The sternebræ are designated by Roman numerals and the costal facets
by ordinary figures.
]
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.
[Illustration:
FIG. 25.—ANTERIOR APERTURE OF THORAX OF HORSE. (After Schmaltz, Atlas
d. Anat. d. Pferdes.)
]
The =anterior extremity= or =manubrium sterni=[5] 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.