HABITS OF BIRDS. 23 



and in reptiles, the bronchiae terminate in little cells, which all end 

 in a cul-de-sac, and the air that enters the lungs cannot pass be- 

 yond them, while in birds, the bronchiae and pulmonary cells 

 communicate with the great cavities, and this fluid, in this manner, 

 penetrates to all parts of the body, even into the interior of the 

 bones and feathers. (Plate 2, fig. I.) These cavities, by means 

 of which the air is distributed to the different parts of the body, 

 are formed of very thin lamina of cellular tissue, and are desig- 

 nated under the name of aerial pouches. 



42. Consequently, the blood comes in contact with the air in 

 passing through the capillary vessels of all the organs, as well as 

 in passing through the capillary vessels of the lungs, and we 

 might say that the respiration of these animals, as well as their 

 circulation, is double. A bird consumes, proportionably, more air 

 than any other animal, and perishes more rapidly when its respi- 

 ration is interrupted. 



43. The cavity of the thorax, which contains the heart and 

 lungs, is not separated from the abdomen by a complete muscular 

 partition, as in the mammalia ; the diaphragm is rudimentary, 

 and only occupies the sides of the body ; but the lungs are 

 adherent to the ribs, so that they are forced to dilate when these 

 bones separate from each other; therefore, the movements of in- 

 spiration and expiration are performed nearly in the same manner 

 as in the mammalia, 



LESSON II. 



Eggs, Incubation Nests. Migration. Classification. 



Habits of Birds. 



1. Birds, like reptiles, fishes, and most of the invertebrate 

 animals, (that is, without vertebrae,) are oviparous, that is, they 

 lay eggs from which their young are hatched. 



2. The Eyg is first formed in an organ, named ovary, and 

 descends (Plate 2. fig. 4.) from it, through a long tube called 

 oviduct: it consists at first of a membranous sack filled with 

 yellow matter, and is not surrounded with the white till it reaches 

 the oviduct, where it receives a more solid envelope which be- 

 comes encrusted with a calcareous matter that constitutes the 



42. What are the consequences of the peculiarity of the respiratory ap- 

 paratus ? 



43. What is the character of the diaphragm in birds ? 



1. Why are birds said to be oviparous ? 



2. How are eggs formed ? 



