THE PHYSIOLOGY OF AN ANIMAL 



207 



From Figure 102 pu, it will be seen that the first branch of 

 the artery passes to the lungs. In the bulbus arteriosus are 

 valves so arranged that the first blood passing from the heart 

 with each beat goes to the lungs; after these are partly filled 

 the next blood passes through the blood vessels shown at 

 ao, down to the arms and to the lower parts of the body; and 

 finally the last of the blood that comes out with each beat of 

 the heart passes up into 

 the head through the ar- 

 tery, co. Thus the most 

 impure blood passes to 

 the lungs, where it is pu- 

 rified, the mixed blood 

 goes to the lower parts 

 of the body, and the 

 purest blood goes to the 

 head and brain. The 

 separation of pure and 

 impure blood in the frog 

 is not complete, but the 

 arrangement just de- 

 scribed is such as to send 

 the most impure blood 

 to the organs which pu- 

 rify it, and the purest 

 blood to the brain where 

 the purest blood is 

 needed. The two auricles are separated from the ventricle by 

 valves, va, opening mechanically in one direction, in such a way 

 that when the heart beats the blood is forced onward and 

 never backward. 



The blood passes out through the arteries and is carried by the 

 numerous branches into the various parts of the body, the small 

 branches breaking up finally into minute twigs called capillaries, 

 that are distributed in great abundance in every active organ. 



FlG. 102. A DIAGRAM OF THE HEART OP 

 THE FROG TO EXPLAIN ITS CIRCULATION 



ao, aorta, passing to the posterior part of the body; 



6a, bulbus arteriosus; 



co, carotid artery going to the head; 



la, left auricle; 



pu, pulmonary artery passing to the lung; 



ra, right auricle; 



v, ventricle; 



va, valves separating auricles and ventricle. The 



arrows show the passage from the auricles to the 



ventricle. 



(Modified from Parker and Haswell.) 



