146 CHARACTERS OF VERTEBRATE ANIMALS 



the mammal (see p. 32), consists, in the very young bird, of 

 two elements only, of which one fuses with the tibia to form 

 a tibio-tarsus, while the other fuses with three of the meta- 

 tarsals, these also uniting with one another, the compound 

 product being known as the " tarso-metatarsus ". It will thus 

 be seen that what corresponds to the ankle-joint of the human 

 foot (p. 32) comes in the middle of the tarsus, and not between 

 it and the bones of the lower leg. The great toe possesses 

 a metatarsal which is distinct from the others, and this digit 

 therefore has a greater power of movement. The remaining 

 point deserving mention here is in regard to the number of 

 phalanges in the various toes. It will be remembered that 

 in Man (p. 31), and the same thing is true for mammals 

 generally, the great toe has two and the remaining toes three 

 phalanges each. A bird has similarly two phalanges in the 

 great toe and three phalanges in the second toe, but the third 

 toe possesses four, and the fourth five, thus giving the regular 

 succession two, three, four, five, each digit possessing one more 

 phalanx than its number in the series. 



Digestive Organs (fig. 99). The pigeon, like all existing 

 birds, is entirely devoid of teeth, and its beak is only useful 

 for picking up food, not being capable of breaking up the hard 

 grain on which the animal largely feeds. A temporary receptacle 

 for food is therefore necessary, and this is found in the large crop 

 into which the gullet swells. A further peculiarity, common to 

 all birds, is found in the nature of the stomach, which consists 

 of two parts, chemical stomach (proventriculus) and gizzard. 

 The former looks like a swelling at the end of the gullet, but 

 is distinguished by the presence of numerous gastric or peptic 

 glands (see p. 37) imbedded in its mucous membrane, and 

 secreting gastric juice. The gizzard is a rounded and some- 

 what flattened structure with a tough lining and exceedingly 

 thick muscular walls. As is well known, it serves to grind up 

 the food, its action being assisted by small stones and other 

 hard bodies which are swallowed for the purpose. The stomach 

 is followed by a long small intestine into which the liver and 

 pancreas pour their secretions ; and a much shorter large intestine, 

 opening into a cloaca, which communicates with the exterior by 

 a cloacal aperture. 



Circulatory Organs (fig. 102). As in a mammal, both blood 



