A second peculiarity of the ’tween-brain lies in the thinness of its roof. The roof is exceedingly thin and is so intimately connected with the pia mater that they cannot be removed separately. The ventral thick floor of the ’tween-brain is directly continuous with the similar floor of the cerebrum; but where the roof of the ’tween-brain joins the roof of the cerebrum along the oblique plane already indicated, the roof is very thin and is intimately connected to the pia mater, and is at the same time folded into the lateral ventricles to form, together with the pia mater, the =choroid plexuses= of the lateral ventricles (Fig. 148, _e_). When the pia mater is removed the thin roof of the brain along the line of junction of the ’tween-brain and the cerebrum is brought away and there _appears_ to be a direct communication between lateral ventricles and the exterior. We may now take up the parts of the diencephalon in detail. a. =Thalamus= (Fig. 141, _t_).--The two thalami are large oblique rounded ridges, forming the sides of the ’tween-brain. They lie just craniad of the cranial corpora quadrigemina (_q_), but separated from them by a broad groove, and are completely covered by the caudally projecting part of the cerebral hemispheres. The medial larger end of each thalamus is near the middle line; thence the thalamus extends caudolaterad, and rises at its lateral extremity into a sharply rounded projection, the =corpus geniculatum laterale= (_x_). From this projection the thalamus is continued ventrad and then craniomediad as a large band of fibres, the =optic tract= (=tractus opticus=) (Fig. 142, _d_), which extends to the optic chiasma (_e_), where it passes into the optic nerves (_f_). On the caudal border of the thalamus, near the median line, is a very faint projection, the =pulvinar= (Fig. 141, _w_); this lies just craniad of the lateral border of the cranial corpus quadrigeminum (_q_). Just ventrad of the corpus geniculatum laterale (_x_) is the prominent rounded =corpus geniculatum mediale= (_s_); this is connected by a prominent ridge, the =brachium quadrigeminum inferius= (_r_), with the caudal corpus quadrigeminum (_p_). In a similar manner the =brachium quadrigeminum superius= passes from the cranial corpus quadrigeminum (_q_) into the thalamus itself.