126 STORY OF THE REPTILES 



lungs and heart (called the diaphragm), by which the 

 lungs can be slightly compressed. In some of them 

 the lower shell and abdominal parts move generally 

 to compress the lungs ; and besides this, they get air 

 into the lungs by swallowing it after the manner of 

 the frogs. In those where the lower shell is fixed to 

 the upper, the bones inside of it which form the 

 shoulder are said to move in breathing. Their lungs 

 are short and have many air-cells. A lizard breathes 

 in the ordinary way, and so very rapidly that the 

 expression, "Panting like a lizard," is proverbial. 

 Snakes and lizards have no hint of a diaphragm. 



Some lizards have expanded places in their wind- 

 pipes in which they store air; and our common 

 " spread-head " snake {Heterodori) has great lung-sacs 

 that extend forward toward the head, to aid it, per- 

 haps, in blowing or hissing. In the crocodilians there 

 is such a large extra sac that there appear to be really 

 three lung-sacs. These are very cellular, and all com- 

 municate with each other. They hold great quantities 

 of air by which the creature can remain immersed for 

 a long time. 



OlEOULATION AND THE HeAET 



While Keptiles are always spoken of as having 

 three-chambered hearts, some of the tortoise-forms 

 have the partition between the two receiving cham- 

 bers (auricles) not complete, and hence, since there 

 is only one ventricle (pumping-out chamber), these 

 hearts are really only two-chambered. In these, how- 

 ever, the old bulb below the heart in the vein, which 



