258 VERTEBKATE ANIMALS. 



ways form a single piece each, and they are always firmly 

 united with the sacral region of the spine. With the single 

 exception, however, of the Ostrich, they do not unite below 

 but remain separate. As in the higher Vertebrates, the lower 

 limb consists of a thigh-bone {femur), a shank, composed of 

 two bones {tibia and Jibula), a tarsus, a metatarsus, and pha- 

 langes, but some of these parts are obscured by coalescence. 

 The thigh-bone or femur (Fig. 139, /) is generally very short, 

 comparatively speaking ; and the chief bone of the leg is the 

 tibia {t), to which a thin and tapering fibula (f) is attached. 

 In the regular typical limb of a Vertebrate animal the tibia 

 and fibula would be followed by a series of small bones, called 

 the tarsus, constituting the ankle-joint (Fig. 97) ; and the tar- 

 sus would in turn be followed by a series of bones constituting 

 the root of the foot, or metatarsus. In Birds, however, the 

 tibia and fibula are followed by a single cylindrical bone, 

 which is called the " tarsc-metatarsus" {m), and which is 

 formed by the amalgamation of the entire metatarsus with the 

 whole or a portion of the tarsus. The most probable view is 

 that only the lower portion of the tarsus is present in the 

 tarso-metatarsus, and that the upper portion of the tarsus is 

 fused with the lower end of the tibia. In this case the ankle- 

 joint is placed in the middle of the tarsus. The tarso-meta- 

 tarsus is followed below by the foot, which consists in most 

 birds of four toes, of which three are directed forward and 

 one backward. In no wild birds are there more than four 

 toes; but some domesticated varieties possess a fifth. In 

 most bird^ with four toes, the toe which is directed backward 

 consists of two phalanges ; the innermost of the three forward 

 toes has three phalanges, the next has four, and the outer- 

 most toe is composed of five. In manj- birds, such as the 

 Parrots, the outermost toe is turned backward, so that there 

 are two toes in front and two behind. In Ihe Swifts, again, 

 all the four toes are turned forward. In many of the swim- 

 ming-birds {Natatores) the hinder toe is wanting or rudimen- 

 tary ; and in the Ostrich both this and the next toe are ab- 

 sent, so that the foot consists of no more than two toes. 



The digestive system in Birds consists of the beak, tongue, 

 gullet, stomach, intestine, and cloaca, with certain accessory 

 glands. There are no teeth, and the beak is employed, in dif- 

 ferent birds, for holding and tearing the prey, for prehension, 

 for climbing, and in some cases as an organ of touch, being in 

 these last instances more or less soft, and supplied with 

 nervous filaments. In many birds, too, the base of the bill is 



