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.
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.