The dorsal wall or roof of the lateral ventricle joins the thin roof of the third ventricle on each side along an oblique curved line (Fig. 141, 1) which follows the cranial or lateral border of the thalamus (Fig. 141, _t_). Along this line the thin roof of the brain is folded in together with the pia mater to form the =choroid plexus= of the =lateral ventricles= (=lamina chorioidea epithelialis=) (Fig. 148, _e_). When this is pulled out there is left a fissure, the “great transverse fissure of the cerebrum” (Fig. 141, 1). Just dorsad of the groove between the thalami there runs a tract of white fibres known as the =fornix= (Fig. 148, _a_). The two halves of the fornix separate at the cranial ends of the thalami and pass ventrad, forming thus the =pillars= of the =fornix= (Fig. 148, _b_; Fig. 143, _v_). Dorsad of the cranial end of the fornix the =corpus callosum= (Fig. 143, _p_) passes from one hemisphere to the other. Caudad of the pillars of the fornix, the lateral ventricles communicate with the third ventricle by way of the =interventricular foramen= (foramen of Monroe). The parts of the cerebrum may now be taken up in detail. [Illustration: FIG. 147.--THE CORPUS CALLOSUM. The dorsal portion of the hemispheres has been dissected away, then sliced off, showing the plate of transverse fibres forming the corpus callosum. _a_, splenium; _b_, genu; _c_, line marking the medial edge of the hemispheres; _d_, line marking the lateral boundary of the supracallosal sulcus; laterad of this line the corpus callosum lies in the substance of the hemispheres, which have been dissected away; _e_, line marking medial limit of cut surface.] The =corpus callosum= (Fig. 147; Fig. 143, _p_; Figs. 149-152, _a_) is a broad transverse band of fibres forming a secondary connection between the medial walls of the two hemispheres, dorsad of the roof of the third ventricle. Its outer surface (Fig. 147) is exposed at the bottom of the fissure which separates the hemispheres. On each side it passes laterad, forming the roof of the lateral ventricle. Its cranial part lies dorsad of the corpus striatum, and its caudal part dorsad of the thalamus. Laterally its fibres radiate into the substance of the hemispheres. At its cranial end the corpus callosum bends ventrad and then caudad (Fig. 143). The part which turns to pass ventrad is the =genu= (Fig. 143, _q_) or knee, while the part which projects caudad is the =rostrum= (_r_). The caudal border of the corpus callosum is also thickened and turned ventrad and is called the =splenium= (Fig. 143, _s_); it lies dorsad of the cranial corpora quadrigemina (_z_). The caudal half of the ventral surface of the corpus callosum is united with the fornix (Fig. 143, _u_). [Illustration: FIG. 148.--FORNIX, HIPPOCAMPUS, AND CORPUS STRIATUM.