428 MANUAL OF ZOOLOGY. 



piece. As in the Reptiles, the lower jaw articulates with the 

 skull, not directly, but through the intervention of a distinct 

 bone the quadrate bone or tympanic bone which always re- 



)Fig. 167. Skull of Spur-winged Goose (Plectropterus Gambensis). 



mains permanently movable, and is never anchylosed with the 

 skull. In no bird are teeth ever developed in either jaw, but 

 both mandibles are encased in horn, forming the beak, and 

 the margins of the bill are sometimes serrated. 



The thoracic cavity is bounded by the dorsal vertebrae, 

 which are usually, as before said, anchylosed to 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. 1 68, 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 carti- 

 lages." These sternal ribs (fig. 168, 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. 168, A), to which are at- 

 tached 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 



