-fiO VEUTEBIIATE ANIMALS. 



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

 extent in the venti'iele. From the ventricle arise the great vessels 

 which carry the blood t(] the lungs and to all jjarts of the body, and 

 it follows that to some extent all these parts are supplied v/ith a 

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

 of pure or arterial blood. In tlie Crocodiles, in which there are two 

 ventricles completely separated from each otlier, the same result is 

 brought about by moans of a communication which takes place 

 between the great vessels whicli sjiring from the ventiicles, in the 

 immediate neighbourhood of the heart. 



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

 that this theoretical mixture of tlie arterial and venous currents in 

 the connnon ventricle only takes place in jM-actice to a limited extent. 

 What actually occurs is that the lungs are supplied mainly with 

 venous Idood, and the head and fore-limbs are supplied mostly with 

 pure arterial blooil, while the body and hind-limbs ai-e supplied 

 with mixed l)lood ; but the arrangements wheieby this is bro)ight 

 about are too complicated for descripti(ju here. 



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

 that thei'e is n<j partition {dinplinKjm or midriff) sejiarating the two 

 cavities of the thorax and abdomen, and that the lunges, 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 communicating with the lungs, as in the Birds. 



Lastly, all Rejitiles are essentially oviparous, some beii}g ovo- 

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

 cases contains more or less calcareous matter. 



The class Ri-ptilin is divided into four living and six extinct orders, 

 as follows, but the latter require no extended notice here : — 



1. Cli''hmiM. (Tortoises and Turtles), 



i. Opliidia (Snakes). 



3. Lai'ertiUd (Lizaixls). 



4. Criii-odiiia (C'l'ocodiles). 

 .^1. Inldhyopterygw \ 

 (■>. Sitiiropterij 



7. Pterosavri 



8. A/'oniodoittia 

 '.). Di'inoi^ciirta 



10. Tlmrioduntia 



•terygui \ 

 '.1-ygia I 



,. > Ext 



