The Ribs (costae) are twenty-four in number, twelve on each side. They articulate posteriorly with the dorsal vertebrae, and anteriorly with the sternum and one another (except the eleventh and twelth, which are free at their anterior ends), thus forming the lateral walls of the thorax.

A rib is a long, narrow, flat bone, consisting of a head and neck attached to the shaft. The head is somewhat expanded, and has on its inner surface a facet for articulation with the vertebrae, the lower half of which corresponds with the facet on the upper margin of the body of the vertebra corresponding with it, so that each rib articulates with the body of the vertebra with which it is in numerical sequence, and with the body of the one above. Thus, the seventh rib articulates with the upper margin of the body of the seventh vertebra and the lower margin of the body of the sixth. Below, the head is constricted to form a neck, and where it joins the shaft there is a tubercle on the posterior surface. The tubercle has a facet to articulate with the transverse process of the vertebra numerically corresponding with it. The shaft is long, flat, and narrow. It is convex laterally, but the curve is much sharper posteriorly than anteriorly, and where the curve is sharpest there is a rough, oblique ridge on the external surface; this is called the angle. On the first rib the tubercle and angle coincide. They gradually separate, being farthest apart on the eighth rib, and approaching one another again on the lower ones. The shaft of the rib is also twisted on itself, so that, if laid on a flat surface, one end is always raised up. This is most marked in the middle ones, and not found at all in the first, second, and twelfth. The upper border is thick and rounded, the lower one thin and sharp, and on the inner surface of the lower border is a groove for vessels and nerves. The anterior end of the shaft is slightly expanded and hollowed out for the attachment of the costal cartilage, by means of which the ribs articulate with the sternum or one another.

The first rib is much smaller than the others, quite flat, and forms nearly a semicircle. On its upper surface, about midway, is a groove for the subclavian artery. On the inner border, about an inch from the anterior extremity, is the scalene tubercle. The surfaces are directed upwards and downwards.

Fig. 32. - A Typical Rib.

The second rib resembles the first in shape and in having no twist, but is a good deal larger. The surfaces are directed obliquely, similarly to those of the other ribs.

The eleventh and twelfth ribs are very much shorter and smaller than the others. Their anterior extremities are pointed and tipped with cartilage, but are free and non-articulating.

Ossification

The shaft is almost completely ossified before birth. Secondary centres for the head and tubercle appear before puberty, and the whole rib is fused by the twenty-fifth year.