mater they are found to spring from the
arachnoid. The observations of Luschka and Cleland have proved that
villous processes invariably grow from the free surface of that
membrane, and that when these villi greatly increase in size they form
the bodies in question. Sometimes the Pacchionian bodies greatly
hypertrophy, occasioning absorption of the bones of the cranial vault
and depressions on the upper surface of the brain.

[Illustration: After D.J. Cunningham's _Text-book of Anatomy_.

FIG. 2.--Front View of the Medulla, Pons and Mesencephalon of a
full-time Human Foetus.]

_Pia Mater._--This membrane closely invests the whole outer surface of
the brain. It dips into the fissures between the convolutions, and a
wide prolongation, named _velum interpositum_, lies in the interior of
the cerebrum. With a little care it can be stripped off the brain
without causing injury to its substance. At the base of the brain the
pia mater is prolonged on to the roots of the cranial nerves. This
membrane consists of a delicate connective tissue, in which the
arteries of the brain and spinal cord ramify and subdivide into small
branches before they penetrate the nervous substance, and in which the
veins conveying the blood from the nerve centres lie before they open
into the blood sinuses of the cranial dura mater and the extradural
venus plexus of the spinal canal.

_Medulla Oblongata._

The _Medulla Oblongata_ rests upon the basi-occipital. It is somewhat
pyramidal in form, about 1-1/4 in. long, and 1 in. broad in its widest
part. It is a bilateral organ, and is divided into a right and a left
half by shallow anterior and posterior median fissures, continuous
with the corresponding fissures in the spinal cord; the posterior
fissure ends above in the fourth ventricle. Each half is subdivided
into elongated tracts of nervous matter. Next to, and parallel with
the anterior fissure is the _anterior pyramid_ (see fig. 2). This
pyramid is continuous below with the cord, and the place of continuity
is marked by the passage across the fissure of three or four bundles
of nerve fibres, from each half of the cord to the opposite anterior
pyramid; this crossing is called the _decussation of the pyramids_. To
the side of the pyramid, and separated from it by a faint fissure, is
the _olivary fasciculus_, which at its upper end is elevated into the
projecting oval-sh