246 



BIRDS. 



out life as separate pieces (fig. 582, b). Anteriorly, the ribs 

 articulate with a series of straight bones, which are called 

 the " sternal ribs," but which in reality are to be looked 

 upon as the ossified " costal cartilages." These sternal ribs 

 (fig. 582, b) are in turn movably articulated to the sternum 



Fig. 582. — A, Breast-bone, shoulder-girdle, and fore-limb of Penguin (after Owen) : 6, 

 Sternum, with the sternal keel ; s, s, Scapulse ; k, k, Coracoid bones ; c, Furculum or merry- 

 thought, composed of the united cla\neles ; h. Humerus ; u. Ulna ; r, Radius ; t, Thumb ; 

 VI, Metacarpus; p. Phalanges of the fingers, b, Ribs of the Golden Eagle : a, a. Ribs giving 

 oft' (b, b) uncinate processes ; c, c, Sternal ribs. 



in front, and " they are the centres upon which the respira- 

 tory movements hinge " (Owen). In front the thoracic 

 cavity is completed by an enormously-expanded sternum or 

 breast-bone, which in some birds of great powers of flight 

 extends over the abdominal cavity as w^ell, in some cases 

 even reaching the pelvis. The sternum of all birds which 

 fly, is characterised by the presence of a greatly-developed 

 median ridge or keel (fig. 582, a), to which are attached the 

 great pectoral muscles which move the wings. As a general 

 rule, the size of this sternal crest allows a very tolerable 

 estimate to be formed of the flying powers of the bird to 

 which it may have belonged ; and in the Ostriches and other 

 birds which do not fly, there is no sternal keel (fig. 583). 



