THE RESPIRATORY SYSTEM. 



377 



finally it resolves itself into tissue-breathing — the appropriation 

 of oxygen and the excretion of carbon dioxide. Since the man- 

 ner in which oxy- 

 gen is intro 

 into the lungs and 

 foul gases expelled 

 from them in some 

 reptiles and amphib 

 ians is largely dif- 

 ferent from the 

 method of respira- 

 tion in the mam- 

 mal, we call atten- 

 tion to this process 

 in an animal readily 

 watched — ^the com- 

 mon frog. ThLs 

 creature, by depress- 

 ing the floor of the 

 mouth, enlarges his 

 air-space in this re- 

 gion and conse- 

 quently the air free- 

 ly enters through 

 the nostrils ; where- 

 upon the latter are 

 closed by a sort of 

 valve, the glottis 

 opened and the air 

 forced into the lungs 

 by the elevation of 

 the floor of the 

 mouth. By a series 

 of flank movements 

 the elasticity of the 

 lungs is aided in 

 expelling the air 

 through the now 

 open nostrils. The 

 respiration of the 

 turtle and some oth- 

 er reptiles is somewhat similar 



Fig. 304. — General view of air-reservoirs of ducli, opened 

 inferiorly, also their relations with principal viscera 

 of trunk (after Sappey). 1, 1, anterior extremity of 

 cervical reservoirs ; 3, thoracic reservoir; 3, anterior 

 diaphragmatic reservoir; 4, posterior ditto; 5, abdom- 

 inal reservoir; a, membrane forming anterior dia- 

 phragmatic reservoir; ft, membrane forming posterior 

 ditto; c, section of thoraco-abdominal diaphragm: d, 

 subpectoral prolongation of thoracic reservoir; e, 

 pericardinm; /, liver; g, g\ia.»iA: h, intestines; m, 

 heart; n, n, section of great pectoral muscle above its 

 insertion into the humerus ; o, anterior clavicle ; p, pos- 

 terior clavicle of right side cut and turned outward. 



In the case of aquatic animals, 



