BIRDS 277 



and spaces. The special function of these air-sacs in not un- 

 derstood; many believe that in some "way they aid the bird in 

 its flight or in respiration. The vocal utterances of birds are 

 produced by the vocal cords of the syrinx or lower larynx, 

 situated at the lower end of the trachea just where it divides 

 into the two bronchial tubes, the tracheal rings being here 

 modified so as to produce a voice-box containing two vocal 

 cords controlled by five or six pairs of muscles. The air passing 

 through the voice-box strikes against the vocal cords, the ten- 

 sion of which can be varied by the muscles. In mammals 

 the voice-organ is at the upper or throat end of the trachea. 



The heart of birds is composed of four distinct chambers, 

 the septum between the two ventricles, incomplete in the 

 Reptilia, being complete in this group. There is thus no 

 mixing of arterial and venous blood in the heart. The sys- 

 temic blood circulation being completely separated from the 

 pulmonic, the circulation is said to be double. The circula- 

 tion of birds is active and intense; they have the hottest 

 blood and the quickest pulse of all animals. In them the 

 brain is compact and large, and more highly developed than 

 in amphibians and reptiles, but the cerebrum has no convolu- 

 tions as in the mammals. Of the special senses the organs 

 of touch and taste are apparently not keen; those of smell, 

 hearing, and sight are well developed. The optic lobes of the 

 brain are of great size, relatively, compared with those of other 

 vertebrate brains, and there is no doubt that the sight of birds 

 is keen and effective. The power of accommodation, or of 

 quickly changing the focus of the eye, is highly perfected. 

 The structure of the ear is comparatively simple, there being 

 ordinarily no external ear, other than a simple opening. The 

 organs of the inner ear, however, are well developed, and 

 birds undoubtedly have excellent hearing. The nostrils open 

 upon the beak, and the nasal chambers are not at all complex, 

 the smelling surface being not very extensive. It is probable 

 that the sense of smell is not, as a rule, especially keen. 



Development and Life History. All birds are hatched from 

 eggs, which undergo a longer or shorter period of incubation 

 outside the body of the mother, and which are, in most cases. 



