354 DIVISION I. VERTEBRAL ANIMALS. — CLASS II. AVES. 



As liefore remarked, tlie birds constitute a large group in the great divis- 

 ion vertebrata. On glancing at the skeleton of a bird, the vertebral structiu'e 

 is at once apparant, but here the resemblance to the mammals almost ends. 

 The skeleton is ligiit, all the larger bones, tiiose of the limbs especially, 

 being iiollow and marrowless. Tiie body is covered with feathers ; the jaws, 

 instead of being provided with teeth, arc incased in a Iiorny sheatli, which is 

 usually smooth, and on the edges sharj) ; the digestive, respiratory, and cir- 

 culatory systems are essentially ditlcrent from those of the mannnals, and 

 the young arc always produced from eggs, which have been expelled frcMU 

 the mother bird, and incubated upon for a length of time which varies in the 

 different families. 



The digestive apparatus of birds, while differing from that of the mam- 

 mals, varies somewhat in the different orders, according to their peculiar 

 diet. Generally .speaking, however, birds have first the crop (a large mus- 

 cular sac, a dilation of the irsophagus, wiiich is situated in front of the 

 collar bone), ''which is abundantly supplied with glands, and acts as a sort 

 of first stomach, in which the food receives a certain amount of preparation 

 before being sul)mitted to the action of the proper digestive organs." A 

 little below the crop is a second enlargement of the canal, "the conunencing 

 portion of which is surrounded by a zone of glands pouring out a solvent or 

 gastric juice. Tliis jjortion is called the vcntriculus succoitariulus." T!ie 

 canal then conducts to the gizzard, which organ is composed of " two firm 

 voluminous muscles, and is lincil witji a tliick, tough membrane." Tlicse 

 muscles exert a sort of opposite grinding motion, with pressure on each 

 other, like two millstones, and the efiect is the reduction of grain, seeds, 

 and other food int(3 a puljiy mass ; but this cannot lie done unless a number 

 of pebbles and coarse sand are swallowed with the food, wiiicii, by the work- 

 ing of the walls, triturate the food among them. In mollusk-feeding ducks 

 the gizzard is enormously powerful, grinding down hard and sharp shells. 

 In granivorous birds it is also hard and powerful, while in predaceous birds 

 it is thin and membraneous. The respirator}- system of birds is also worthy 

 of more than a passing notice. M. Jacquemin, in an essay read before the 

 French Academy, gives us the following facts. 



After observing that the air enters not only into the lungs and about the 

 parieties of the chest, but that it penetrates also by certain openings (foram- 

 ina) into eight pneumatic bags, or air-cells, occupying a considerable por- 

 tion of the pectero-abdominal cavity, and thence into the upper and lower 

 extremities, he draws these conclusions: 1. That the pneumatic bags are 

 so situated as to be ready conductors of the air into the more solid part of 

 the bird's Ijody ; and tliat the air, by surrounding the most weighty viscera, 

 may support the bird in flight, and contribute to the facility of its motions 



