BIRDS. 



are usually, as before said, anchylosed with one another to a 

 greater or less extent. Laterally, the thorax is bounded by 

 the ribs, which vary in number from six to ten pairs. In most 

 birds each rib carries a peculiar process the "uncinate pro- 

 cess " which arises from its posterior margin, is directed up- 

 wards and backwards, and passes over the rib next in succes- 

 sion behind, where it is bound down by ligament. The first 

 and last dorsal ribs carry no uncinate processes, and in some 

 cases the processes continue throughout life as separate pieces 

 (fig. 329, B). Anteriorly, the ribs articulate with a series of 



B 



Fig. 329. A, Breast-bone, shoulder-girdle, and fore-limb of Penguin (after Owen): 

 b Sternum, with the sternal keel ; j J Scapulae; k k Coracoid bones; c Furculum or 

 merry-thought, composed of the united clavicles ; h Humerus ; Ulna ; r Radius ; t 

 Thumb ; ? Metacarpus ; / Phalanges of the fingers. B, Ribs of the Golden Eagle : a a 

 Ribs giving off( b) uncinate processes ; c c Sternal ribs. 



straight bones, which are called the " sternal ribs," but which 

 in reality are to be looked upon as the ossified " costal carti- 

 lages." These sternal ribs (fig. 329, B) are in turn movably 

 articulated to the sternum in front, and " they are the centres 

 upon which the respiratory movements hinge " (Owen). In 

 front the thoracic cavity is completed by an enormously-ex- 

 panded sternum or breast-bone, which in some birds of great 

 powers of flight extends over the abdominal cavity as well, 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. 329, A), to which are 



