In a dorsal view (Fig. 141) the roof is seen to be marked by two pairs of elevations, the =corpora quadrigemina= (_p_ and _q_). The cranial pair (_q_) (known as the colliculi superiores) are circular in outline, surrounded on all sides except the cranial one by a deep groove. From the cranial side a tract of fibres (=brachium quadrigeminum superius=, or arm of the cranial corpus) extends craniad and disappears beneath the thalamus (_t_). Between the anterior or cranial corpora quadrigemina lies the =pineal body= or epiphysis (corpus pineale) (Fig. 143, _y_; Fig. 141, 2), a portion of the roof of the ’tween-brain. The caudal corpora (Fig. 141, _p_) are larger than the cranial ones, and ovoid in shape with the long axis vertical. They are united in the median line, and the velum medullare anterius (_m_) stretches between their caudal borders. The brachium of the caudal corpus quadrigeminum (brachium quadrigeminum inferius) (_r_) extends craniad and disappears beneath a considerable elevation, the =corpus geniculatum mediale= (_s_). Crossing this brachium is seen a small tract of fibres which extends ventrad, crosses the pedunculus cerebri, and reaches the medial border of the latter. It is the =tractus transversus peduncularis= (Fig. 142, _b_). Ventrad of the caudal corpus quadrigeminum is seen a triangular area of oblique fibres which corresponds in position to the human lemniscus. The =pedunculi cerebri= (peduncles of the cerebrum) form the ventral part of the midbrain. They appear in a ventral view of the entire brain (Fig. 138) as two broad tracts of fibres (_g_) emerging from beneath the pons and diverging from one another as they pass craniad, finally disappearing beneath the cerebral hemispheres. Each is made up of many fibre-bundles, which are apparent in surface view (Fig. 142). The peduncles are separated by a small triangular space, which is marked by a median longitudinal sulcus. In this space, just caudad of the mammillary bodies, is a small area through which a number of blood-vessels pass into the brain. This is known as the =posterior perforated area= (or substance) (Fig. 142, _j_). The cerebral peduncles (_a_) are crossed by the tractus transversus peduncularis (_b_) (see above). The third cranial nerve (Fig. 138, _III_) (=N. oculomotorius=) leaves the brain at the medial border of the cerebral peduncle (_g_), just caudad of the tractus transversus peduncularis. The =aqueductus cerebri= (Fig. 143, _j_; Fig. 153, _d_) (or aqueduct of Sylvius) is the continuation craniad of the fourth ventricle. It is a narrow passage, one or two millimeters in diameter, lying dorsad of the pedunculi cerebri and ventrad of the corpora quadrigemina. [Illustration: FIG. 142.--VENTRAL SURFACE OF THE MIDBRAIN AND ’TWEEN-BRAIN, WITH THE PONS.