33 s ELEMENTARY ZOOLOGY 



As birds have no teeth with which to masticate their 

 food, a special region of the alimentary canal, the gizzard, 

 is provided with strong muscles and a hard and rough 

 inner surface by means of which the food is crushed. 

 Seed-eating birds have the gizzard especially well devel- 

 oped, and some birds take small stones into the gizzard 

 to assist in the grinding. The lungs of birds are more 

 complex than those of batrachians and reptiles, being 

 divided into small spaces by numerous membranous par- 

 titions. They are not lobed as in mammals, and do not 

 lie free in the body-cavity, but are fixed to the inner dorsal 

 region of the body. Connected with the lungs are the 

 air-sacs already referred to, which are in turn connected 

 with the air-spaces in the hollow bones. By this arrange- 

 ment the bird can fill with air not only its lungs but all 

 the special air-sacs and spaces and thus greatly lower its 

 specific gravity. The vocal utterances of birds are pro- 

 duced 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 con- 

 taining 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 in this group complete. 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 



