The =sternum= or breast-bone is a median segmental bone which completes
the skeleton of the thorax ventrally, and articulates with the
cartilages of the sternal ribs laterally. It consists of six to eight
bony segments (Sternebræ) connected by intervening cartilage in the
young subject. Its form varies with that of the thorax in general and
with the development of the clavicles in animals in which they are
present. Its anterior extremity, the =manubrium sterni= or =presternum=,
is specially affected by the latter factor, being broad and strong when
the clavicles are well developed and articulate with it (as in man),
relatively small and laterally compressed when they are absent (as in
the horse) or rudimentary (as in the dog). The cartilages of the first
pair of ribs articulate with it. The =body= or =mesosternum= (Corpus
sterni) presents laterally, at the junction of the segments, concave
facets (Incisuræ costales) for articulation with the cartilages of the
sternal ribs. The posterior extremity or =metasternum= presents the
=xiphoid= (or ensiform) =cartilage= (Processus xiphoideus); this is thin
and plate-like, as in the horse and ox, or narrow and short, as in the
pig and dog.
=Development.=—The cartilaginous sternum is formed by the fusion
medially of two lateral bars which unite the ventral ends of the first
eight or nine costal cartilages, and is primitively unsegmented. The
manubrium ossifies from a single center, but the centers for the other
segments appear to be primitively paired. The sternum never becomes
completely ossified; details in regard to persisting cartilage will be
given in the special descriptions. The layer of compact tissue is for
the greater part very thin and the spongy substance is open-meshed and
very vascular.