[Illustration: FIG. 149.--CAUDAL SURFACE OF TRANSVERSE SECTION OF BRAIN THROUGH THE GENU OF THE CORPUS CALLOSUM. FIG. 150.--CAUDAL SURFACE OF TRANSVERSE SECTION OF BRAIN THROUGH THE OPTIC CHIASMA AND THE REGION OF THE INTERVENTRICULAR FORAMEN (FORAMEN OF MONROE). Fig. 149.--Gray matter dotted; white matter with a few lines. 1, right hemisphere; 2, left hemisphere. _a_, corpus callosum, section through genu and rostrum; _b_, cavity of lateral ventricles. Fig. 150.--1, right hemisphere; 2, left hemisphere. _a_, corpus callosum; _b_, fornix (cut); _c_, cavity of lateral ventricles; _d_, fimbria (_d′_, cut surface; _d_, caudal uncut surface of the part that curves ventrad); _e_, pillars of fornix; _f_, anterior commissure; _g_, parts of third ventricle; _h_, optic chiasma; _i_, parts of corpus striatum.] [Illustration: FIG. 151.--CAUDAL SURFACE OF TRANSVERSE SECTION OF THE BRAIN THROUGH THE TUBER CINEREUM AND INFUNDIBULUM, CRANIAD OF THE HYPOPHYSIS. _a_, corpus callosum; _b_, fornix (continuous at the sides with the hippocampus); _c_, cavity of lateral ventricles; _d_, hippocampus; _e_, choroid plexus of the third ventricle (roof of the diencephalon); _f_, dorsal part of cavity of the third ventricle; _g_, section of thalamus; _h_, massa intermedia of the thalami, dividing the third ventricle into a dorsal (_f_) and a ventral (_i_) portion; _i_, ventral part of third ventricle; _j_, part of optic tracts; _k_, thin wall of infundibulum, with part of cavity of third ventricle; _l_, line separating roof of thalamus (diencephalon) from floor of hemispheres (telencephalon).] The =hippocampus= (Fig. 148, _d_; Fig. 152, _d_) is an elongated rounded elevation of the floor of the lateral ventricle. It is continuous mediad with the fornix (Fig. 148, _a_) and extends thence along the inferior horn of the lateral ventricle to its end in the temporal lobe. It is somewhat narrower at its lateral end. It is thus curved into a semicircle in conformity with the inferior horn of the ventricle. Its dorsal surface is convex and looks into the lateral ventricle; its ventral surface is concave and rests upon the thalamus and the optic tract. Along the craniolateral edge of the hippocampus is a broad, clearly marked fibre-tract, the =fimbria=; this runs parallel to the choroid plexus of the lateral ventricle and beneath it, and is continuous at its medial end with the fornix. The =corpus striatum= (Fig. 148, _f_; Fig. 141, _y_; Fig. 150, _i_) is a fusiform or ovoid elevation of the floor of the lateral ventricle. One of its narrow ends lies about opposite the middle of the thalamus (Fig. 141, _t_) and it extends thence obliquely mediad nearly parallel with the thalamus. In the groove between it and the thalamus is the choroid plexus of the lateral ventricles (Fig. 148, _e_). Internally the corpus striatum is made up of several layers of different texture (Fig. 150, _i_), whence its name.