. The Cambridge natural history. Zoology. Fig. 20.—Sternum of Rudolp-lii's Wliale (Bui uenoptera borealis), showiug its relation to the inferior extremities of the first pair of rite. x-i\j. (Fiom Flov/er's Fig. 21.—Sternum of a young Dugong {Halicore indicus). X ^. From a specimen in the Leyden Museum. ^5, Presternum ; xs, xiphistei- num. {From Flower's Oste- ology). 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, ribs which are not attached to it, it would seem that the sternum has underg
. The Cambridge natural history. Zoology. Fig. 20.—Sternum of Rudolp-lii's Wliale (Bui uenoptera borealis), showiug its relation to the inferior extremities of the first pair of rite. x-i\j. (Fiom Flov/er's Fig. 21.—Sternum of a young Dugong {Halicore indicus). X ^. From a specimen in the Leyden Museum. ^5, Presternum ; xs, xiphistei- num. {From Flower's Oste- ology). 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, ribs which are not attached to it, it would seem that the sternum has undergone a considerable reduc- tion 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 j\famnralia in the reduction of the sternum and of the numljer of ribs which reach it. The Episternum.—The Mammalia are as a rule to be dis- tinguished from lower Vertebrates by the absence of an episternum, or interclavicle as it is also called. In the ilonotremata, however, there is a large T-shaped bone which does not overlie the ster- num 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 JMonotremata are not, however, the only mammals in which this structure is to be seen. The Mole in the embryonic condition is VOL. X D. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original Harmer, S. F. (Sidney Frederic), Sir, 1862- ed; Shipley, A. E. (Arthur Everett), Sir, 186
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Keywords: ., bookcentury1800, bookdecade1890, booksubjectzoology, bookyear1895
