394 



METAZOAN PHYLA 



The lungs of birds are not capable of dilation since the thoracic 

 skeleton forms a rigid framework. Breathing is allowed by the presence 

 of a large number of air sacs (Fig. 269) lying among the muscles and 

 about the viscera in various parts of the body and communicating with 

 the bronchi of the lungs. Air is drawn into the windpipe, through the 

 lungs, and on into the air sacs by the action of the muscles of the thorax 



Cere 



Ear opening 



Esoph- 

 agus " 



Air sacs 



Syrinx 

 Auricle 



Ventricle 



Fig. 269.- 



-Dissection of a pigeon, Columha livia Linnaeus. (Based upon a Pichlcr chart, by 

 the courtesy of Martinus Nijhoff.) 



and abdomen. Thus the air in the lungs is practically entirely changed 

 with each respiration, which results in a very perfect aeration of the 

 blood and is responsible for the high temperature of the body. Birds 

 maintain temperatures varying from 40.5°C. (105°F.) to as high as 

 46°C. (115°F.). While flying the movements of the wings contribute 

 to respiration by compressing and dilating the air sacs, and thus the 

 bird breathes more easily when in flight than at other times. The posses- 

 sion of air sacs is also shared with many reptiles, especially the lizards, 



