March 13, 2010
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Thoracic Vertebra

The thoracic vertebrae (figs. 265, 274, 275), twelve in number, show a gradual increase in size from above downwards. All are distinguished by the presence of facets on the sides of the bodies, and all but the last two, sometimes three, by facets on the transverse processes ; the former articulate with the heads of the ribs and the latter with the tubercles of the ribs.

The first, ninth, tenth, eleventh and twelfth thoracic vertebra= present certain peculiarities and must be considered separately. The others, although showing individual differences of a minor degree, conform to a common type.

Figure 274

General features.-- The body of a typical thoracic vertebra (fig. 265) resembles in shape a conventional heart from a playing-card, and its anteroposterior is nearly equal to its transverse measurement. On each side it bears two costal facets; the superior facets are usually the larger and are placed on the upper border near the root of the pedicle; the inferior facets are situated on the lower border of the body just in front of the inferior vertebral notch. The vertebral foramen is relatively small, and its circular outline may be associated with the fact that the pedicles show no lateral deviation as they pass backwards from the body. This also accounts for the shortness of the lamina, which are broad and thick and overlap each other from above. The spine is long and is directed downwards and backwards. The superior articular processes are thin plates of bone which project upwards at the junction of the laminar and pedicles; their articular facets are almost flat and are directed backwards and a little laterally and upwards. The inferior articular processes are fused to the lateral ends of the lamina; their articular facets are directed forwards and slightly downwards and medially. The transverse process is a substantial, club-shaped projection which springs from the vertebral arch at the junction of the lamina and pedicle. It is directed laterally and backwards and bears on its anterior aspect, near its extremity, a facet for articulation with the tubercle of the numerically corresponding rib.

The first thoracic vertebra is distinguished by the character of the upper facets on the sides of the body, which are circular in outline, as each articulates with the whole of the head of the first rib. The lower facets are small and semilunar in shape. The spine is thick, long and horizontal; it can be identified easily in the living subject, for it forms a visible projection below the spine of the vertebra prominens.

The ninth thoracic vertebra may possess all the features of a typical thoracic vertebra, but it often fails to articulate with the head of the tenth rib, and the lower facets oil the body are then absent (fig. 275).

The tenth thoracic vertebra articulates with the head of the tenth rib only. The facet is placed at the upper border of the body and encroaches a little on to the pedicles ; it is usually incomplete but, when the tenth rib fails to articulate with the ninth thoracic vertabra, it is complete and circular in outline. The transverse process may, or may not, bear an articular facet for the tubercle of the tenth rib.

The eleventh thoracic vertebra, articulates with the head of the eleventh rib only. The circular facet is placed close to the upper border of the body and extends backwards on the lateral aspect of the pedicle. The transverse process is small but can be gripped between the finger and thumb; it is not marked by an articular facet.

The twelfth thoracic vertebra articulates with the head of the twelfth rib only. The facet, roughly circular in outline, lies below the upper border of the body and extends over the lateral aspect of the pedicle. The body is large and approximates closely to the lumbar type. The transverse process is small and insignificant and is not marked by an articular facet; 'it is subdivided into superior, lateral and inferior tubercles. The inferior articular processes are turned laterally and the articular facets are convex from side to side, like those of a lumbar vertebra.

Particular features of the thoracic vertebrae - The bodies of the upper thoracic vertebra show a gradual transition from the cervical to the thoracic type, -while the bodies of the lower thoracic vertebra show a similar transition from the thoracic to the lumbar type. The body of T. 1 is cervical inform, and its transverse is nearly twice as great as its anteroposterior measurement. The body of T. 2 retains the cervical type, but its breadth is less and the disproportion between its two measurements is diminished. The body of T. 3 is actually the smallest of the thoracic bodies, but its anterior aspect, instead of being flattened like the bodies of T. 1 and T. 2, is rounded of and shows a forward convexity. From this point the bodies gradually increase in size and, owing to an increase in the anteroposterior measurement that of T. 4 is typically heart-shaped. The bodies of T. 5 to T. 8 show a gradual increase in the anteroposterior measurement while the transverse measurement shows little alteration. These four vertebrae, when seen on transverse section, are asymmetrical, for the left side of each body shows a flattening produced by the pressure of the descending thoracic aorta. The remaining vertebrae increase in size more rapidly, the increase affecting all the measurements of the body, so that T. 12 approximates closely to the shape of a typical lumbar vertebra.

