280 VERTEBRATE ANIMALS. 



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

 whicli form the pelvic arch, which cari'ies the hind -limbs. The 

 vertebi'ie uf the tail are more or less movable upon one another; 

 and in almost all living l.>irds, wdieu fully grown, the last joint of 

 the tail (tig. 204, B, s) is a long, slender, ploughshare-shajjed bone, 

 which is really composed of several vertebrae united together. It 

 is usually set on at an angle nearly pei'pendicular to the axis of 

 the body, and it serves to s\i]i]iort the great tail-feathers, which act 

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

 which su]iplies the secretion with which the feathers ai'e lubricated. 

 The skull in Birds 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, wdiicli 

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

 halves, an upper half or "u]>i>er mandible," and a " lower mandi- 

 ble." The lower maTidiblc, like the lower jaw of all the Saurop- 

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

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

 are also amalgamated together. In no living Bird are ti-cth ever 

 developed in either maudilile ; but both mandibles ai'e sheathed in 

 horn, con.stituting the "beak," and the margins of this sheath are 

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

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

 bv the intervention of a movable "ipiadrate bone." In many 

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

 as to be ca]ia1)le of movement. 



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 doreal vertebrae, 

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

 num. The ribs vary in nundjer fr-oni seven ti> 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 ril) next in succession behind. 

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

 called the "sternal riljs," and tliese, in turn, are movably articu- 

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

 nal ribs arc "tlie centres upon which the respiratory movenjenta 

 hinge." In front the cavity of the che.st is completed by an enor- 

 mously expanded breast-lwaie or .ifmiirm (fig. 202, A), whjch, in most 

 birds of any powers of flight, extends moi'e or less over the abdom- 

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

 |>ower fif flight is characterised by the presence of a prominent 

 ridgi! or " k.'el " Oig. 202, A, !>), t) which are attached the great 



