78 THE BOOK OF NATURE STUDY 



The air-sacs do not, as was supposed, serve the purpose of render- 

 ing the body lighter during flight, but rather to contain reserves 

 of breathing air during flight. 



In some birds, it should be remarked, yet other air-containing 

 vessels have been developed. In the emu, for instance, a great 

 air-bag is given off from a slit in the windpipe, near the middle of 

 the neck, and this serves apparently to increase the sound of the 

 voice. In some other birds, again, the gullet is made to serve the 

 purpose of a wind-bag, and is inflated at the will of the bird, when 

 it desires to draw increased attention to itself. The pouter pigeon, 

 for example, inflates the crop to an enormous extent, and so also 

 do some near relatives of the grouse tribe, natives of North America. 



In birds like the gannet of our coasts the whole body is encased 

 in a mass of small air-containing cavities placed just under the 

 skin. 



Birds of the ostrich tribe, it is interesting to notice, still retain 

 the internal air-sacs, though they have long since lost the power 

 of flight. 



The Heart. The heart (Model D. 10) of birds contains four 

 chambers as in mammals. Thereby a perfect oxidation of the 

 blood is possible, and hence a higher temperature than in reptiles, 

 wherein the heart has but three chambers. But the blood of 

 birds is of a higher temperature than in mammals, while it differs 

 further in that the oxygen-carrying cells or red corpuscles have a 

 nucleus therein agreeing with the blood cells of Reptilia. 



