The Anterior median fissure is wider, but of less depth than the posterior, extending into the cord for about one-third of its thickness, and is deepest at the lower part of the cord. It contains a prolongation from the pia mater; and its floor is formed by the anterior white commissure, which is perforated by numerous blood-vessels, which pass to the centre of the cord.

The Posterior median fissure is much more delicate than the anterior, and more distinct at the upper part of the cord and below. It extends into the cord to about one half of its depth. It contains a very slender process of the pia mater and numerous blood-vessels, and its floor is formed by a thin layer of white substance, the posterior white commissure. Some anatomists state, that the bottom of this fissure corresponds to the grey matter, except in the cervical region, and at a point corresponding to the enlargement in the lumbar region. On either side of the anterior median fissure, a linear series of foramina may be observed, indicating the points where the anterior roots of the spinal nerves emerge from the cord. This is called, by some anatomists, the anterior lateral fissure of the cord, although no actual fissure exists in this situation. And on either side of the posterior median fissure, along the line of attachment of the posterior roots of the nerves, a delicate fissure may be seen, leading down to the grey matter which approaches the surface in this situation: this is called the posterior lateral fissure of the spinal cord. On the posterior surface of the spinal cord, on either side of the posterior median fissure, is a slight longitudinal furrow, marking off two slender tracts, the posterior median columns. These are most distinct in the cervical region, but are stated by Foville to exist throughout the whole length of the cord. The fissures divide each half of the spinal cord into four columns, an anterior column, a lateral column, a posterior column, and a posterior median column.

The Anterior column includes all the portion of the cord between the anterior median fissure and the anterior lateral fissure, from which the anterior roots of the nerves arise, and is continuous with the anterior pyramid of the medulla oblongata.

The Lateral column, the largest segment of the cord, includes all the portion between the anterior and posterior lateral fissures. It is continuous with the lateral column of the medulla. By some anatomists, the anterior and lateral columns are included together, under the name of the antero-lateral column, which forms rather more than two-thirds of the entire circumference of the cord.

The Posterior column is situated between the posterior median and posterior lateral fissures. It is continued, above, into the restiform body of the medulla.

The Posterior median column is that narrow segment of the cord which is seen on each side of the posterior median fissure, usually included with the preceding, as the posterior column.

If a transverse section of the spinal cord be made, it will be seen to consist of white and grey nervous matter. The white matter is situated at the circumference, the grey matter in the interior.

The Grey matter presents two crescentic masses, placed one in each lateral half of the cord, with their convexities towards one another, and joined by a transverse band of grey matter, the grey commissure. Each crescentic mass has an anterior and posterior horn.

The posterior horn is long and narrow, and approaches the surface at the posterior lateral fissure, near which it presents a slight enlargement. The grey matter, in this situation, is pale and soft, and was called by Rolando, the substantia cinerea gelatinosa, being surrounded by a layer of reddish-brown substance.

The anterior.horn is short and thick, and does not quite reach the surface, but extends towards the point of attachment of the anterior roots of the nerves. Its margin presents a dentate, or stellate appearance. Owing to this peculiar arrangement of the grey matter, the anterior and posterior horns projecting towards the surface, each half of the cord is divided, more or less completely, into three columns, anterior, middle, and posterior; the anterior and middle being joined, as the anterior horn does not quite reach the surface, to form the antero-lateral column.

The Grey commissure, which connects the two crescentic masses of grey matter, is separated from the bottom of the anterior median fissure by a thick layer of white substance, the anterior white commissure; and, from the bottom of the posterior fissure by the posterior white commissure. The existence of the latter commissure is doubted by some anatomists. The grey commissure consists of a transverse band of grey matter, and of white fibres, derived from the opposite half of the cord and the posterior roots of the nerves. The white commissure is formed, partly of fibres from the anterior column, and partly from the fibrils of the anterior roots of the spinal nerves, which decussate as they pass across from one to the other side.

The mode of arrangement of the grey matter, and its amount in proportion to the white, vary in different parts of the cord. Thus, the posterior horns are long and narrow, in the cervical region; short and narrower, in the dorsal; short, but wider, in the lumbar region. In the cervical region, the crescentic portions are small, the white matter more abundant than in any other region of the cord. In the dorsal region, the grey matter is least developed, the white matter being also small in quantity. In the lumbar region, the grey matter is more abundant than in any other region of the cord. Towards the lower end of the cord, the white matter gradually ceases. The crescentic portions of the grey matter gradually blend into a single mass, which forms the only constituent of its extreme point.

The white matter of the cord forms about seven-eighths of its entire substance. It is composed of parallel fibres collected into compressed, longitudinal bundles, between which blood-vessels, supported by a delicate process of pia mater, pass transversely into the substance of the cord.

In the foetus, until after the sixth month, a canal, continuous with the general ventricular cavity of the brain, extends throughout the entire length of the spinal cord, formed by the closing-in of a previously open groove.

In the adult, this canal can only be seen at the upper part of the cord, extending from the point of the calamus scriptorius, in the floor of the fourth ventricle, for about half an inch down the centre of the cord, where it terminates in a cul de sac, the remnant of the canal being just visible in a section of the cord, us a small, pale spot, corresponding to the centre of the grey commissure, its cavity having become obliterated. In some cases this canal remains pervious throughout the whole length of the cord.