GENERAL CHARACTERISTICS. 1 19 



The brain of birds is comparatively very large. The 

 eyes are so' constructed, that with equal facility they can 

 distinguish objects near or remote. Besides the two or- 

 dinary eyelids, there is a third one placed at the inner 

 angle, and which can instantly be drawn over the eye 

 like a curtain. It is called the nictitating membrane, 

 and serves a most important office in the protection of 

 the eye. The ear has but a single small bone, formed of 

 one branch that adheres to the tympanum, and of another 

 terminating in a plate that rests upon the fenestra ovalis ; 

 the cochlea is a slightly arcuated cone, but the semi- 

 circular canals are large. Nocturnal birds alone have 

 an external conch. The organ of smell is concealed in 

 the base of the bill. The tongue has but little muscular 

 substance, and the taste is probably not very delicate. 



The plumage of birds is rendered water-proof by the 

 oil with which they dress their feathers, and which is fur- 

 nished by a special gland at the hind part of the body. 



Birds moult their feathers twice a year. In some, the 

 winter plumage differs in its colors from that of summer. 

 In a majority of cases the colors of the male are more 

 brilliant than those of the female ; and when this is the 

 case, the young of both sexes resemble the adult female. 

 When the adult male and female are of the same color, 

 their young have colors peculiar to themselves. 



Birds lay eggs, and sit upon them to hatch them. The 

 egg in the ovary consists merely of the part we call yolk ; 

 it imbibes the external fluid called the white in the upper 

 part of the oviduct, and becomes covered with a shell at 

 the bottom of the same canal. The young bird of every 

 kind has a horny point at the extremity of the bill, with 

 which it breaks the shell, and which falls off a few days 

 after it is hatched. This may be seen by every one on 

 the bill of the young chicken. Most birds build nests in 

 which to lay their eggs, and it is an interesting fact that 



