y a dense layer of
transverse fibres, called _cor'pus cal-lo'sum_.

734. In the interior of the brain there are several cavities, two of
which are of considerable size, and are called the lateral ventricles.
They extend from the anterior to the posterior part of the brain, and
wind their way into other parts of the cerebral organ.

_Observation._ In the disease called "dropsy of the brain,"
(hydrocephalus internus,) the serum, or water, is usually deposited in
these ventricles. This is effused from the many small blood-vessels of
the membrane in these cavities.

735. The brain is of a pulpy character, quite soft in infancy and
childhood; but it gradually becomes more and more consistent, and in
middle age it assumes the form of determinate structure and
arrangement. It is more abundantly supplied with blood than any organ
of the system. No lymphatics have been detected, but it is to be
presumed that they exist in this organ.

736. The CEREBELLUM is about seven times smaller than the cerebrum.
Like that organ, it is composed of white and gray matter, but the
gray constitutes the larger portion. Its surface is formed of parallel
plates separated by fissures. The white matter is so arranged, that
when cut vertically, the appearance of the trunk and branches of a
tree (_ar'bor vi'tae_) is presented. It is situated under the posterior
lobe of the cerebrum, from which it is separated by a process of the
dura mater, called the _ten-to'ri-um_.

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733. Describe the appearance of the brain when a horizontal section
has been made. What is the gray border often called? What connects the
hemispheres? 734. Describe the ventricles of the brain. In the disease
called "dropsy of the brain," where is the water deposited? 735. What
is the character of the brain in childhood? In adults? 736. How does
the cerebellum compare in size with the cerebrum?

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[Illustration: Fig. 122. The under surface, or base, of the brain and
origin of the cranial nerves. 1, 1, The anterior lobes of the cerebrum.
2, 2, The middle lobes. 3, 3, The posterior lobes, almost concealed by
the cerebellum. 4, 4, The cerebellum. 7, 7, The longitudinal fissure that
divides the brain into two hemispheres. 8, The first pair of nerves. 9,
9, The second pair of nerves. 10, The decussation, or crossing, of its
fibres. 13, 13, The third pair of nerves. 14, The pons varolii. 15, 15,
The fourth pair of nerves. 16, 16,