CHAPTER IT. 



OF THE CLASS OF BIRDS. 



322. The class of Birds, which includes all the animals 

 furnished with an internal skeleton and expressly adapted for 

 flight, is one of the most distinct and clearly characterised of all 

 the subdivisions of the Animal kingdom ; whether we consider 

 merely the external configuration of these beings, or attend exclu- 

 sively to the peculiarities of their internal structure, or observe the 

 manner in which their functions are executed. To define this 

 group, it would be sufficient to say, that Birds are oviparous 

 vertebrated animals, whose circulation is double and complete; 

 but to give an exact idea of its principal characters, we must add, 

 that the respiration of birds is atmospheric and double (that is, 

 instead of being carried on in the lungs alone, as is that of the 

 Mammalia and Reptiles, it takes place both in these organs and 

 in the whole interior of the body), that their blood is warm like 

 that of the Mammalia ; that their anterior members have the form 

 of wings ; and that their skin is covered with feathers. 



323. The conformation of these animals varies but little ; and 

 harmonises with the mode of locomotion for which they are 

 peculiarly designed. They seldom attain a large size ; and the 

 presence of a considerable quantity of air within their body, 

 renders them very light. The feathers, which cover the whole 

 body of the Birds, are productions very analogous to the hairs 

 of Mammalia ; but of a more complicated structure. We may 

 generally distinguish in them a horny tube, which forms the 

 lower part, and which is pierced at its extremity ; a stem which 

 surmounts this tube ; and lamina, or vanes, on each side of the 

 stem, which are themselves furnished with small barbs; and 

 these sometimes appear, in their turn, fringed on their edge. The 

 secreting organ intended to form the feather, is termed the 



