II Ribs. Costæ (Figs. 1 And.)15
The cat has thirteen pairs of ribs. One of the fifth pair (Fig. 15) may be taken as typical. It is a curved flattened rod of bone attached at its dorsal end to the vertebral column, and at its ventral end to a cartilage (costal cartilage, Fig. 15, f) which serves to unite it to the sternum.
The most convex portion of the bone is known as the angle (e). Each rib presents a convex lateral and a concave medial surface, a cranial and a caudal border. The borders are broad dorsad and narrow ventrad, while the surfaces are narrow dorsad and broad ventrad. The rib has thus the appearance of having been twisted.
The rib ends dorsad in a globular head or capitulum (a), by which it articulates with the costal demifacets of two contiguous thoracic vertebræ. Between the capitulum and angle on the lateral surface is an elevated area, the tubercle, marked by the smooth tubercular facet (c) for articulation with the transverse process of a vertebra. The constricted portion between the head and tubercle is known as the neck (collum) (d). The angle is marked by a projecting process (e) (angular process) on its lateral border, for attachment of a ligament.
The ribs increase in length to the ninth (the ninth and tenth are of the same length) and then decrease to the last. They decrease in breadth behind the fifth. The first is nearly in a dorsoventral plane, while the others have their dorsal ends inclined slightly craniad.
The tubercles become less prominent as we pass caudad and are absent on the last two or three ribs, which do not articulate with the transverse process.
The first nine ribs (true ribs or costæ veræ) are attached separately to the sternum by their costal cartilages. The last four (false ribs or costæ spuriæ) are not attached separately to the sternum. The costal cartilages of the tenth, eleventh, and twelfth are united to one another at their sternal ends. They may be united also to the ninth costal cartilage or to the sternum by a common cartilage of insertion, or they may be quite free from the sternum. The thirteenth costal cartilages are free (floating ribs).
Ligaments of the Ribs.—The articular surfaces between the head of the rib and the centra, and between the tubercle and the transverse process of the vertebra, have each an articular capsule. There are also a number of small ligamentous bands from the tuberosity and the neck of the rib to the transverse process of the vertebra.