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 fluid that cushions the spinal cord.

A cross section of the spinal cord reveals white matter arranged around a butterfly-shaped area of gray matter. The white matter consists of myelinated fibres, or axons, that form nerve tracts ascending to and descending from the brain. The white matter is grouped into discrete sectors called funiculi. The gray matter contains cell bodies, unmyelinated motor-neuron fibres, and interneurons connecting the two sides of the cord. Gray-matter cells form projections called horns. Fibres exiting the spinal cord from the dorsal and ventral horns join in paired tracts to form the spinal nerves. Information travels up the ascending tracts of neurons and is sorted by the brain. Responses are induced by nerve impulses traveling down the descending tracts that stimulate motor neurons or that initiate glandular secretion.

human spinal cordLower cervical segment of the human spinal cord. In this photograph of a cross section, the white matter is shown stained dark.