The cartilages of the true ribs, with the exception of the first, articulate with the sternum by synovial joints. The cartilage of the first rib is directly united with the stcrmlm, and the joint between this rib and the sternum is a primary cartilaginous joint:
The ligaments of the synovial joints are; capsular, intra-articular, sternocostal, costoxiphoid.
The capsular ligaments surround the joints between the sternum and the cartilages of the ribs from the second to the seventh inclusive. They are very thin, intimately blended with the sternocostal ligaments, and strengthened at the upper and lower parts of the articulations by a few fibers which connect the cartilages to the side of the sternum.
The sternoeostal ligaments are broad, thin, membranous bands which radiate from the, front and back of the sternal ends of the cartilages of the true ribs to the anterior and posterior surfaces of the sternum. Their superficial fibers intermingle with the fibers of the ligaments above and below them, with those of the opposite side, and on the front of the sternum with the tendinous fibers of origin of the pectoralis major, forming a thick fibrous membrane which envelops the bone, and is more distinct at its lower than at its upper part.
The intra-articular ligaments are constantly present only between the second costal cartilages and the sternum. The cartilage of the second rib is connected with the sternum by means of an intra-articular ligament, attached laterally to the cartilage of the rib, and medially to the fibrocartilage which unites the manubrium and body of the sternum. Occasionally the cartilage of the third rib is connected with the first and second pieces of the body of the sternum by an intra-articular ligament. Still more rarely, similar ligaments are found in the other four joints of the series. In the lower two an intra-articular ligament sometimes obliterates the joint-cavity, so as to convert the articulation into a syndesmosis. After middle life the articular surfaces lose their polish, become roughened, and the synovial membrane apparently disappears. In old age, the cartilages of most of the ribs become continuous with the sternum, and the joint cavities are consequently obliterated.
The costoxiphoid ligaments connect the anterior and posterior surfaces of the seventh costal cartilage, and sometimes those of the sixth, to the front and back of the xiphoid process. They vary in length and breadth in different subjects; those on the back of the joint are less distinct than those in front.
Movements.-Slight gliding movements are permitted in the sternocostal joints.
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