Elements of the comparative anatomy Elements of the comparative anatomy of vertebrates elementsofcompar00wied Year: 1886 STERNUM. 53 muscles. In contrast to these, the Ratitae (' Cursorial Birds ') are characterised by a broad, slightly-arched, shield-like sternum without a keel. In both cases the sternum arises in two bands connected with ribs, a greater number often taking part in its formation than are present in the adult. According to Lindsay, the two parts of this costal sternum, corresponding to the breast-bone of Reptiles and Mammals, become connected by a portion, the inetasternuin,
Elements of the comparative anatomy Elements of the comparative anatomy of vertebrates elementsofcompar00wied Year: 1886 STERNUM. 53 muscles. In contrast to these, the Ratitae (' Cursorial Birds ') are characterised by a broad, slightly-arched, shield-like sternum without a keel. In both cases the sternum arises in two bands connected with ribs, a greater number often taking part in its formation than are present in the adult. According to Lindsay, the two parts of this costal sternum, corresponding to the breast-bone of Reptiles and Mammals, become connected by a portion, the inetasternuin, which gives rise to the median and posterior portion of the sternum of the adult. In the Ratitee, the inetasternum, which probably arises from the fused edges of the costal portion, remains partly cartilaginous in the adult. In the Cariuatse, it gives rise to a median ventral outgrowth, the keel. This generally arises at the time of the fusion of the two halves, but in some cases there appears to be a tendency for it to become differentiated from the rest of the sternum. It is either ossified by the fusion of a pair of lateral bony centres, or else by means of a separate endosteal (see p. 63) centre. Thus the keel of the Bird's sternum is probably of late phylogenetic development, arising in correlation with the large development of the pectoral muscles, and having no relationship to the interclavicle of Reptiles, as is often asserted. A far greater number of ribs are as a rule concerned in the formation of the breast-bone of Mammals than is the case in ABC FIG. 40.—A, STERNUM OF Fox ; B, OF WALRUS ; AND C, OF MAN. Mb, manubrium ; C, body ; Pe, xiphoid process ; E, ribs. Reptiles and Birds. Consisting at first of a simple cartilaginous plate, it later becomes segmented into definite bony regions, the number of which originally corresponds to the affixed ribs (Fig. 40, A, B). But in other cases, as, for instance, in Primates, the individual bony segments usually run togethe
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