284 VERTEBRATE ANIMALS. 



the metatarsal bone of the great toe being, when present, distinct 

 and rudimentary. The upper portion of the tarsus is amalgamated 

 with the lower end of the tibia, to which, therefore, the name of 

 " tibio-tarsus " is often applied. The ankle-joint is thus placed in 

 the middle of the tarsus. In most of the long-legged birds it is 

 by the great elongation of the tarso-metatarsus that the enormous 

 length of the legs is produced. The tarso-metatarsus is followed 

 below by the foot, which consists in most birds of four toes, of 

 which three are directed forwards and one backwards. In no wild 

 Birds are there more than four toes ; but some domesticated varie- 

 ties possess a. fifth. In all birds with four toes, the toe which is 

 directed backwards (the great toe or " hallux ") consists of two pha- 

 langes ; the innermost of the three forward toes has usually three 

 phalanges, the next has four, and the outermost toe is composed 

 of five. In many Birds, such as the Parrots, the outer toe is turned 

 backwards, so that there aie two toes in front and two behind. In 

 the Swifts, again, all the four toes are turned forwards. In many 

 of the Swimming Birds {X'ttatores) the hinder toe is wanting or 

 rudimentary ; 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 different 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 surrounded by a circle of naked skin, constituting what 

 is called the " cere," and this too serves as an organ of touch. The 

 tongue of birds can rarely be looked upon as an organ of taste, since 

 it is generally cased in horn, like the mandibles. It is principally 

 employed as an organ of prehension, but it is soft and fleshy in the 

 Parrots, and in them, doubtless, acts as an organ of taste. Sahvary 

 glands are always present, but they are rarely of large size, and are 

 often of extremely simple structure. In accordance with the length 

 of the neck, the gullet is usually very long in birds, and is generally 

 very dilatable. In the flesh-eating and grain-eating Birds the gullet 

 is dilated (fig. 20.5, c) into a pouch which is called the " crop," and 

 is sitxiated in the lower jjart of the neck, just in front of the merry- 

 thought. This may be simply a dilatation of the tube of the gullet, 

 or it may be a single or double potich. The function of the crop is 

 to detain the food, for a longer or shorter ]ieiiod according to its 

 nature, before it is submitted to the action of the proper digestive 

 organs. In the Pigeons, the food which has been previously softened 

 in the crop is returned to the mouth, and supplied to the young in 



