[Illustration: FIG. 152.--CAUDAL SURFACE OF TRANSVERSE SECTION OF THE BRAIN THROUGH THE PINEAL BODY AND THALAMI. FIG. 153.--CAUDAL SURFACE OF TRANSVERSE SECTION OF THE BRAIN THROUGH THE CRANIAL PAIR OF CORPORA QUADRIGEMINA. Fig. 152.--_a_, corpus callosum; _b_, fornix; _c_, inferior horn of lateral ventricles; _d_, hippocampus; _e_, thalamus; _f_, third ventricle; _g_, pineal body; _h_, space between the floor of the hemisphere (hippocampus) and roof of the ’tween-brain (thalamus). Fig. 153.--_a_, cranial corpora quadrigemina; _b_, hippocampus; _c_, part of inferior horn of lateral ventricle; _d_, aqueductus cerebri (aqueduct of Sylvius); _e_, space between the outer surface of the midbrain and the lower surface of the hemisphere; _f_, cross-section of midbrain; _g_, pedunculi cerebri.] The =choroid plexus= of the lateral ventricles (=lamina chorioidea epithelialis=) (Fig. 148, _e_).--The line of junction of the roof of the third ventricle (i.e., the choroid plexus of the third ventricle) and the cerebral hemispheres is along the lateral (or cranial) border of the thalamus (Fig. 141, _t_). Along this line (Fig. 141, 1) the brain-wall remains very thin and becomes closely associated with the pia mater, so that on the removal of the pia there is left the “transverse fissure” which leads into the lateral ventricle. In an early stage, before the cerebrum has grown far caudad, the fissure is exposed in dorsal view, but after the hemispheres have covered the thalami the fissure appears to be in the lower face of the hemisphere. Through this fissure the pia mater extends upward into the lateral ventricles as a fold separated from the ventricle by the thin brain-wall. This fold is vascular and is known as the choroid plexus of the lateral ventricle (Fig. 148, _e_). It is a fringe-like fold lying on the fimbria and extending from the foramen of Monroe (at _h_) nearly to the end of the inferior horn of the lateral ventricle. The =lateral ventricles= are the extensions into the hemispheres of the originally unpaired cavity of the forebrain. Each consists of a portion parallel to the basis cranii and lying at the side of the septum pellucidum between the corpus callosum dorsad and the corpus striatum, hippocampus, and fornix ventrad, and of two horns, an =anterior= and an =inferior= horn. The inferior horn (Fig. 152, _c_) is a narrow cleft, crescent-shaped in cross-section, which follows the dorsal surface of the hippocampus (Fig. 148, _d_; Fig. 152, _d_) along the temporal lobe to its end almost against the tuber cinereum. It thus passes first caudad and then ventrad and finally mediad. The choroid plexus and hippocampus project into it from its floor. The =anterior= horn extends ventrad and then slightly caudad in the frontal lobe, following the corpus striatum (Fig. 148, _f_), on which it lies.