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The Nervous System - Advanced Version / The Central Nervous System
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 The central nervous system is divided into two major parts: the brain and the spinal cord. In the average adult human, the brain weighs about 3 pounds. The brain contains about 100 billion nerve cells (neurons) and trillons of "support cells" called glia. The spinal cord is about 43 cm long in adult women and 45 cm long in adult men and weighs about 35-40 gm. The vertebral column or spine, the collection of bones (back bone) that houses the spinal cord, is about 70 cm long. So the spinal cord is much shorter than the vertebral column.
Centrally located in the human organism are accumulations of nerve cells forming the brain or cerebrum and the spinal cord or medulla.
Originating from these central groups, bundled nerve cell processes run out to form the peripheral nervous system.
The signals received from the peripheral nervous system are analyzed, can be stored, and motor signals are constructed. These functions, which are predominantly accomplished by the brain, are extremely complicated and require the cooperation of thousands of nerve cells.
The most important activity of the nervous system is performed in the brain or cerebral cortex. Here is the center of all bodily functions.
A large vascular net supplies the central nervous system with oxygen and nutrients. The central nervous system itself is embedded in bone cavities formed in the area of the head by the skull or cranium and in the area of the neck and trunk by vertebral arches.
These bony cavities protection to the brain and the spinal medulla. Within these cavities, the protection is reinforced by three layers of connective tissue called the meninges, (see figure to left), which, in the head, envelope the brain, and in the vertebral column, the spinal cord or medulla.
Between the middle and the inner connective tissue layers are spaces filled with brain and spinal fluid called the cerebrospinal fluid. This fluid protects the central nervous system largely from mechanical damage.
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