spinal cord and the higher nerve centres. It has in it certain cell centres
that give origin to six of the cranial nerves.

[Illustration: FIG. 25.--Photograph of model of horse's brain: longitudinal
section; lateral view; cerebrum; cerebellum; and isthmus.]

The third division of the brain is the _cerebellum_. This is a single mass
supported by the isthmus. It is situated posterior to the cerebrum, from
which it is separated by a transverse fold of the membranes covering the
brain. This mass of nerve tissue is much smaller than the cerebrum. The
white nerve tissue forms central nuclei which send out branches that ramify
in every direction. The centre of the muscular sense is said to be located
in this division of the brain. A second function is the maintenance of body
equilibrium through its connection with the nerve of the middle ear.

_The spinal cord_ commences at the posterior opening (occipital foramen) of
the cranial cavity, and terminates posteriorly in the lumbar region at the
upper third of that portion of the spinal canal belonging to the sacrum. It
is thick, white in color, irregularly cylindrical in shape, slightly
flattened above and below and reaches its largest diameter in the lower
cervical and lumbar regions. The spinal canal is lined by the outer
membrane that envelops the cord, which aids in fixing this organ to the
wall of the canal. The spinal cord is formed by white and gray nerve
tissue. The gray tissue is situated within the white, and it is arranged in
the form of two lateral comma-shaped columns connected by a narrow
commissure of gray matter. The extremities of the lateral gray columns mark
the origin of the superior and inferior roots of the spinal nerves. The
white tissue of the cord is also divided into lateral portions by superior
and median fissures. The inferior fissure does not extend as far as the
gray commissure, leaving the lateral inferior columns connected by a white