The =mesencephalon= or =mid-brain= connects the rhombencephalon with the
fore-brain. In the undissected brain it is covered dorsally by the
cerebral hemispheres. It consists of a dorsal part, the corpora
quadrigemina, and a larger ventral part, the cerebral peduncles, which
are visible on the base of the brain. It is traversed longitudinally by
a narrow canal, the cerebral aqueduct, which connects the fourth
ventricle with the third.
The =corpora quadrigemina=[191] are four rounded eminences which lie
under the posterior part of the cerebral hemispheres. They consist of
two pairs, separated by a transverse groove. The =anterior pair=
(Colliculi nasales) are larger and much higher than the posterior pair.
They are gray in color, almost hemispherical, and are separated by a
narrow furrow which leads forward to the subpineal fovea. A wide groove
intervenes between them and the optic thalami. The =posterior pair=
(Colliculi caudales) are relatively small and are paler than the
anterior pair. They are marked by a wide median depression, and are
limited below by a transverse furrow (Sulcus postquadrigeminus), at
either side of which the =trochlear= (fourth) =nerve= emerges. Laterally
each is prolonged to the inner geniculate body by a band of white matter
termed the inferior brachium.[192]
The =cerebral peduncles= or crura cerebri (Pedunculi cerebri) appear on
the base of the brain as two large rope-like stalks which emerge from
the pons close together and diverge as they extend forward to enter the
cerebrum. At the point of disappearance the =optic tract= winds
obliquely across the peduncle. About half an inch further back a small
tract (Tractus peduncularis transversus) curves across the peduncles,
and behind this near the median line is the superficial origin of the
=oculomotor= (third) =nerve=. The triangular depression between the
diverging peduncles is the =interpeduncular space= (Fossa
interpeduncularis). It is covered to a large extent by the =pituitary
body=, a discoid brown mass which is connected with the base of the
brain by a hollow stalk, the =infundibulum=. The posterior part of the
space is pierced by numerous minute openings which transmit
blood-vessels, and is therefore termed the =locus perforatus posticus=.
The objects in the space belong to the diencephalon, and will be
described later. The lateral aspect of the peduncle is marked by a
groove (Sulcus lateralis mesencephali) which indicates the division into
a dorsal part, the =tegmentum=, and a ventral part the =basis
pedunculi=; these are separated by a layer of dark gray matter, the
=substantia nigra=. The triangular area (Trigonum lemnisci) above the
lateral groove is faintly marked by fibers passing obliquely upward and
backward to the anterior cerebellar peduncle; these belong to the fillet
or lemniscus, an important tract that connects the thalamus and corpora
quadrigemina with the sensory reception nuclei of the opposite side of
the medulla.
The =aqueduct of the cerebrum= or of Sylvius (Aquæductus cerebri) is the
canal which extends through the mid-brain from the fourth to the third
ventricle. It is surrounded by a layer of gray matter (Stratum griseum
centrale), in the ventral part of which are the nuclei of origin of the
oculomotor and trochlear nerves, and laterally nuclei of the
mesencephalic roots of the trigeminal nerves.