CENTRAL NERVOUS SYSTEM
The central nervous system (CNS) consists of the brain and spinal cord. The brain is almost entirely enclosed in the skull, but it is connected with the spinal cord, which lies in the canal formed by the vertebral column.

Brain
The brain has two main divisions, the cerebrum and the cerebellum. The cerebrum is the largest and most superiorly situated portion of the brain. It occupies most of the cranial cavity. The outer surface is called the cortex. This portion of the brain is also called "gray matter" because the nerve fibers are unmyelinated (not covered by a myelin sheath), causing them to appear gray. Beneath this layer is the medulla, often called the white matter of the brain because the nerves are myelinated (covered with a myelin sheath), giving them their white appearance.

CEREBRUM.-The cortex of the cerebrum is irregular in shape. It bends on itself in folds called convolutions, which are separated from each other by grooves, also known as fissures. The deep sagittal cleft, a longitudinal fissure, divides the cerebrum into two hemispheres. Other fissures further subdivide the cerebrum into lobes, each of which serves a localized, specific brain function (fig. 1-43). For example, the frontal lobe is associated with the higher mental processes such as memory, the parietal lobe is concerned primarily with general sensations, the occipital lobe is related to the sense of sight, and the temporal lobe is concerned with hearing.

CEREBELLUM.-The cerebellum is situated posteriorly to the brain stem (which is made up of the pons, mid-brain, and medulla oblongata) and inferior to the occipital lobe. The cerebellum is concerned chiefly with bringing balance, harmony, and coordination to the motions initiated by the cerebrum.

PONSANDMEDULLAOBLONGATA.- Two smaller divisions of the brain vital to life are the pons and the medulla oblongata. Together, the pons and medulla form the brain stem (fig. 1-43). The pons consists chiefly of a mass of white fibers connecting the other three parts of the brain (the cerebrum, cerebellum, and medulla oblongata).

The medulla oblongata is the inferior portion of the brain, the last division before the beginning of the spinal cord. It connects to the spinal cord at the upper level of the first cervical vertebra (C-1). In the medulla oblongata are the centers for the control of heart action, breathing, circulation, and other vital processes such as blood pressure.

MENINGES.-The outer surface of the brain and spinal cord is covered with three layers of membranes called the meninges. The dura mater is the strong outer layer; the arachnoid membrane is the delicate middle layer; and the pia mater is the vascular inner-most layer that adheres to the surface of the brain and spinal cord. Inflammation of the meninges is called meningitis. The type of meningitis contracted depends upon whether the brain, spinal cord, or both are affected, as well as whether it is caused by viruses, bacteria, protozoa, yeasts, or fungi.

CEREBROSPINAL FLUID.-Cerebrospinal fluid is formed by a plexus, or network, of blood vessels in the central ventricles of the brain. It is a clear, watery solution similar to blood plasma. The total quantity of spinal fluid bathing the spinal cord is about 75 ml. This fluid is constantly being produced and reabsorbed. It circulates over the surface of the brain and spinal cord and serves as a protective cushion as well as a means of exchange for nutrients and waste materials.