In Insectivora the sternum is well developed but variable. The presternum is commonly large and is sometimes as in the Hedgehog (Erinaceus) bilobed in front, sometimes as in the Shrew (Sorex) trilobed. It is especially large in the Mole (Talpa) and is expanded laterally and keeled below.
In the Chiroptera the presternum is strongly keeled and so is sometimes the mesosternum.
Among Primates, in Man and the Anthropoid Apes the sternum is rather broad and flattened; the mesosternum consists of four segments which are commonly fused together and the xiphisternum is imperfectly ossified.
The Ribs.
Free ribs are borne as a rule only by the thoracic vertebrae; ribs may be found in other regions, especially the cervical and sacral, but these are almost always ankylosed to the vertebrae. As a general rule the first thoracic rib joins the presternum, while the succeeding ones are attached between the several segments of the mesosternum. Some of the posterior ribs frequently do not reach the sternum; they may then be attached by fibrous tissue to the ribs in front, or may end freely (floating ribs). There are generally thirteen pairs of ribs, and in no case do they have uncinate processes.
In Monotremes (fig. 102, B) each rib is divided not into two but into three parts, an intermediate portion being interposed between the vertebral and sternal parts. The sternal ribs are well ossified, and some are very broad and flat. The intermediate portions are unossified, those of the anterior ribs are short and narrow, but they become longer and wider further back.
In Marsupials there are almost always thirteen pairs of ribs, whose sternal portions are very imperfectly ossified. Notoryctes has fourteen pairs of ribs, eight of which are floating: the first rib is very stout, and is abruptly bent on itself to join the sternum. It has no distinct sternal portion. All the other ribs are slender.
Of the Edentates the Sloths have very numerous ribs; twenty-four pairs occur in Choloepus, and half of these reach the sternum. In the Armadillos there are only ten or twelve pairs of ribs, but the sternal portions are very strongly ossified. The first rib is remarkably broad and flat, and is not divisible into vertebral and sternal portions.
In the Sirenia there are a very large number of ribs noticeable for their great thickness and solidity, but not more than three are attached to the sternum.
Cetacea. In the Whalebone whales the ribs are remarkable for their very loose connection both with the vertebral column and with the sternum. The capitula are scarcely developed, and the attachment of the tubercula to the transverse processes is loose. The first rib is the only one connected with the sternum. In the Toothed whales the anterior ribs have capitula articulating with the centra, as well as tubercula articulating with the transverse processes; in the posterior ones, however, only the tubercula remain. Seven pairs of well-ossified sternal ribs generally meet the sternum. In the Physeteridae most of the ribs are connected to the vertebrae by both capitula and tubercula.