235. CENTRAL NERVOUS SYSTEM
The central nervous system is composed of the brain and the spinal cord
a. Meninges. Surrounding the brain and spinal cord are three layers of
protective membranes known as the meninges. The outer layer, called the dura mater,
forms a tough lining just inside the skull and the vertebrae. The middle layer is the
arachnoid. The inner layer, the pia mater, is closely in contact with the surface of the
brain and cord. The subarachnoid space between the arachnoid and the pia mater
contains the cerebrospinal fluid.
b. Brain. The brain consists of four main partsthe cerebrum, the cerebellum,
the pons, and medulla oblongata (figure 215).
(1) Cerebrum. The cerebrum is made up of the right and left cerebral
hemispheres. The outside surface is called the cerebral cortex. It is the socalled "gray
matter" because it is made up of nerve cell bodies that are pigmented. The cortex is
thrown into folds called convolutions or gyri, separated from each other by grooves or
fissures known as sulci. These grooves divide the surface of the cerebrum into lobes ,
chief of which are the frontal lobe, the parietal lobe, the temporal lobe, and the occipital
lobe (figure 216). Certain areas of the cerebrum are localized for certain functions. In
the frontal lobe, anterior to the central fissure, is the motor area, which controls
voluntary movements of the body; the speech center; and the writing center. A great
portion of the frontal lobe is believed to be the seat of intelligence, memory, and the
association of ideas. In the parietal lobe, posterior to the central fissure, is the general
sensory area that receives sensations of heat, cold, touch, pressure, pain, and position.
In the temporal lobe are the centers for the sensations of hearing, tasting, and smelling.
In the occipital lobe is the visual center.
(2) Cerebellum . The cerebellum lies below the posterior part of the
cerebrum behind the pons and the medulla oblongata. Its surface is not convoluted like
that of the cerebrum, but is characterized by a cortex of gray matter having sulci of
varying depth. The cerebellum serves as a coordinator of muscular activity, a regulator
of muscle tone, and a center for reflex action and for equilibrium. The cerebellum exerts
considerable control over the posture of the individual. When the cerebellum is
removed from lower animals, they cannot walk.