2i8 THE ANIMALS AND MAN 



being here modified so as to produce a voice-box contain- 

 ing two vocal cords controlled by five or six pairs of muscles. 

 The air passing through the voice-box strikes against the 

 vocal cords, the tension 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 cham- 

 bers, 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 systemic blood-circulation being completely separated 

 from the pulmonic, the circulation is said to be double. 

 The circulation 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 de- 

 veloped than in batrachians and reptiles, but the cerebrum 

 has no convolutions 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 ex- 

 ternal ear, other than a simple opening. The organs of 

 the inner ear, however, are well developed, and birds un- 

 doubtedly 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 in- 

 cubation outside the body of the mother, and which are, in 

 most cases, laid in a nest and incubated by the parents. 



