Chapter 14
XIV. The Thoracic Vertebrae. Vertebrae thoracales. The thoracic vertebrae are intermediate in size between those of the cervical and lumbar regions.
They increase in size from above downward, the upper vertebrae being much smaller than those in the lower part of the region. They are distinguished by the presence of facets on the sides of the bodies for articulation with the heads of the ribs, and facets on the transverse processes of all, except the 11th and 12th, for articulation with the tubercles of the ribs.
The bodies in the middle of the thoracic region are heart-shaped, and as broad in the anterior-posterior as in the transverse direction. At the ends of the thoracic region they resemble respectively those of the cervical and lumbar vertebrae. They are slightly thicker behind than in front, flat above and below, convex from side to
side in front, deeply concave behind, and slightly constricted laterally and in front. They present on either side two costal demi-facets, one above near the root of the pedicle, the other below, in front of the inferior vertebral notch. These are covered with cartilage in the fresh state, and when vertebrae are articulated with one another, form, with the intervening intervertebral fibrocartilages, oval surfaces
for the reception of the heads of the ribs. The pedicles are directed backward and slightly upward, and the inferior vertebral notches are of a large size, deeper than in any other region of the vertebral column. The laminae are broad, thick, and imbricated, that is to say, they overlap those of sub-adjacent vertebrae, like tiles on a roof.
The vertebral foramen is small and of a circular form. The spinous process is long, triangular on coronal section, directed obliquely downward, and ends in a tuberculated extremity. These processes overlap from the fifth to the eighth, but are less oblique in direction above and below.
The superior articular processes are thin plates of bone projecting upward from the junctions of the pedicles and laminae. Their articular facets are practically flat, and are directed backward and a little lateral ward and upward. The inferior articular processes are fused to a considerable extent with the laminae,
and project but slightly beyond their lower borders. Their facets are directed forward and a little medialward and downward. The transverse processes arise from the arch behind the superior articular processes and pedicles. They are thick, strong, and of considerable length, directed obliquely backward and lateral
inward, and each ends in a clubbed extremity, on the front of which is a small, concave surface for articulation with the tubercle of a rib. The first, ninth, tenth, eleventh, and twelfth thoracic vertebrae present certain peculiarities and must be specially considered. The first thoracic vertebra has, on either side of the body, an entire articular facet
for the head of the first rib, and a demi-facet for the upper half of the head of the second rib. The body is like that of a cervical vertebra. Being broad transversely, its upper surface is concave and lipped on either side. The superior articular surfaces are directed upward and backward.
The spinous process is thick, long, and almost horizontal. The transverse processes are long, and the upper vertebral notches are deeper than those of the other thoracic vertebrae. The ninth thoracic vertebra may have no demi-facets below. In some subjects, however, it has two demi-facets on either side.
When this occurs, the tenth has only demi-facets at the upper part. The tenth thoracic vertebra has, except in the cases just mentioned, an entire articular facet on either side, which is placed partly on the lateral surface of the pedicle. In the eleventh thoracic vertebra, the body approaches in its form and size to that of the lumbar vertebrae.
The articular facets for the heads of the ribs 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 thoracic region. The spinous process is short and nearly horizontal in direction. The transverse processes are very short, tuberculated at their extremities, and have no articular
facets. The twelfth thoracic vertebra has the same general characteristics as the eleventh, but may be distinguished from it by its inferior articular surfaces being convex and directed like those of the lumbar vertebrae. By the general form of the body, laminae and spinous process, in which it resembles the
lumbar vertebrae, and by each transverse process being subdivided into three elevations, the superior, inferior, and lateral tubercles, the superior and inferior correspond to the mammillary and accessory processes of the lumbar vertebrae. Traces of similar elevations are found on the transverse processes of the tenth and eleventh thoracic vertebrae.
The lumbar vertebrae. Vertebrae lumbalis. The lumbar vertebrae are the largest segments of the movable part of the vertebral column and can be distinguished by the absence of a foramen in the transverse process and by the absence of facets on the sides of the body.
The body is large, wider from side to side than from before backward, and a little thicker in front than behind. It is flattened or slightly concave above and below, concave behind, and deeply constricted in front and at the sides. The pedicles are very strong, directed backward from the upper part of the body.
Consequently, the inferior vertebral notches are of considerable depth. The laminae are broad, short, and strong. The vertebral foramen is triangular, larger than in the thoracic, but smaller than in the cervical region. The spinous process is thick, broad, and somewhat quadrilateral.
It projects backward and ends in a rough, uneven border, thickest below where it is occasionally notched. The superior and inferior articular processes are well defined, projecting respectively upward and downward from the junctions of pedicles and laminae. The facets on the superior processes are concave and look backward and medialward.
Those on the inferior are convex and are directed forward and lateralward. The former are wider apart than the latter, since in the articulated column the inferior articular processes are embraced by the superior processes of the subadjacent vertebra. The transverse processes are long, slender, and horizontal in the upper three lumbar vertebrae. They incline a little upward in the lower two.
In the upper three vertebrae they arise from the junctions of the pedicles and laminae, but in the lower two they are set farther forward and spring from the pedicles and posterior parts of the bodies. They are situated in front of the articular processes instead of behind them, as in the thoracic vertebrae, and are homologous with the ribs.
Of the three tubercles noticed in connection with the transverse processes of the lower thoracic vertebrae, the superior one is connected in the lumbar region with the back part of the superior articular process, and is named the mammillary process. The inferior is situated at the back part of the base of the transverse process and is called the accessory process.
The fifth lumbar vertebra is characterized by its body being much deeper in front than behind, which accords with the prominence of the sacrovertebral articulation, by the smaller size of its spinous process, by the wide interval between the inferior articular processes, and by the thickness of its transverse processes, which spring from the body as well as from the pedicles.
End of section 14
