260 VERTEBEATE ANIMALS. 



one, it follows that the venous and arterial streams must mix to some 

 extent in the ventricle. From the ventricle arise the great vessels 

 which carry the blood to the lungs and to all parts of the body, and 

 it follows that to some extent all these parts are supplied with a 

 mixed fluid, consisting partly of impure or venous blood, and partly 

 of pure or arterial blood. In the C'rocodiles, in which there are two 

 ventricles completely separated from each other, the same result is 

 brought about by means of a communication which takes place 

 between the great vessels which spring from the ventricles, in the 

 immediate neighbourhood of the heart. 



As a matter of fact, the heart of the Reptiles is so constructed 

 that this theoretical mixture of the arterial and venous currents in 

 the common ventricle only takes place in practice to a limited extent. 

 What actually occurs is that the lungs are supplied mainly with 

 venous Ijluud, and the head and fore-limbs are sujjplied mostly with 

 pure arterial blood, while the body and hind-limbs are supplied 

 with mixed blood ; but the arrangements whereby this is brought 

 about are too complicated for description here. 



As regards the structure of the lungs, it is merely to be noted 

 that there is no jiartition {diaphragm or midriff) separating the two 

 cavities of the thorax and abdomen, and that the lungs, therefore, 

 often attain a great proportionate size, sometimes extending through 

 almost the whole length of the cavity of the trunk. There are also 

 no air-sacs couuuunicating with the lungs, as in the Birds. 



Lastly, all Reptiles are essentially oviparous, some being ovo- 

 viviparous. The egg-shell is usually parchment-like, but in other 

 cases contains more or less calcareous matter. 



The class Ri'ptilia is divided into four living and six extinct orders, 

 as follows, bvit the latter require no e-vtended notice here : — 



1. CJii-'tiinia (Toitoises and Turtles). 



2. Opiddtii (Snakes). 



3. Ijircrtillii (Lizards). 



4. Crocdililia (C'rocodiles). 

 .5. Ichthiiitpti'i-tjciiu N 

 6. Siiiirojiterijgia 



7. rti'rosduria, 



8. Anomodontia 



9. DfiidixKuria 

 10. Theriudii iitia 



Extinct. 



