VENTRICULUS TERTIUS.
The third ventricle is a median inpaired cleft-like cavity, that communicates in front with the lateral ventricles by means of the foramen interventriculare or foramen of Monro, and behind with the fourth ventricle by means of the aquaeductus cerebri or Sylvian aqueduct. The front wall is formed, in the lower part by the lamina terminalis, in the upper part by the commissura anterior and the columnae fornicis, while the back wall is formed by the commissura habenularum and the commissura posterior (Fig. 58). The side walls are contributed by the medial surfaces of the thalami and of the hypothalami, separated by the sulci hypothalamici. The floor of the third ventricle, in the hind part, is formed by the cerebral peduncles and the intervening posterior perforated substance; in the front part it includes the corpora mamillaria, the tuber cinereum, with the dibulum and hypophysis, and the chiasma opticum. The immediate roof of the ventricle consists of the lamina chorioidea epithelialis, which is fused with the overlying tela rioidea ventriculi tertii, behind is attached to the dorsal surface of the habenula and of the corpus pineale, and laterally passes into the stria medullaris. The tela chorioidea ventriculi tertii, or velum interpositum, represents an expansion of the pia cerebri between the ventral surface of the corpus callosum and the fornix, on the one hand, and the dorsal surface of the diencephalon, on the other. The tela in form resembles an equilateral triangle, whose apex lies in front, behind the columnae fornicis, and whose base is behind, beneath the splenium corporis callosi (Figs. 55, 6z and 63). It consists of two laterally continuous sheets, of which the dorsal one is attached to the under surface Of the callosum and the fornix, while the ventral one in the middle overlies the lamina chorioidea epithelialis of the third ventricle and at the sides covers the larger part of the dorsal surface of the thalamus. Laterally, where the
two sheets are continuous, richly vascular villi-like tufts of the dorsal sheet project into the lateral ventricle to constitute the plexus chorioideus. Similarly, villi from the ven tral sheet project into the third ventricle, where they appear as two narrow stripes close to the mid-line and together constitute the plexus chorioidea ventriculi tertii. The choroid plexus of the lateral ventricle is inserted laterally in the lamina affixa—taenia medially at the free edge of the fornix—taenia fornicis. The two stripes of the plexus chorioidea ventriculi tertii are attached laterally to the stria medullaris taenia thalami. The choroid plexus of the lateral ventricles and the band-like plexus of the third ventricle come together at the foramen interventriculare. Between the dorsal and ventral sheets of the tela chorioidea lies arachnoidal connective tissue. In this run, in the mid-line and close together, two veins, the venae cerebri internae, into which empty in front the vena sefiti pellucidi, from the septum pellucidum, the vena terminalis, from beneath the stria terminalis, and the vena chorioidea, from the choroid plexus of the lateral ventricles. Behind, at the hind end of the tela chorioidea, the venae cerebri internae unite to form the vena cerebri magna of Galen (Fig. 55).
Certain outpouchings of the third ventricle claim mention. Of these the recessus suprapinealis, the recessus pinealis, the aditus ad aquaeductum cerebri, the recessus in fundibuli and the recessus opticus have been noted. In front, is the recessus triangu laris, between the columnae fornicis and the commissura anterior (Fig. 56).