BIRDS. 49 



CLASS II.: AVES (BIRDS). 



Warm-blooded Vertebrates, which breathe by lungs, 

 are covered with feathers, have no teeth but a horny 

 beak, and lay hard -shelled eggs, which are hatched 

 by the warmth of their body. They are adapted for 

 movement in the air, though not all to the same 

 degree. The fore limbs are modified into wings, in 

 which, however, the parts found in Mammals can be 

 recognized. We distinguish in the first place a small 

 thumb, and then in most cases a two-jointed fore- 

 finger and a small second finger. Secondary quills 

 (Fig. 28, BB) are attached to the ulna, and primary 

 quills (A, 1 — 10) to the two metacarpals and the 

 finger joints, while the thumb bears the bastard wing 

 (C). The tail feathers, or rectrices, are attached to the 

 last joint of the tail. The body is clothed with stiff, 

 tolerably long contour feathers, which conceal the soft 

 short dovm from view. The bones are hollow and 

 filled with air. Their cavities are connected with 

 air-sacs, which are found in all parts of the body, and 

 fill themselves with air from the lungs when the bird 

 begins to fly. In this way its specific gravity is 

 reduced. The body firm, especially the hinder part 

 of it, which is almost immovable ; the neck, which may 

 consist of many vertebrae (even as many as twenty-two), 

 can, on the contrary, be turned in many directions. 

 Birds walk entirely on their toes ; the metatarsals are 

 fused with one of the rows of tarsals into a " tarsus " 

 bone. Tarsus and toes are covered with horny scales. 



A bird's egg (Fig. 29) consists of a germinal disc 

 {h) from which the young bird develops, and which 

 rests on the yolk, made up of substances serving for 

 the nutriment of the developing bird : the yellow {a) 

 and the white (b) yolk, as well as the albumen (c, c^ 

 "white of egg") in which lie two twisted cords 



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