STERNUM 61 



Later it may become calcified (ReptiJes), or converted into true 

 bone (Birds, Mammals). In Reptiles, Birds, and Monotremes the 

 coracoids, as in Amphibians, always come into direct connection 

 with the lateral edges of the sternum (comp. Figs. 41, 44, and 48). 

 The sternum is greatly developed in Birds, and consists of a 

 broad more or less fenestrated plate, provided in the vast majority 

 of Carinatae with a projecting keel, which forms an additional 

 surface for the origin of the wing-muscles (Fig. 41). In contrast 

 to these, the cursorial Ratitse are characterised by a broad, more 

 or less arched, shield-like sternum without a keel. In some 

 flightless Carinatse, however, the keel is rudimentary or even ab- 

 sent, and a keel may occasionally appear, though not constantly, 



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). 



