Sternum
the breast bone; in man, the flat bone, occupying the front of the chest, and formed by the meeting of the visceral arches. It is flattened from before background, and presents a slight vertical curve with the convexity in front. It is divided into the manubrium or presternum, the mesosternum, and the ensiform or xiphoid process of metasternum. All mammals and birds possess a sternum, and the presence or absence of a keel on that bone in birds is used as a means of classification. Fishes, amphibians, and ophidians have no sternum, and in saurians the broad portion is generally expanded. Some suppose that the plastron of the Chelonia is a highlydeveloped sternum; others hold that it is a mere integumentary ossification. The name sternum is also given to the plate on each segment of the breast of a crustacean and an arachnidan, but these are integumentary, and have no relation to a true sternum.