AYES. 255 



bones generally so amalgamated in the adult, that it forms a 

 bony case of a single piece, the lower jaw alone remaining 

 movable. The head is jointed to the spine by no more than 

 a single articulating surface or condyle. The beak, ■which 

 forms such a conspicuous feature in birds, is composed of two 

 halves, an upper half or "upper mandible," and a "lower 

 mandible." The lower mandible, like the lower jaw of all the 

 Sawopsida, is at first composed of several pieces, but these 

 are all undistinguishably united in the adult, and the two 

 halves of the jaw are also amalgamated together. In no adult 

 bird are teeth ever developed in either mandible ; but both 

 mandibles are sheathed in horn, constituting the " beak," and 

 the margins of this sheath are sometimes serrated. 



The most characteristic points, however, in the skeleton of 

 the birds, are to be found in the structure of the limbs. The 

 cavity of the chest or thorax is bounded behind by the dorsal 

 vertebrae, on the sides by the ribs, and in front by the breast- 

 bone or sternum. The ribs vary in number from seven to 

 eleven pairs, and in most birds each rib gives off a peculiar 

 process (Fig. 138, B), which passes over the rib next in suc- 

 cession behind. In front the ribs are jointed to a series of 

 straight bones, which are called the " sternal ribs," and these, 

 in turn, are movably articulated to the breastbone in front. 

 According to Owen, these sternal ribs are " the centres upon 

 which the respiratory movements hinge." In front the cavity 

 of the chest is completed by an enormously-expanded breast- 

 bone or sternum (Fig. 128, A), which, in most birds of any 

 powers of flight, extends more or less over the abdominal 

 cavity as well. The sternum of all birds which possess the 

 power of flight is characterized by the presence of a prominent 

 ridge or "keel" (Fig. 138, A, h), to which are attached the 

 great muscles (pectoral muscles) which move the wings. As 

 a general rule, the size of this crest or keel gives a tolerably 

 just estimate of the flying powers of the bird to which it be- 

 longed. The keel is, of course, most largely developed in 

 those birds which possess the power of flight in its greatest 

 perfection ; and in those which do not fly, such as the Ostrich, 

 there is no sternal keel at all. The pectoral arch or shoulder- 

 girdle of birds consists of the shoulder-blades {scapulae), the 

 clavicles or collar-bones, and of two bones, which are distinct 

 in birds, and are called the " coracoid bones." The shoulder- 

 blades (s s) are usually long and narrow bones. The coracoid 

 bones (kk) correspond with the part of the shoulder-blade 

 which is known in most of the Mammals as the " coracoid 



