INTRODUCTION, 



Class AVES— Bikds. 



Vertebrate, warm-blooded, oviparous animals, breath^ 

 ing by lungs, the heart witli two ventricles and two auri- 

 cles; the anterior limbs changed into wings; body clad 

 with feathers ; bill covered with a horny substance. 



Birds being intended for flight, or progression through 

 a thin and but slightly resisting medium, have their whole 

 organization specially directed for this purpose. They 

 are seldom of large size. The bulk of their body is 

 greatly increased by their thick covering of feathers, of the 

 lightest specific gravity, and it is rendered still lighter by 

 many of the bones being hollow. The muscles that move 

 the wings, which are the greatly lengthened anterior 

 extremities, thickly furnished with strong and unyielding 

 plumes, are of great power ; and the sternum to which 

 these muscles are attached is highly developed ; whilst, at 

 the same time, their respiratory function is vastly in- 

 creased by the admission of the external air, not only to 

 their lungs, but to air-cells in various parts of the body, 

 and even in the bones, by which means the aeration of the 

 blood is vastly accelerated, the blood raised to a higher 

 temperature than in Mammals, and, consequently, the 

 muscular power and excitability is greatly increased. 



The bill or beak of Birds consists of the upper man- 

 dible, or maxilla of some, and the lower one or mandible. 

 In the upper one we find the nares, or nostrils, of various 

 form, and covered, or not, by hair, or hair-like feathers, 

 and sometimes pierced in the hard substance of the bill, 



