STERNUM 



73 



tremes, the coracoids, as in Amphibians, come into direct connec- 

 tion with the lateral edges of the sternum (Figs. 53, 56, and 103), 

 and in other Mammals, the clavicles, when present, are connected 

 with it directly or indirectly. 



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

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

 of Carinata3 with a projecting keel, which forms an additional 

 surface for the origin of the wing-muscles (Fig. 53). 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 

 absent, and the vestige of a keel may occur, though not constantly, 



FIG. 57. A, STERNUM OF Fox; B, OF WALKUS ; AND C, OF MAN. 



From the ventral side. 



C, body ; J/Z>, manubrium ; Pe, xiphoid process ; B, ribs. 



in certain Ratitse. The presence or absence of a keel is not, there- 

 fore, a constant character separating these two groups of Birds 

 from one another. 1 



A far greater number of ribs are as a rule concerned in the 

 formation of the sternum of Mammals than is the case in Reptiles 

 and Birds. Consisting at first of a simple cartilaginous plate, it 

 later becomes segmented into definite bony portions (stcrnclircK) 

 the number of which may correspond to the affixed ribs (Fig. 57, 

 A, B) : in other cases as, for instance, amongst Primates (c), the 

 individual bony segments may run together to form a long plate 

 (corpus sterni}. Its proximal end forms a more or less distinct 

 manubrium, and the distal end a partly cartilaginous xiphoid or 

 ensiform 



1 A keel was also present in the Pterosauria, and may be developed 

 wherever a larger surface fur the origin of the pectoral muscles is required (e.g. 

 Cheiroptera). 



