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. The spinal cord and the brain together make up the system of nerve tissue in vertebrates called the central nervous system, which controls both voluntary movements, such as those involved in walking and in speech, and involuntary movements, such as breathing and reflex actions. It also is the centre of emotion and cognition.

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. (See also nervous system, human.)

Superficial arteries and veins of face and scalp, cardiovascular system, human anatomy, (Netter replacement project - SSC)
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central nervous system, system of nerve tissue in vertebrates that consists of the brain and spinal cord. The central nervous system controls both voluntary movements, such as those involved in walking and in speech, and involuntary movements, such as breathing and reflex actions. It also is the centre of emotion and cognition. It is one of the two main parts that comprise the human nervous system, the other being the peripheral nervous system (the nerves that carry impulses to and from the central nervous system).

The brain and spinal cord are surrounded by protective membranes known as the meninges, and both float in a crystal-clear cerebrospinal fluid. The central nervous system lies largely within the axial skeleton, wherein the brain is encased in a bony vault, the neurocranium, while the cylindrical and elongated spinal cord lies in the vertebral canal, which is formed by successive vertebrae connected by dense ligaments.

See also nervous system.

Human nervous system
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human nervous system: The central nervous system
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