The spinal cord extends downward from the medulla through the protecting bony rings of the spinal column. The cord is cylindrical in shape, with an outer covering of supporting cells and blood vessels, and an inner, H-shape core of nerve fibers. The spinal cord extends through four-fifths the length of the spine, and is a little longer in men than in women, averaging sixteen and one-half inches in length. It weighs just about one ounce.

Twelve pairs of nerves branch off the spinal cord and pass through the base of the skull into the brain. Thirty-one other pairs branch off the spinal cord throughout its length. These nerve branches run to all the organs of the body, where they branch again and again, until the smallest branches are nerves which are so thin that they cannot be seen with the unaided eye.

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Nerves that extend upwards from the spinal cord to the brain pass through the medulla where they cross. Thus, the left side of the brain controls the right side of the body, while the right side of the brain controls the left side of the body.