STERNUM, a portion of the skeleton of animals. It is present in the articulates (arthropoda) and crustaceans, as well as arachnides and insects, and in those vertebrates having exoskeletons, as tortoises, although Mivart and many others hold that the plas tron does not form (or contain) a sternum—in other words, that the chelonia are ster numle3s. A true sternum, however, belongs to the warm-blooded vertebrates, 'and' reaches its highest development in the birds. The somites of locusts have well-developed sternal sections included between the pleura or side portions of the segments, especially in the abdomen. All these sternal sections are collectively called the sternum. It can, however, be said to be only analagous to the mammal sternum, or the sternum of birds. It has nearer relations to the sternal pieces of the tortoises, because the plastron which these pieces constitute is a part of the exoskeleton, and, as above mentioned, is by many not regarded as a true sternum. In some mammals, and especially in man, there might seem to he more analogy (if analogies are to be strained) between the sternum and the " exoskeleton sternum," than between this latter and the sternum of birds, for in them the relations with the rest of the endoskeleton are much more extensive. Man's ster num is little else than" a point of attachment for certain pleural pieces, the ribs and rib cartilages, and a covering or shield for the central part of the thorax. In the clielonia the whole of the under shell, or plastron, is regarded by some as a sternum, and com posed of pairs of pieces, and one single piece, the entosternal. The pairs are the epi stern ale, the most anterior, arched portion of the plastron, and including within the arch the entosternal ; the hyosternals and hyposternals, respectively anterior lateral and pos terior lateral, and not joined together as pairs in the median line. Posteriorly, and
joined together at their extremities, are the xiphisternals, small curved pieces, forming an arch, like the anterior episternals. In birds of flight the sternum is the most impor tant bone in the body; it is enormously expanded, and gives attachment to the powerful pectoral muscles which move the wings. In birds of great powers of flight it extends over the abdominal cavity, and sometimes reaches the pelvis, In all birds which fly it has a median ridge, called the keel, which is prominent in proportion to the powers of flight, examples of which are seen in the sternum of the pigeon, the duck, and the wild goose. In ostriches and other birds which do not fly the sternum has no keel. In the mannnalia it is composed of several pieces, usually three, the manubrium, or anterior portion, the mesosternum, or middle portion, and the xiphisternum, or posterior portion (in man, xipboid cartilage). In most mammals the sternum is long and narrow, but in some, as in cetacea, it is broad. The Greenland whale has only a manubrium, while the &gong has this piece and the xiphisternum, the mesosternum being absent. In some marnmals, burrowing and flying animals, as the moles and bats, the sternum has a keel, but it is placed differently than in birds, being more anterior. The middle and posterior parts of the sternum in birds are those which are most developed, the manubriinn being greatly subordinate or dwarfed.