92 GENERAL STRUCTURE OF BIRDS. 



a manner as to exhibit the chief of them. The cavity of 

 the trunk is completely divided, by the muscular partition 

 termed the diaphragm, into two portions the thorax, con- 

 taining the heart and lungs ; and the abdomen, containing 

 the digestive apparatus. It is chiefly by the alternate con- 

 traction and relaxation of this muscle, that the act of 

 breathing is performed in Mammals, as will be explained 

 hereafter ( 331). 



78. In BIRDS there is a much closer conformity to one 

 general plan than we find among Mammals. The covering of 

 feathers, by which we ordinarily distinguish the members of 

 this class, prevails universally ; and there is no wide depar- 

 ture from the typical form. This class belongs to the 

 oviparous division of the Vertebrata ; the young being reared 

 from eggs. But it is distinguished from Reptiles, which are 

 also oviparous and air-breathing, by being warm-blooded; 

 and by having a very energetic instead of a very slow circu- 

 lation. The warmth of the maternal body, moreover, is im- 

 parted to the egg in the act of incubation ; and without the 

 heat thus communicated (unless it be supplied from some 

 other source) the embryo cannot be developed. The covering 

 of feathers is given, not only to keep in the heat of the body, 

 which is even greater than that of Mammals, but also 

 to afford the required surface for the wings, on which the 

 Bird is supported and propelled through the air. The 

 feathered portion of the wings is stretched out upon the 

 bones which answer to those of our arm, and is moved by its 

 muscles. The wings are very small, or are entirely absent, in 

 the Ostrich and a few other birds, which present the nearest 

 approach to the Mammalia in their internal structure ; and 

 these cannot rise from the ground, but run swiftly along it, 

 by means of their powerful legs. In the Penguin, also, the 

 wings are small ; and they are used as fins, by the assistance 

 of which this bird, which can neither walk nor fly with 

 rapidity, can swim very quickly through the water. 



79. Generally speaking, Birds are characterized by the 

 extraordinary power of motion which they possess, and by 

 the great acuteness of the sense of sight, by which their 

 movements are chiefly directed. They are also remarkable for 

 their instinctive actions, which are chiefly related to their care 

 of their young, for whom they usually construct a protective 



