The spinal cord is a continuation of the brain extending from the medulla to the second lumbar vertebra ; its lower end tapers to a point. Before its termination it breaks up into a number of fibres, which, from their resemblance to a horse's tail, are called cauda equina. It lies in the spinal canal, which is a bony tube formed by the vertebrae, being arranged one above the other. It is divided into right and left parts by two fissures. As in the medulla, the gray matter of the cord is in the interior, and is surrounded by the white.

It is from 16 to 18 inches in length, and its diameter is about 3/4 of an inch. Like the brain, it is enveloped in three membranes : the pia mater closely invests it and is adherent to its surface, the arachnoid and dura mater form loose investments.

Fig. 23.—Section of Spinal Cord and Nerve Roots.

1, Anterior fissure ; 2, posterior fissure ; 3, anterior root ; 4, posterior root ; 5, ganglion.

The cord and its covering membranes are supplied by small arteries, which enter the spinal canal through the spaces between the vertebrae. In the cervical and lower dorsal regions of the spinal cord there are two swellings, called the cervical and lumbar enlargements, from which proceed the nerves that supply the upper and lower limbs. From the under surface of the brain twelve pairs of nerves are given off. From the spinal cord thirty-one pairs of nerves are given off. The nerve fibres which conduct impulses to a nerve centre are called afferent, or sensory. Those which conduct impulses from a nerve centre are called efferent, or motor. By the former we feel pain or any other sensation. The latter are distributed entirely to the muscles, and by stimulating them produce motion. Some nerves have both sensory and motor fibres running through them, and convey impulses in both directions ; these are called mixed nerves.