The Thoracic Vertebrae are intermediate in size between those in the cervical and those in the lumbar region, and increase in size from above downward, the upper vertebrae in this segment of the spine being much smaller than those in the lower part of the region. The dorsal vertebrae may be at once recognized by the presence on the sides of the body of one or more facets or half-facets for the heads of the ribs.

The bodies of the thoracic vertebrae resemble those in the cervical and lumbar regions at the respective ends of this portion of the spine; but in the middle of the thoracic region their form is very characteristic, being heart-shaped, and as broad in the antero-posterior as in the lateral direction. They are thicker behind than in front, flat above and below, convex and prominent in front, deeply concave behind, slightly constricted in front and at the sides, and marked on each side, near the root of the pedicle, by two demi-facets, one above, the other below. These are covered with cartilage in the recent state, and, when articulated with the adjoining vertebrae, form, with the intervening fibro-cartilage, oval surfaces for the reception of the heads of the corresponding ribs. The pedicles are directed backward, and the inferior intervertebral notches are of large size, and deeper than in any other region of the spine. The laminae are broad, thick, and imbricated - that is to say, overlapping one another like tiles on a roof. The spinal foramen is small, and of a circular form. The spinous processes are long, triangular on transverse section, directed obliquely downward, and terminate in a tubercular extremity. They overlap one another from the fifth to the eighth, but are less oblique in direction above and below. The articular processes are flat, nearly vertical in direction, and project from the upper and lower part of the pedicles; the superior being directed backward and slightly outward and upward, the inferior forward and a little inward and downward. The transverse processes arise from the same parts of the arch as the posterior roots of the transverse processes in the neck, and are situated behind the articular processes and pedicles; they are thick, strong, and of great length, directed obliquely backward and outward, presenting a clubbed extremity, which is tipped on its anterior part by a small concave surface, for articulation with the tubercle of a rib. Besides the articular facet for the rib, three indistinct tubercles may be seen rising from the transverse processes, one at the upper border, one at the lower border, and one externally. In man they are comparatively of small size, and serve only for the attachment of muscles. But in some animals they attain considerable magnitude, either for the purpose of more closely connecting the segments of this portion of the spine or for muscular and ligamentous attachment.

The peculiar thoracic vertebrae are the first, ninth, tenth, eleventh, and twelfth .

The First Thoracic Vertebra presents, on each side of the body, a single entire articular facet for the head of the first rib and a half facet for the upper half of the second. The body is like that of a cervical vertebra, being broad transversely; its upper surface is concave, and lipped on each side. The articular surfaces are oblique, and the spinous process thick, long, and almost horizontal.

The Ninth Thoracic has no demi-facet below. In some subjects, however, the ninth has two demi-facets on each side; when this occurs the tenth has only a demi-facet at the upper part.

The Tenth Thoracic has (except in the cases just mentioned) an entire articular facet on each side, above, which is partly placed on the outer surface of the pedicle. It has no demi-facet below.

In the Eleventh Thoracic the body approaches in its form and size to the lumbar. The articular facets for the heads of the ribs, one on each side, are of large size, and placed chiefly on the pedicles, which are thicker and stronger in this and the next vertebra than in any other part of the dorsal region. The spinous process is short, and nearly horizontal in direction. The transverse processes are very short, tubercular at their extremities, and have no articular facets for the tubercles of the ribs.

The Twelfth Thoracic has the same general characters as the eleventh, but may be distinguished from it by the inferior articular processes being convex and turned outward, like those of the lumbar vertebrae; by the general form of the body, laminae, and spinous process, approaching to that of the lumbar vertebras; and by the transverse processes being shorter, and marked by three elevations, the superior, inferior, and external tubercles, which correspond to the mammillary, accessory, and transverse processes of the lumbar vertebrae. Traces of similar elevations are usually to be found upon the other dorsal vertebrae (vide ut supra).