STERNUM. 



53 



muscles. In contrast to these, the Ratitas (" Cursorial Birds ") are 

 characterised by a broad, slightly-arched, shield-like sternum without 

 a keel. 



In both cases the sternum arises in two bauds 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 metasternum, which gives rise to the median and posterior portion of 

 the sternum of the adult. In the Ratitae, the metasternum, which probably 

 arises from the fused edges of the costal portion, remains partly cartilaginous 

 in the adult. In the Carinatae, 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 



FIG. 40. A, STERNUM OF Fox ; B, OF WALKUS ; AND C, OF MAN. 

 Mb, manubrium ; C, body ; Pe, xiphoid process ; R, 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 together to form a long plate 

 (corpus sterni), of which the proximal end becomes differen- 

 tiated into the so-called manubrium, and the distal end into the 

 xiphoid process (processus ensiformis). The latter (Fig. 40, 

 C, Pe] owes its origin in the embryo to the ventral fusion of a true 

 pair of ribs, arising independently or as a direct continuation of 

 the primary sternal tracts, from which it later becomes segmented 



