280 VERTEBEATE ANIMALS. 



" sacrum " ; and this, in turn, is united on both sides with the bones 

 which form tlie pelvic arch, which carries the hind -limbs. The 

 vertebrae of the tail are more or less movable upon one another ; 

 and in almost all living birds, when fully grown, the last 'joint of 

 the tail (fig. 20 I, B, s) is a long, slender, ploughshare-shaped bone, 

 which is really comjiosed of several vertebrae united together. It 

 is usually set on at an angle nearly jjerpendicular to the axis of 

 the body, and it serves to support the great tail-feathers, which act 

 as a rudder during flight. It also serves to support the oil-gland, 

 which supplies the secretion with which the fea,thers are lubricated. 

 The skull in Biiils has its several bones generally so amalgamated 

 in the adult, that it forms a bony case in which the sutures can no 

 longer be recognised. 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 mandi- 

 ble." The lower mandible, like the lower jaw of all the Saitrop- 

 sida, is at first composed of several pieces, but these are all undis- 

 tinguishably united in the adult, and the two halves of the jaw 

 are also amalgamated together. In no living Bird are tfeth ever 

 developed in either mandible ; but both mandibles are sheathed in 

 horn, constituting the " beak," and the margins of this sheath are 

 sometimes serrated. In some curious fossil Birds, however, there 

 are teeth in the jaws. The lower jaw is articulated to the skull 

 by the intervention of a movable "quadrate bone." In many 

 Birds, also, tlie upper mandible of the beak is so joined to the skull 

 as to be capable of movement. 



The most characteristic points, however, in the skeleton of the 

 Birds, are to he found in the structure of the limbs. The cavity 

 of the chest or thorax is bounded behind by the dorsal vertebra;, 

 on the sides by the ribs, and in front by the breast-bone or ster- 

 num. The ribs vary in number from seven to eleven pairs, and in 

 most Birds most of the ribs give off a peculiar process (" uncinate pro- 

 cess," fig. 202, B) which passes over the rib next in succession behind. 

 In front the rilis are jointed to a series of straight bones, which are 

 called the "sternal ribs," and these, in turn, are movably articu- 

 lated to the breast-bone in front. According to Owen, these ster- 

 nal ribs are " the centres upon which the respiratory movements 

 hinge.'' In front the cavity of the chest is completed by an enor- 

 mously expanded breast- bone or sternum (fig. 202, A), which, inmost 

 biids of any powers of flight, extends more or less over the abdom- 

 inal cavity as well. The sternum of all Birds which possess the 

 power of flight is characterised by the presence of a prominent 

 ridge or "keel" (fig. 202, A, h), to which are attached the great 



