276 THE VITAL FUNCTIONS. 



distributed to those organs; and in proportion 

 as these arteries enlarge, the branchial arteries 

 diminish ; until, on their becoming entirely ob- 

 literated, the course of the blood is wholly 

 diverted from them, and flows through the 

 enlarged lateral trunks (o, p,) of which the 

 junction constitutes the descending aorta. This 

 latter vessel now receives the whole of its blood 

 directly from the heart ; which, from being 

 originally a branchial, has become a systemic 

 heart. 



The heart of the Chelonian reptiles, such as the 

 ordinary species of Tortoises and Turtles, has 

 two distinct auricles ; the one, receiving the blood 

 from the pulmonary veins ; the other, from those 

 of the body generally ; so that the mixture of 

 aerated and vitiated blood takes place, not in the 

 auricle, but in the ventricle itself. When all the 

 cavities are distended with blood, the two auricles 

 being nearly of the same size as the ventricle, 

 the whole has the appearance of a union of 

 three hearts. The circulatory system of the 

 Ophidia is constructed on a plan very similar 

 to that of the Chelonia. 



In the Saurian reptiles, the structure becomes 

 again more complicated. In the Chameleon each 

 auricle of the heart has a large venous sinus, 

 appearing like two supplementary auricles.* 



* Houston ; Trans. Roy. Irish Acad, xv, 189. 



