CHARACTERS OF AVES. 485 



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 

 remains 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 quadrate 

 bone (os carre of the French) is in contact at one end with an 

 elongated bone (jugal bone), the other end of which comes 

 against the palate. By the depression of the lower jaw, assisted 

 by proper muscles, the quadrate bone is brought forward, and 

 the jugal bone is thus made to elevate the palate and upper 

 jaw. With one or two exceptions, the upper mandible is thus 

 movable in all birds. 



Fig. 211. Skull of Spur- winged Goose (Plectropterus Gambensis}. 



The thoracic cavity is bounded behind by the dorsal verte- 

 brae, which are usually, as before said, anchylosed to one ano- 

 ther 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 

 process " which arises from its posterior margin, is directed 

 upwards 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. 212, 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. 212, 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 grea*; 



