The spinal cord occupies the upper two-thirds of the vertebral canal. It extends from the margin of the foramen magnum of the occipital bone to the level of the upper border of the second lumbar vertebra. At its upper end it is continuous with the medulla oblongata of the brain, at its lower end it tapers, forming a pointed extremity called the conus medullaris, from the end of which comes a slender thread called the filum ter-minale. The cord is a cylindrical structure, slightly flattened before and behind, and is considerably smaller than the canal, which allows movements of the vertebral column to take place without jarring the cord.

The cord has three coverings continuous with those of the brain-viz., dura mater, arachnoid mater, and pia mater. The cord is suspended within the dura mater by two ligaments, which project laterally in its whole length, called the ligamenta dentic-ulata. These extend outwards, and are attached to the inner surface of the dura mater by tooth-like projections.

The cord is not of uniform thickness throughout, but is considerably thickened in the lower cervical and upper lumbar regions. The cord similarly to the brain has a longitudinal fissure running down its posterior surface, which grooves it nearly to the centre. There is a shallower anterior furrow, and two very shallow grooves, one on each side of the posterior crus. These are called the postero-lateral grooves.

On transverse section the cord is seen to consist also of grey and white matter, but the grey matter is entirely embedded in the white, and has a minute central canal running through it, which communicates above with one of the ventricles of the brain. The grey matter forms a column which extends the whole length of the cord, and in transverse section resembles in shape the letter H, the four ends of which are called horns.

There are thirty-one pairs of spinal nerves, which are attached by two roots each to the lateral aspects of the cord opposite the anterior and posterior horns of grey matter on each side respectively. These nerves come through the intervertebral foramina, and are named according to the vertebrae below which they emerge, except in the case of the cervical ones. The first nerve comes out between the occipital bone and the atlas, so that there are eight pairs of cervical nerves, twelve pairs of dorsal nerves, five pairs of lumbar nerves, five pairs of sacral nerves, and one pair of coccygeal nerves. As the cord itself ends in the upper lumbar region, the lower nerves lie in the vertebral canal, forming what is known as the cauda equina, and emerging in order through the intervertebral foramina.

The spinal nerves are attached to the cord by two roots-anterior and posterior-which join together inside the vertebral canal so that a mixed nerve - i.e., formed by the junction of the two roots-emerges from the intervertebral foramen. It immediately divides into anterior and posterior divisions.

The posterior divisions supply the skin at the back of the head, neck, shoulder, buttock, and trunk, and the longitudinal muscles of the back. Each divides into two parts-an internal and external trunk. In the upper half of the body the internal trunks are cutaneous, the external ones muscular; in the lower half of the body the reverse is the case. The posterior divisions of the first and second cervical nerves vary a little.

First cervical nerve (suboccipital) does not divide into internal and externaltrunks, and gives off no cutaneous branches. Muscular branches to complexus, rectus capitis posticus major and minor, obliquus superior and inferior.

Second cervical nerve (great occipital) supplies the skin at the back of the head as far as the vertex, and communicates with the other cutaneous nerves of that region.

Muscular branches to complexus, obliquus inferior, and other muscles on the back of the neck.