. Elements of the comparative anatomy of vertebrates. Anatomy, Comparative. A B C Tig. 45.—A, Stbknum of Fox ; B, of Walrus; and C, of Man. 3fb, manubrium ; O, body ; Pe, xiphoid process ; R, ribs. in certain Ratitae. The presence or absence of a keel is not, there- fore, a constant character separating these two groups of Birds from one another.^ A greater number of ribs are as a rule concerned in the forma- tion of the sternum of Mammals than is the case in Reptiles and Birds. Consisting at first of a simple cartilaginous plate, the sternum later becomes segmented into definite bony portions
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. Elements of the comparative anatomy of vertebrates. Anatomy, Comparative. A B C Tig. 45.—A, Stbknum of Fox ; B, of Walrus; and C, of Man. 3fb, manubrium ; O, body ; Pe, xiphoid process ; R, ribs. in certain Ratitae. The presence or absence of a keel is not, there- fore, a constant character separating these two groups of Birds from one another.^ A greater number of ribs are as a rule concerned in the forma- tion of the sternum of Mammals than is the case in Reptiles and Birds. Consisting at first of a simple cartilaginous plate, the sternum later becomes segmented into definite bony portions (sternebrse) the number of which may correspond to the affixed ribs (Fig. 45, A, B). But in other cases, as, for instance, amongst Primates (C), the individual bony segments may run together to form a long plate (corpus sterni). The anterior end of the sternum becomes differentiated into the so-called manubrium, and the posterior end into the xiphoid or ensiform process. The latter owes its origin in the embryo to the ventral fusion of a true pair of ribs. ^ A keel was also present in the flying Reptile Plesiosmirus, and may be developed wherever a larger surface for the origin of the pectoral muscles is required {e.g., Cheiroptera).. 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 work.. Wiedersheim, Robert, 1848-1923; Parker, William Newton, 1857-1923. London, Macmillan