The upper and lower borders of the bodies in front and behind give attachment to the anterior and posterior longitudinal ligaments, respectively; and the margins of the costal facets give attachment to the capsular and radiate ligaments of the joints of the heads of the ribs. The bogus cervicis (longus colli) muscle arises from the bodies of the first three thoracic vertebrae lateral to the anterior longitudinal ligament. The psoas major and minor muscles arise from the lateral aspect of the twelfth thoracic vertebra near its lower border.

The pedicles increase in thickness from above downwards. The superior vertebral notch is scarcely recognizable except in the first thoracic vertebra, but the inferior notch is deep and conspicuous. The upper borders of the lamina and the lower parts of their anterior surfaces serve for the attachment of the ligamenta flava ; their dorsal aspects give insertion to the rotatoreas muscles.

The transverse processes gradually diminish in length from above downwards. In the upper six (sometimes five) the costal facets are concave and face forwards and laterally; in the others the facets are flattened and face upwards, laterally, and slightly forwards. The tuberculated extremity of the process gives attachment: to the lateral costotransverse ligament (ligament of the tubercle of the rib): its lower border, to the superior (anterior) costotransverse ligament: its anterior surface, anedial to the facet, to the inferior costotransverse ligament (ligament of the neck of therib) : and its base to the posterior costotransverse ligament. In addition, the upper and lower borders of the transverse process provide attachment for intertrasverse muscles or their fibrous vestiges, and the posterior aspect for the deep muscles of the back, the levator costae arising from the dorsal aspect of the tuberculated extremity under cover of the longer muscles.

Figure 275

The spines overlap from the fifth to the eighth, which are the longest and most nearly vertical of the thoracic spines. Above and below they are less oblique in direction.* They give attachment to the supraspinous and interspinous ligaments, and to the trapezius, rhomboideus major and minor. latissimus dorsi, the serratus posterior auperior and inferior, and many of the deep muscles of the back.

The first thoracic vertebra resembles a cervical vertebra in the shape of it's body. In addition, the posterolateral parts of its upper border are raised, as they are in the cervical region, and this projection forms the anterior border of the superior vertebral notch, which is a distinctive feature of this vertebra. The upper facet on the side of the body is not always complete, as the head of the first rib often articulates with the intervertebral disc between the seventh cervical arid the first thoracic vertebrae. Immediately below the facet there is frequently a small, deep depression in the bone.

In the eleventh and twelfth thoracic vertebrae the spines are characteristically triangular, with blunted apices formed by their extremities. In each case, the lower border of the spine is horizontal, or nearly so, and the upper border is oblique. In the region of the transverse process of the twelfth thoracic vertebra three little tubercles can be distinguished. Of these the superior is the largest and ,jets upwards. It corresponds to the mamillary process of a lumbar vertebra, but it is not so closely connected with the superior articular process. The lateral tubercle is small and corresponds to the true transverse process. The inferior tubercle, directed downwards, corresponds to the accessory process of a lumbar vertebra.

In distinguishing between these two vertebrae the student should be guided by (1) the character of the inferior articular processes: (2) the size and character of the transverse process; and (1) the distance between the costal facet and the upper border of the vertebra.

^* In quadrupeds the majority of the spines of the thoracic vertabrae project dorsally and caudally, while those in the harnba.r region are directed, dorsally and headwaxds. The change in inclination is effected in one of the lower thoracic vertebra', the spine of which points almost straight dorsally. This vertebra, is known as the anticlinal, and in man its representative is the eleventh thoracic.