REPTILIA. 



237 



same result is brought about by means of a communication 

 which takes place between the great vessels which spring 

 from the ventricles, in the imme- 

 diate neighborhood of the heart. 



From this brief description it 

 will be seen that the peculiarity 

 of the circulation in Reptiles con- 

 sists in the fact, that the lungs 

 and all parts of the body are sup- 

 plied with mixed blood ; whereas 

 in the higher Vertebrates the 

 lungs are supplied with pure 

 venous blood, and the various tis- 

 sues of the body with pure arte- 

 rial blood. 



As regards the structure of 

 the lungs, it is merely to be noted 

 that there is no partition (dia- 

 phragm or midriff) separating 

 the two cavities of the thorax and 

 abdomen, and that the lungs, 

 therefore, often attain a great pro- 

 portionate size, sometimes ex- 

 tending through almost the whole 

 length of the cavity of the trunk. 

 There are also no air-sacs commu- 

 nicating with the lungs, as in the 

 Birds. 



Lastly, all Reptiles are essen- 

 tially oviparous, some being ovo- 

 viviparous. The egg-shell is usu- 

 ally parchment-like, but in other 

 cases contains more or less cal- 

 careous matter. 



The class Reptilia is divided into four living and five ex- 

 tinct orders, as follows, but the latter require but brief notice : 



1. Chelonia (Tortoises and Turtles). 



2. Ophidia (Snakes). 



3. Lacertilia (Lizards). 



4. Crocodilia (Crocodiles). 



5. Ichthyopterygia 1 



6. Sauropterygia 



7. Pterosauria V Extinct. 



8. Anomodontia 



9. Deinosauria 



venous blood from the body ; a' Left 

 auricle, receiving arterial blood from 

 the lungs; v Arterio-venous ventri- 

 cle, containing mixed blood, which is 

 driven by (p) the pulmonary artery to 

 the lungs, and by (0) the aorta to the 

 body. (The venous system is left light, 

 the arterial system is black, and the 

 vessels containing mixed blood are 

 cross-shaded.) 



