SPINAL CORD, the name given in anatomy to the great cord or rod of nervous matter which is inclosed within the backbone or spine of vertebrates. The spinal cord in man, which is from 15 to 18 inches long, has direct connec tion with the brain by means of the medula oblongata, and passes down the back till it terminates in a fine thread at the level of the first lumbar vertebra. Lodged in the bony vertebrae it varies in thickness throughout, and like the brain is invested by membranes called respectively pia 'nutter and dura mater. Situated between these two are the deli cate layers of the arachnoid membrane, inclosing a space which contains the cerebrospinal fluid. Besides these pro tective coverings there is also a packing of fatty tissue which further tends to diminish all shocks and jars. The spinal nerves, to the number of 31 on each side, pass out from the cord at regular in tervals, pierce the dura mater, escape from the backbone, and ramify thence through the soft parts of the body.
Eight pairs pass off in the region of the neck called the cervical nerves, 12 pairs are dorsal, 5 are lumbar, and 5 sacral, while the last pair comes off behind the coccyx. In its structure the spinal cord consists of gray and white matter. The gray matter, which is characterized by large cells, is gathered in the center into two crescent-shaped masses connected at the central part of the cord. The white matter, consisting mainly of fibers, is outside of and surrounds these gray crescents. In its functions the spinal cord forms a tract along which sensory impressions may pass to the brain, and along which motor impulses may travel to the muscles. It is besides a great re flex center. See BRAIN: NERVE: SPINE.