GENERAL CHARACTERS OP REPTILES. 537 



system conveying arterial blood, and that conveying venous blood ; 

 so that these two liquids mingle, and the body only receives 

 blood which has been imperfectly arterialised by the act of respi- 

 ration. The heart is almost always composed of two auricles 

 (Fig. 312), opening into a single ventricle. It follows from this 



Pulmonary artery ^^T" Pulmonar y arte ^ 

 Pulmonary vein Pulmonary vein. 



" Left auricle. 



Venacava ....... r ^ Single ventricle. 



Left aortic trunk- 



Right aorta 







Abdominal aorta. 



FTG. 312 HEART OF TORTOISE. 



that the arterial blood coming from the lungs, received into the 

 left auricle, and the venous blood flowing from the different parts 

 of the body into the right auricle, are mixed in this common 

 ventricle. One portion of this mixture returns by the aorta to 

 the different organs, which it is destined to nourish ; whilst the 

 other part is carried to the lungs by vessels which arise directly 

 from the common ventricle, or even from the aorta. In Croco- 

 diles, however, the heart is formed (Fig. 313) almost in the same 

 manner as in Birds and Mammalia, and presents a partition 

 which separates the right from the left ventricle ; it follows, 

 therefore, that this organ presents two distinct ventricles and 

 two auricles, and that the arterial blood is not mixed there with 

 the venous blood ; but a particular arrangement of the arteries 

 effects this mixture at some distance from the heart, and the 

 vessels of all the posterior half of the body only receive im- 

 perfectly- arterialised blood. In fact, the venous blood received 

 into the right ventricle does not go entirely to the lungs, as in 

 warm-blooded Vertebrata ; for, at the side of the opening of the 

 pulmonary arteries (ap\ is found another vessel (a) which like- 

 wise arises from the right ventricle, and which, after bending back 



