h from the brain to the cord.
Dorsal to this is a layer of pigmented grey matter, called the
_substantia nigra_, and dorsal to this again is the tegmentum, which
is a continuation upward of the formatio reticularis of the medulla,
and passing through it are seen three important nerve bundles. The
superior cerebellar peduncle is the most internal of these and
decussates with its fellow of the opposite side so that the two
tegmenta are continuous across the middle line. More externally the
mesial fillet is seen, while dorsal to the cerebellar peduncle is the
posterior longitudinal bundle. If the section happens to pass through
the superior corpus quadrigeminum a characteristic circular area
appears between the cerebellar peduncle and the fillet, which, from
its tint, is called the red nucleus. More dorsally still the section
will pass through the Sylvian aqueduct or passage from the third to
the fourth ventricle, and this is surrounded by a mass of grey matter
in the ventral part of which are the nuclei of the third and fourth
nerves. The third nerve is seen at the level of the superior corpus
quadrigeminum running from its nucleus of origin, through the red
nucleus, to a groove on the inner side of the crus called the
_oculo-motor_ groove, which marks the separation between the crusta
and tegmentum. Dorsal to the Sylvian aqueduct is a layer called the
_lamina quadrigemina_ and on this the corpora quadrigemina rest. The
superior pair of these bodies is overlapped by the pineal body and
forms part of the lower visual centres. Connexions can be traced to
the optic tract, the higher visual centre on the mesial surface of the
occipital lobe, the deep origin of the third or oculo-motor nerve as
well as to the mesial and lateral fillet. The inferior pair of
quadrigeminal bodies are more closely in touch with the organs of
hearing, and are connected by the lateral fillet with the cochlear
nucleus of the auditory nerve.

[Illustration: From Cunningham, _Text-book of Anatomy_.

FIG. 8.--Transverse Section through the Human Mesencephalon at the
level of the superior Quadrigeminal Body.]

_Surface of the Brain._

The peripheral part of each hemisphere, which consists of grey matter,
exhibits a characteristic folded appearance, known as gyri (or
convolutions) of the cerebrum. These gyri are separated from each
other by _fissures_ and _sulci_, some of whic