Between the two thalami there exists a groove, the =sulcus hypothalamicus=. Over this groove lies the roof of the third ventricle, forming the =choroid plexus= of the third ventricle (Fig. 143, _w_). The medial surface of the two thalami are flat and extend directly ventrad, forming part of the lateral boundary of the third ventricle (Fig. 143, _h_). The two medial surfaces meet over a considerable area across the narrow cavity of the third ventricle, and unite, forming the =massa intermedia= (Fig. 143, _f_) or intermediate mass of the thalamus (“middle commissure”). This connection of the thalami of the two sides is thus not a primitive one, forming no part of the roof or floor of the central cavity, but is a secondary connection due to a growing together of a part of the two sides of the ventricle across its cavity. Along the dorsal edge of the medial border of each thalamus passes a distinct white strand, the =stria medullaris= (Fig. 141, _u_); the two striæ meet in an arch caudad, lying beneath the pineal body (2). The thalamus is separated craniolaterad by a groove (1) from the corpus striatum (_y_), on the floor of the cerebral hemisphere. _b._ The =roof= of the third ventricle is thin and united with the pia mater, as already stated. The pia mater bears many blood-vessels, and the two are folded into the groove between the optic thalami, forming the =lamina chorioidea epithelialis=, or choroid plexus of the third ventricle (Fig. 143, _w_). The roof is attached to the dorsomedial borders of the thalami and becomes continuous with the choroid plexus of the lateral ventricles (Fig. 148, _e_) at the craniolateral borders of the thalami. The =pineal body= (=corpus pineale=) or epiphysis (Fig. 141, 2; Fig. 143, _y_) is a small conical body which is formed as an outgrowth of the caudal part of the roof of the third ventricle. It lies on the roof of the brain between the two cranial corpora quadrigemina (Fig. 141, _q_). It is hollow, containing an extension of the third ventricle. From its craniolateral angles two white strands, the =habenulæ=, extend into the striæ medullares (Fig. 141, _u_) of the thalami. Just ventrad of the pineal body is a transverse band of white fibres, lying in the caudal part of the roof of the third ventricle. This interconnects the two thalami, and forms the =posterior commissure= (Fig. 143, _z′_) (commissura posterior). From this commissure a thin sheet of tissue extends to the pineal body. _c._ The =floor= of the third ventricle appears in a ventral view of the brain (Fig. 138, Fig. 142) as a somewhat diamond-shaped space craniad of the pedunculi cerebri (Fig. 142, _a_) and bounded along its cranial margin by the =optic tracts= (Fig. 142, _d_). The optic tracts come from the thalami, as already described; they converge and unite to form the optic chiasma (Fig. 142, _e_), from which the optic nerves (_f_) diverge.