GENERAL DISCUSSION.--The nervous system may be divided into central and
peripheral portions. The _central portion_ comprises the brain or
encephalon and the spinal cord. These organs are lodged in the cranial
cavity and spinal canal. The nerves and ganglia comprise the _peripheral
portion_. The nerves form white cords that are made up of nerve fibres. The
ganglia are grayish enlargements formed by nerve cells and supporting
tissue, situated at the origin of the nerve trunk or along its course.

_The brain_ is an oval mass of nerve tissue elongated from before to
behind, and slightly depressed from above to below. It terminates
posteriorly in the spinal cord. It is divided into three portions:
_cerebrum, isthmus_ and _cerebellum_ (Fig. 25).

_The cerebrum_ forms the anterior portion. It is divided into two lateral
lobes or hemispheres by a deep longitudinal fissure. The surface of the
cerebral hemispheres is gray and roughened by pleats or folds separated by
grooves or fissures. The gray or cortical layer is distinct from the white
or connecting structure. The cortical layer is made up of nerve cells or
areas which control the voluntary muscles of the body. It is connected with
the special senses of touch, temperature and muscle-sense. The gray layer
is connected with the posterior portion of the brain, the isthmus or
medulla oblongata, by the white nerve tissue.

_The isthmus_ or _medulla oblongata_ is elongated from before to behind and
connects the cerebral hemispheres with the spinal cord, anteriorly and
posteriorly. It is divided into several different portions, and is made up
largely of white connecting fibres with nuclei of gray matter scattered
through them. The isthmus is hollowed out by a system of small ventricles
that extend from the cerebral hemispheres to the spinal cord, where they
terminate in a small, central canal. The isthmus is the highway between the