General features.-The costal cartilages (fig. 308) are bars of hyaline cartilage which extend forwards from the anterior ends of the ribs and contribute very materially to the elasticity of the walls of the thorax. The first seven pairs are connected with the sternum; the eighth, ninth, and tenth are each articulated with the lower border of the cartilage immediately above ; the last two are pointed, and end in the muscular wall of the abdomen. The costal cartilages vary in their length, breadth and direction. They increase in length from the first to the seventh, then gradually decrease to the twelfth. They diminish in breadth from the first to the last, like the intervals between them. They are broad at their attachments to the ribs, and taper towards their medial extremities, with the exception of the first and second which are of the same breadth throughout, and the sixth, seventh, and eighth which are enlarged where their margins are in contact. The first cartilage descends a little, the second is horizontal, the third ascends slightly, while the others are angular, continuing the course of the ribs for a short distance, and then inclining upwards to the sternum or preceding cartilage.
Particular features.- Each costal cartilage has two surfaces, two borders, and two ends. The anterior surface is convex, and looks forwards and upwards: that of the first gives attachment to the costoclavicular ligament and the subclavius muscle ; those of the first six or seven at their medial ends; to the pectoralis major muscle. The others are covered by, and give partial attachment to, some of the flat muscles of the anterior abdominal wall. The posterior surface is concave, and directed backwards and downwards; that of the first gives attachment to the sternothyroid, those of the second to the sixth inclusive to the sternocostalis (transversus thoracis), and the six inferior ones to the transversus abdominis and the diaphragm. The superior border is concave, and the inferior convex ; they afford attachment to the internal intercostal muscles and the anterior intercostal membranes. The interior borders of the sixth, seventh, eighth. and ninth cartilages present heel-like projections at the points of greatest convexity ; a similar heel-like projection occurs on the lower border of the fifth right cartilage in 72 per cent. and on the fifth left cartilage in 50 per cent. (Fawcett). On these projections are oblong facets which articulate respectively with facets on slight projections from the superior borders of the sixth, seventh; eighth, ninth, and tenth cartilages. The lateral end of each cartilage is continuous with the osseous tissue of the corresponding rib. The medial end of the first is continuous with the sternum; the medial ends of the six succeeding cartilages are rounded and articulate with the shallow costal notches on the lateral margins of the sternum. The medial ends of the eighth, ninth. and tenth costal cartilages are pointed and each is connected with the cartilage immediately above. Those of the eleventh and twelfth are pointed and free.
In old age the costal cartilages are prone to undergo superficial ossification.
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