360 



ZOOLOGY. 



as we have said, there is always a direct communication 

 between the vascular system carrying red blood and that 

 carrying dark blood, and thus the organs receive a fluid im- 

 perfectly acted on by respiration. The heart is almost always 

 composed of two auricles (Fig. 361), opening into a single 

 ventricle. The arterial blood coming from the lungs, re- 

 ceived into the left auricle, and the venous blood coming from 

 different parts of the body and collected in the right auricle, 

 are both poured into the single ventricle ; in this they are 

 mingled together. A portion of this mixture returns by 

 the aorta into the different organs it is intended to nourish, 

 whilst another part proceeds to the lungs by vessels which 

 spring immediately from the common ventricle or from the 

 aorta itself. In crocodiles, the heart (Fig. 362) more resem- 



Pulmonary Artery. ,^ 



Pulmonary Vein, 

 Eight Auricle 



Vena Cava, 

 Left Arch of the Aorta... 



Eight Aorta. 



* Pulmonary Artery. 

 ^Pulmonary Vein. 



Left Auricle. 

 Single Ventricle. 



Ventral Aorta. 



Fig. 361. Heart of the Turtle. 



bles that of birds and mammals, having a septum separating 

 the right ventricle from the left ; but a peculiar disposition of 

 the arteries causes the mixture of the dark and red blood to 

 take place at some, distance from the heart, and thus the pos- 

 terior half of the body receives blood imperfectly arterialized. 

 In fact the right ventricle, instead of sending off one artery, 

 the pulmonary, sends off two, one of which winding behind the 

 heart, unites with the descending aorta, but not until all the 

 vessels have been given off which go to the head and fore- 

 part of the body. With regard to the distribution of the 

 arteries in reptiles, we shall limit ourselves to the remark, 

 that there exist two or more aortic arches, bending to the 

 right and left, and reuniting to form a single trunk (Fig. 5 1). 



