Spinal Cord
The spinal cord is a long, fragile tubelike structure that begins at the end of the brain stem and continues down almost to the bottom of the spine (spinal column). The spinal cord consists of nerves that carry both incoming and outgoing messages between the brain and the rest of the body. It is also the center for reflexes, such as the knee jerk reflex (see Section 6, Chapter 77). Like the brain, the spinal cord is covered by three layers of tissue (meninges). The spinal cord and meninges are contained in the spinal canal, which runs through the center of the spine. In most adults, the spine is composed of 26 vertebrae, which are the individual bones of the back. Just as the skull protects the brain, vertebrae protect the spinal cord. The vertebrae are separated by disks made of cartilage, which act as cushions, reducing the forces generated by movements such as walking and jumping.
 See the figure How the Spine Is Organized.
Like the brain, the spinal cord consists of gray and white matter. The butterfly-shaped center of the cord consists of gray matter. The front "wings" (called horns) contain motor nerves, which transmit information from the brain or spinal cord to muscles, stimulating movement. The back horns contain sensory nerves, which transmit sensory information from other parts of the body through the spinal cord to the brain. The surrounding white matter contains columns of nerve fibers that carry sensory information to the brain from the rest of the body (ascending tracts) and columns that carry impulses from the brain to the muscles (descending tracts).
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