AVES. 279 
The skull in birds has its several bones generally so amal- 
gamated 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 Sauropszda, 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. Inno adult bird are /ee¢h ever developed in either 
mandible ; but both mandibles are sheathed in horn, constitut- 
ing 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 breastbone or 
sternum. ‘The ribs vary in number from seven to eleven pairs, 
and in most birds each rib gives off a peculiar process (fig. 151, 
B), which passes over the rib next in succession 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 articu- 
lated to the breastbone in front. According to Owen, these 
sternal ribs are “the centres upon which the respiratory move- 
ments hinge.” In front the cavity of the chest is completed by 
an enormously-expanded breastbone or sternum (fig. 151, 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 characterised by the pre- 
sence of a prominent ridge or “keel” (fig. 151, A, 8), 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 
atolerably just estimate of the flying powers of the bird to which 
it belonged. The keel is, of course, most largely developed in 
those birds which possess the power of flight in its greatest per- 
fection ; 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 (scapu/e), the clavicles or 
collar-bones, and of two bones, which are distinct in birds, and 
are called the “ coracoid bones.” The shoulder-blades (ss) are 
