the pyramidal and olivary tracts of the medulla oblongata; at its
superior surface are the two crura cerebri; each lateral surface is in
relation to a hemisphere of the cerebellum, and a peduncle passes from
the pons into the interior of each hemisphere; the posterior surface
forms in part the upper portion of the floor of the fourth ventricle,
and in part is in contact with the corpora quadrigemina.

The pons consists of white and grey matter: the nerve fibres of the
white matter pass through the substance of the pons, in either a
transverse or a longitudinal direction. The transverse fibres go from
one hemisphere of the cerebellum to that of the opposite side; some
are situated on the anterior surface of the pons, and form its
superficial transverse fibres, whilst others pass through its
substance and form the deep transverse fibres. The longitudinal fibres
ascend from the medulla oblongata and leave the pons by emerging from
its upper surface as fibres of the two crura cerebri. The pons
possesses a median raphe continuous with that of the medulla
oblongata, and formed like it by a decussation of fibres in the mesial
plane. In a horizontal section through the pons and upper part of the
fourth ventricle the superficial transverse fibres are seen most
anteriorly; then come the anterior pyramidal fibres, then the deep
transverse pontine fibres, then the fillet, while most posteriorly and
close to the floor of the fourth ventricle the posterior longitudinal
bundle is seen (see fig. 5).

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

FIG. 5.--Section through the Lower Part of the Human Pons Varolli
immediately above the Medulla.]

The grey matter of the pons is scattered irregularly through its
substance, and appears on its posterior surface; but not on the
anterior surface, composed exclusively of the superficial transverse
fibres.

[Illustration: From Cunningham, _Text-book of Anatomy._

FIG. 6.--Mesial section through the Corpus Callosum, the
Mesencephalon, the Pons, Medulla and Cerebellum. Showing the third and
fourth ventricles joined by the aqueduct of Sylvius.]

_The Cerebellum._

The _Cerebellum_, LITTLE BRAIN, or AFTER BRAIN occupies the inferior
pair of occipital fossae, and lies below the plane of the tentorium
cerebelli. It consists of two hemispheres or lateral lobes, and of a
median or central lobe, which in h