The Ribs

The ribs (Costæ) are elongated curved bones which form the skeleton of the lateral thoracic walls. They are arranged serially in pairs which usually correspond in number to the thoracic vertebræ. Each articulates dorsally with the spine and is continued ventrally by a costal cartilage. Those which articulate with the sternum by means of their cartilages are termed sternal or “true” ribs (Costæ sternales s. veræ); the remainder are asternal or “false” ribs (Costæ asternales s. spuriæ). Ribs at the end of the series which have their ventral ends free in the abdominal wall are named floating ribs (Costæ fluctuantes). The intervals between the ribs are termed intercostal spaces (Spatia intercostalia).

A typical rib consists of a shaft and two extremities. The shaft (Corpus costæ) is band-like and varies much in length, breadth, and curvature. In the case of some ribs the curvature is not uniform, but is most accentuated at a certain point, termed the angle of the rib (Angulus costæ); this occurs at a variable distance from the vertebral end, and is usually marked by a rough ridge. The direction also varies; the first rib is almost vertical, while the remainder slope backward in increasing degree. The external surface is convex, and the internal flattened from edge to edge; on the latter, close to the posterior border, is the costal groove (Sulcus costalis), which fades out ventrally. It contains the intercostal vein. The anterior and posterior borders are thin and sharp on some ribs, rounded on others.

The vertebral extremity (Extremitas vertebralis) consists of the head, neck, and tubercle. The head (Capitulum costæ) is the actual end of the rib, and is rounded and somewhat enlarged. It presents two facets (Facies articularis capituli costæ) for articulation with the bodies of two adjacent thoracic vertebræ; these surfaces are separated by a groove in which the conjugal ligament is attached. The neck (Collum costæ) joins the head to the shaft. It varies in length and diameter. Its outer surface is rough, its inner smooth. The tubercle (Tuberculum costæ) projects backward at the junction of the neck and shaft. It has a facet (Facies articularis tuberculi costæ) for articulation with the transverse process of the posterior vertebra of the two with which the head articulates. The tubercle gradually approaches the head in the posterior ribs, and eventually fuses with it.

The sternal extremity (Extremitas sternalis) is commonly slightly enlarged, and has a rough depression in which the costal cartilage is embedded.

Development.—The ribs are ossified in cartilage from three centers—one each for the shaft (and sternal end), head, and tubercle; the third center does not occur in the last two ribs.