THE VERTEBRATES: BIRDS 



217 



eating birds have the gizzard especially well developed, 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 partitions. They are not lobed 

 as in mammals, and do not lie free in the body-cavity, but 



Fig.: 113. Russet-backed thrush, Turdus ustulatus. (Photograph from 

 life by Eliz. and Jos. Grinnell.) 



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 arrangement 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 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 



