[Illustration: FIG. 141.--DORSAL SURFACE OF MYELENCEPHALON, MESENCEPHALON, AND DIENCEPHALON. The cerebellum and the greater part of the cerebral hemispheres have been removed. _a_, posterior sulcus of cord; _b_, posterior lateral sulcus; _c_, fasciculus gracilis; _d_, clava; _e_, fasciculus cuneatus; _f_, area ovalis; _g_, fasciculus cuneatus lateralis; _h_, fossa rhomboidea or floor of fourth ventricle; _i_, projection formed by origin of auditory nerve; _j_, facial nerve; _k_, cut end of brachium conjunctivum; _l_, cut end of brachium pontis (_l′_, brachium pontis); _m_, velum medullare anterius; _n_, fourth nerve; _o_, depression marking caudal end of aqueductus cerebri (aqueduct of Sylvius); _p_, caudal corpora quadrigemina (colliculi posteriores); _q_, cranial corpora quadrigemina (colliculi anteriores); _r_, brachium quadrigeminum inferius; _s_, corpus geniculatum mediale; _t_, thalamus; _u_, striæ medullares; _v_, third ventricle; _w_, pulvinar; _x_, corpus geniculatum laterale; _y_, corpus striatum; _z_, outlines of olfactory bulbs. 1, boundary between hemispheres and ’tween-brain; 2, pineal body.] The fourth cranial nerve (Fig. 141, _n_; Fig. 138, _IV_) (=N. trochlearis=) arises from the brain at the craniolateral angle of the velum medullare anterius. B. MESENCEPHALON. 3. _Mesencephalon._--The mesencephalon or midbrain includes the =corpora quadrigemina= (Fig. 141) and the =pedunculi cerebri= (Fig. 142). In the primary midbrain there is a pronounced thickening of the walls accompanied by a reduction of the central canal. The midbrain does not thus become very large and is concealed in the dorsal view by the cerebellum and cerebrum, though its floor appears in the ventral view just craniad of the pons (Fig. 138, _g_). Its narrow canal is the =aqueductus cerebri= (=aqueduct of Sylvius=) (Fig. 143, _j_). Its roof forms the =corpora quadrigemina=, and its floor the =pedunculi cerebri=.