614 BIRDS. 



early in life. The right oviduct is represented by a small 

 rudiment close to the cloaca. 



The ovary is covered with follicles containing ova at 

 various stages of ripeness. As these ova become dilated 

 with yolk and otherwise mature, they burst from the ovary, 

 and are caught by the dilated end of the oviduct which 

 opens into the coelom. The first part of the duct is narrow, 

 and there the ova may be fertilised; the second part is 

 wide and glandular, secreting the white of egg; in the 

 third region, which is muscular and glandular, the shell 

 membrane and shell are made. 



In sexual union the cloaca of the male is closely apposed 

 to that of the female ; only in a few cases (in ducks and 

 geese, and in the Ratitae) is there a copulatory organ. 

 The eggs are incubated by the parents for a fortnight, a 

 high temperature of about 40° C. being sustained through- 

 out. 



Habits and Functions of Birds. 



Flight. — As birds are characteristically flying animals, 

 many of their peculiarities may be interpreted in adaptation 

 to this mode of motion. 



(a) Shape and general structure of the body. — The 

 resistance offered by the air to the passage of a body through 

 it depends in part on the shape of the body, and the boat- 

 like shape of the bird is such that it offers relatively little 

 resistance. The attachment of the wings high up on the 

 thorax, the high position of such light organs as lungs and 

 air-sacs, the low position of the heavy muscles, the sternum, 

 and the digestive organs, the consequently low centre of 

 gravity, are also structural facts of importance. But it must 

 be remembered that the frictional resistance of the air is slight. 



(b) The muscles of flight. — The pectoralis major brings 

 the wing downward, forward, and backward, keeping the 

 bird up and carrying it onward. As it has most work to do, 

 it is by far the largest. The pectoralis minor raises the 

 wing for the next stroke. Besides these two main muscles, 

 there are others of minor importance, the deltoides externus 

 and three coraco-brachials, which help to raise the wing. 

 On an average these muscles weigh about one-sixth of the 

 whole bird, but the proportion is often much greater, 



