I30 THE COMMON FROG. ' [chap. 



called venous blood, and is conveyed by the veins to 

 the right auricle, whence it passes into the right ven- 

 tricle, which sends it to the lungs for purification. 



The oxygenated, or arterial blood, is returned from 

 the lungs to the left auricle, and hence it is directly 

 transmitted to the left ventricle, whence it is driven 

 through the great artery (the aorta) into other arteries, 

 and so distributed all over the body. The aorta 

 passes downwards in front of the backbone, when it 

 is called the descending aorta. Before turning down- 

 wards, hovv-ever, it gives off great arteries to the arms 

 and head, the carotid arteries carrying blood to the 

 latter. 



Now it is very important that the blood should not 

 proceed in a direction the reverse of that indicated, 

 and to prevent such misdirection, or regurgitation, 

 special valves are placed at different openings ; these 

 valves freely allowing the blood to flow in the proper 

 direction, but instantly opposing an effectual obstacle 

 to a contrary flux. 



The openings of the auricles into the ventricles are 

 guarded by valves, as also is the opening of the left 

 ventricle into the aorta, and that of the right ventricle 

 into the artery going to the lungs. 



The valve which guards the entrance into the right 

 ventricle is called t^'iciispid, and consists of three flaps 

 attached by delicate tendinous cords in such a way 

 as to hinder the tending backwards of the flaps into 

 the right auricle, and so allowing the blood to flow 

 back into that chamber. 



The valve which guards the entrance into the left 

 ventricle is called mitral (from a fancied resemblance 



