The articulations of the ribs with the vertebral column may be divided into two sets, one connecting the heads of the ribs with the bodies of the vertebrae, another uniting the necks and tubercles of the ribs with the transverse processes.
These articulations constitute a series of plane joints. They are formed by the articulation of the heads of the typical ribs with the facets on the contiguous margins of the bodies of the thoracic vertebrae, and with the intervertebral with a single vertebra; in each of the other joints, an intra-articular ligament divides the joint cavity into two distinct parts. The ligaments of the joints are: capsular, radiate, intra-articular.
The capsular ligaments connect the heads of the ribs with the circumferences of the articular cavities formed by the intervertebral discs and the adjacent vertebrae. Some of their upper fibers pass through the intervertebral foramen to the back of the intervertebral disc, while the posterior fibers are continuous with the inferior costotransverse ligament (Ligament of the neck of the rib). The radiate ligament connects the anterior part of the head of each rib with the sides of the bodies of two vertebrae, and the intervertebral disc between them. It is attached to the anterior part of the head of the rib, just beyond the articular surface. The superior fibers ascend and are connected with the body of the vertebra above; the inferior fibers descend to the body of the vertebra below; the middle fibers, the smallest and least distinct, are horizontal and attached to the intervertebral disc.
2. COSTOTRANSVERSE JOINTS (fig. 520)
The articular portion of the tubercle of a rib forms a plane joint with the transverse process of the vertebra to which it corresponds numerically. In the eleventh and twelfth ribs this articulation is wanting.
The ligaments of the joints are; capsular, inferior costotransverse, superior costotransverse. lateral costotransverse.
The capsular ligament is a thin membrane attached to the circumference of the articular surfaces, and lined with synovial membrane.
The superior (anterior) costotransverse ligament is attached below to the crest on the upper border of the neck of the rib, and passes obliquely upwards and laterally to the lower border of the transverse process immediately above.
The first rib has no superior costotransverse ligament. The neck of the twelfth rib is connected to the base of the transverse process of the first lumbar vertebra by a band of fibers, named the lumbocostal ligament; it is in series with the superior costotransverse ligaments.
The inferior costotransverse ligament (ligament of the neck of the rib) consists of short but strong fibers, connecting the rough surface on the back of the neck of the rib with the anterior surface of the adjacent transverse process. A rudimentary ligament may be present at the eleventh and twelfth ribs.
The lateral costotransverse ligament (ligament of the tubercle of the rib) is a short, thick, strong fasciculus, which passes obliquely from the apex of the transverse process to the rough non-articular portion of the tubercle of the rib. The ligaments attached to the upper ribs ascend from the transverse processes; they are shorter and more oblique than those attached to the lower ribs, which descend slightly.
Movements.-The heads of the ribs are so closely connected to the bodies of the vertebrae by the radiate and intra-articular ligaments that only slight gliding movements of the articular surfaces on one another can take place. Similarly, the strong ligaments binding the necks and tubercles of the ribs to the transverse processes limit the movements of the costotransverse joints to slight gliding, the nature of which is determined by the shape and direction of the articular surfaces (fig. 521). The articular surfaces on the tubercles of the upper six ribs are oval in shape and convex from above downwards; they fit into corresponding concavities on the anterior surfaces of the transverse processes, so that upward and downward movements of the tubercles are associated with rotation of the rib-neck on its long axis. On the seventh, eighth, ninth and tenth ribs the articular surfaces on the tubercles are flat, and are directed obliquely downwards, medially and backwards. The surfaces with which they articulate are placed on the upper aspects of the transverse processes; when, therefore, the tubercles are drawn up they are at the same time carried backwards and medially. The joints of the heads of the ribs and the costotransverse joints move simultaneously aid in the same directions, the total effect being that the neck of the rib moves as if on a single joint, of which the two articulations form the ends. In the upper six ribs the neck of the rib moves but slightly upwards and downwards; its chief movement is one of rotation. round its own long axis, rotation downwards of the front of the neck of the rib being associated with depression, rotation upwards with elevation of the anterior end of the rib and its costal cartilage. In the seventh, eighth, ninth and tenth ribs the neck of the rib moves upwards, backwards and medially, or downwards, forwards and laterally; very slight rotation accompanies these movements.
Muscles producing the movements.-These are discussed with the mechanism of respiration.
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