piece, apparently comparable to a xiphoid process, has been detected.
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Fig. 20.—Sternum of Rudolphi's Whale (Balaenoptera borealis), showing its relation to the inferior extremities of the first pair of ribs. × 1⁄10. (From Flower's Osteology.) |
Fig. 21.—Sternum of a young Dugong (Halicore indicus). × ¼. From a specimen in the Leyden Museum, ps, Presternum; xs, xiphisternum. (From Flower's Osteology). |
From the instances which have been described, as well as from the mode of development of the sternum and from the number of free ribs, i.e. ribs which are not attached to it, it would seem that the sternum has undergone a considerable reduction in its size. This reduction may be possibly accounted for by the need for respiratory activity, which is clearly increased by a less-marked fixity of the walls of the thoracic cavity. In the case of the Whales one can hardly help coming to that conclusion. The arrangement in the Monotremata does not, however, point in the same direction; for these animals are precisely like the higher Mammalia in the reduction of the sternum and of the number of ribs which reach it.
The Episternum.—The Mammalia are as a rule to be distinguished from lower Vertebrates by the absence of an episternum, or interclavicle as it is also called. In the Monotremata, however, there is a large T-shaped bone which does not overlie the sternum as in reptiles, but is anterior to it. The relations of this bone to the clavicles seem to leave no doubt that it is the equivalent of the Lacertilian interclavicle or episternum. The Monotremata are not, however, the only mammals in which this structure is to be seen. The Mole in the embryonic condition is
