SPINAL CORD

major nerve tract of vertebrates, extending from the base of the brain through the canal of the spinal column. It is composed of nerve fibres that mediate reflex actions and that transmit impulses to and from the brain. Like the brain, the spinal cord is covered by three connective-tissue envelopes called the meninges. The space between the outer and middle envelopes is filled with cerebrospinal fluid, a clear, colourless, noncellular fluid that cushions the spinal cord against jarring shock. In cross-section the spinal cord appears as a white oval with a gray, butterfly-shaped area in the centre. The white matter consists of myelinated (sheathed) fibres, or axons (see axon); the gray matter contains cell bodies, unsheathed motor-neuron fibres, and interneurons connecting either the two sides of the cord or the dorsal and ventral ganglia (see ganglion). In humans, 31 paired spinal nerves originate in the spinal cord: 8 cervical, 12 thoracic, 5 lumbar, 5 sacral, and 1 coccygeal. Information about internal and external conditions travels up the ascending tracts of neurons. This information is sorted by the brain, and the proper responses are induced by nerve impulses traveling down the descending tracts to stimulate motor neurons or to initiate glandular secretion. (See also nervous system, human.)