424 ANIMAL RESPIRATION 



LUNGS OF REPTILES (Reptilia) 



Reptiles (as also Birds and Mammals) have abandoned skin- 

 breathing, and rely entirely upon their lungs, which in the smaller 

 members of the class, such as many Lizards, resemble in structure 

 those of the Frog. But in the larger Lizards, Turdes, Tortoises, 

 and Crocodiles these organs have become more complex, with the 

 object of providing a larger breathing surface. Instead of a simple 



honey-combing of the internal 

 surface, complex folds have come 

 into existence (fig. 548). In this 

 way the wall of the lung has 

 become more or less spongy, 

 with corresponding reduction of 

 the central cavity. A new method 

 of renewing the air in the lungs 

 has also come into existence, 

 for well-developed ribs are here 



Fig. 548.— Diagrammatic longitudinal sections through prCSCnt, attachcd tO 3. breaSt- 

 Lungs, showing increase of breathing surface by in- , , . , . . . . 



growth of folds: /,, main branch of windpipe. A, Newt; bone Delow, and jomted On tothe 

 :: SSir^r^crt'codr' ■ " "-^"^ "'^"' °' ^""'^" backbone above. By muscular 



action the ribs and breastbone 

 can be swune forwards and downwards, so as to increase the 

 volume of the front part of the body, by which air is caused to 

 flow into the lungs. When the muscles concerned cease to con- 

 tract, these parts move back, mainly as a result of elasticity, to 

 their original position, and the impure air passes out to the ex- 

 terior. It may, then, broadly be said that renewal of air in the 

 lungs depends in Reptiles (as also in Birds and Mammals) on 

 the mobility of the chest-region (thorax). In cases where the 

 lungs are complicated and spongy the movements of breathing 

 cause direct renewal of air in the larger passages only, that in 

 the smaller ones being purified by diffusion. 



In an average Reptile the internal nostrils open further back 

 than in Amphibians, and thus the feeding and breathing tracts 

 are rather better separated. Crocodiles exhibit a great advance 

 upon this condition, for the internal nostrils do not communicate 

 with the mouth-cavity at all, but with a pharynx (fig. 549), into 

 the floor of which the top of the windpipe projects (see p. 70), 



