366 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 



COMPARATIVE MORPHOLOGY OF VERTEBRATE RESPIRA- 

 TION. 



Mammals. — The respiratory organs and the mode 

 of breathing among mammals do not differ from those 

 of man. All mammals have a diaphragm, and therefore 

 both kinds of respiration. On account of the horizontal 

 position of the body, of course the body cavity is divided 

 into an anterior and posterior instead of an upper and 

 lower chamber. 



Birds. — In birds we find large lungs of minutely 

 spongy structure, and therefore a perfect aeration of the 

 blood. They are, therefore, hot-blooded animals. Yet 

 birds have no diaphragm. This, however, is of little dis- 

 advantage, as their thorax is large and abdomen small, 

 and their thoracic respiration very perfect. It is interest- 

 ing to note, however, that some birds, such as the os- 

 trich, seem to have the beginnings of a diaphragm. In 

 these, certain muscular fibers arise from the backbone 

 and lowest ribs behind and take hold on the lung and 

 pull it downward, and undoubtedly assist in breathing. 



Reptiles. — Reptiles have no diaphragm ; but since 

 their ribs surround the whole body cavity to the hips, 



Fig. 248. — Skeleton of a lizard (Heloderma), showing how the ribs surround 

 the whole body cavity. 



there seems to be no use for a diaphragm in them (Fig. 

 248). Probably the shortening of the rib series, in order 

 to give greater freedom to the loin created the necessity 



