelium. The border of the dorsal lamina is thick ened along the line of attachment to the thalamus and forms the stria medullaris thalami (pineal stria). The thickening is due
to the presence of a bundle of fibers derived from the columna of the fornix and the intermediate stria of the olfactory tract. Between the medullary strice at the posterior end there is a transverse band the commissura habenularum, through which the fibers of the strie partially decussate to the nucleus ha benulm in the thalamus.
The interior of the pineal body is made up of closed follicles surrounded by ingrowths of connective tissue. The follicles are filled with epithelial cells mixed with calcareous matter, the brain-sand (acervulus cerebri). Calcareous deposits are found also on the pineal stalk and along the chorioid plexuses. The function of the pineal body is unknown. Des Cartes face tiously suggests that it is the abode of the spirit (the sand) of man. In reptiles there are two pineal bodies, an anterior and a posterior, of which the posterior remains undeveloped but the anterior forms a rudimentary, cyclopean eye. In the Hatteria, a New Zealand lizard, it projects through the parietal foramen and presents an imperfect lens and retina and, in its long stalk, nerve fibers. The human pineal body is probably homologous with the posterior pineal body of reptiles.
The chorioid tela of the third ventricle (velum interpositum, Figs. 48, 5o and 54) is the triangular fold of pia mater spread over the dorsum of the inter-brain. It lies underneath the for nix and the chorioid epithelial lamina which stretches from the body of the fornix lateralward to the stria terminalis. Its apex is just behind the anterior commissure, and its base, directed backward, is continuous, by the upper layer, with the pia mater of the occipital lobes; and, by the inferior layer, it is continuous with the pia on the posterior surface of the mid-brain and cerebellum. Each border constitutes the chorioid plexus of the lateral ventricle and is seen (through the epithelium) in the floor of its central part. The median part of the inferior lamina of the chorioid tela invests the roof epithelium of the third ventricle and the lateral portion covers the medial half of the upper surface of each thalamus. This layer forms the two chorioid plexuses of the third ventricle which depend from its median portion. Between the inferior and superior laminw is enclosed some connective tissue through which the internal cerebral veins run backward to the base of the tela; there they unite and form the great cerebral vein (Galeni).