The Costal Cartilages

The first costal cartilage is an inch or more (2.5 to 3 cm.) in length. The upper part is very wide and thick. The sternal end is small. The two articulate with each other as well as with the sternum. The cartilages of the other sternal ribs increase progressively in length and become more rounded. The sternal end is expanded and has an elliptical convex facet for articulation with the sternum. The cartilages of the asternal ribs are long, slender, and pointed. The ninth is the longest, and is very firmly attached to the eighth; behind this they diminish progressively in size, and are attached to each other by elastic tissue. Except in the case of the first, the cartilage does not continue the direction of the rib, but forms with the latter an angle which is open in front, and decreases in obtuseness from second to last.

More or less extensive ossification is to be regarded as a normal occurrence, especially in the cartilages of the sternal ribs.

The Sternum

The sternum of the horse is shaped somewhat like a canoe; it is compressed laterally, except in its posterior part, which is flattened dorso-ventrally. It is inclined obliquely so that the posterior end is about six to eight inches (15 to 20 cm.) lower than the anterior.

The dorsal surface has the form of a very narrow isosceles triangle with the apex in front. It is concave longitudinally, flattened transversely.

The lateral surfaces are convex above, slightly concave below, and diminish in extent behind. Each presents on its upper part seven articular cavities (Foveæ costales), which receive the sternal ends of the second to the eighth costal cartilages inclusive. These cavities are situated in series at the intersternebral junctions. The first four are elliptical in outline with the long diameter vertical, and are separated by considerable regular intervals. The others are progressively smaller, more circular, and closer together. The area below these cavities gives attachment to the pectoral muscles.

The dorso-lateral borders separate the dorsal and lateral surfaces. They give attachment to the lateral branches of the sternal ligament.

The ventral border forms the prominent keel-like crest of the sternum (Crista sterni) which may be felt in the living animal; it fades out behind.

The anterior extremity or manubrium sterni can be distinctly felt in the central furrow of the breast. It consists of a laterally compressed cartilaginous prolongation, commonly called the cariniform cartilage. Its lateral surfaces are flat and furnish attachment to muscles of the breast and neck. The ventral border is rounded, and is continued backward on the body of the bone. The dorsal border is concave and has an articular cavity for the first pair of costal cartilages.

The posterior extremity is formed by the xiphoid or ensiform cartilage (Processus xiphoideus). This is a thin plate, connected in front with the last bony segment by a relatively thick, narrow neck, and expanding in nearly circular form behind and laterally. Its dorsal surface is concave, and gives attachment to the diaphragm. The ventral surface is convex. The free margin is very thin.

Development.—At birth the sternum of the horse consists of seven bony segments or sternebræ united by intersternebral cartilages. The last two sternebræ fuse in the second month, but the others do not usually unite completely even in old age. The sternebræ consist of very vascular spongy bone covered by a very thin layer of compact substance. The adult sternum thus consists to a very considerable extent of persisting cartilage, viz., the intersternebral cartilages, the ventral keel, and the extremities; in old age these undergo partial ossification.