214 



STRUCTURE OF THE VERTEBRATES 



bronchioles are given off, each ending in a spherical alveolus. 



A progressive degeneration of one lung is found in the lizards 

 and snakes. In the primitive condition the lungs are bilaterally 

 symmetrical; but in the elongated lizards one lung is definitely 

 smaller than the other, and in the snakes one is either vestigial 

 or lacking. 



Although the lungs of flying birds are not so greatly lobulated 



Fig. 125. Comparative Anatomy of the Lungs. There is a steady increase 



in number of bronchi and alveoH. Each division of the Alhgator kmg is 



equivalent to a complete lizard lung. 



as those of the mammals, their lungs are specialized for more 

 efficienc}^ The organ is not of unusual size, but greater efficiency 

 in gas exchange is effected by three pairs of air sacs. These sacs 

 are continuations of bronchial ducts, which expand after pass- 

 ing through the lungs to form membranous sacs in the pleural, 

 abdominal, and wing regions. As the air is drawn into the lungs 

 it passes across the alveoli and into the sacs, greatly decreasing 

 the specific gravity of the animal. The ramifications of the sacs 



