THE SOUECES OF ENERGY 



65 



The fish respires by taking oxygen into the blood through the 

 gills, and also by giving out carbonic acid in the same way (we 

 return presently to this respiratory process, and only consider 

 just now the path of the blood). In the fish, then, the heart is 

 a fairly simple organ consisting of two main chambers, the 

 auricle and the ventricle. Each of these is a muscular, hollow 

 organ which expands and contracts rhythmically. The blood 

 which has returned from the body via the caval veins is poured 

 into the auricle, which then contracts. There are valves at the 

 entrance to the auricle and others at the opening of the latter 



From fhe body^ 



Proper right 



To body 



Proper left. 



Fig. 15. — The Coknections between the Heart and the Lungs in 

 A Warm -Blooded Animal. 



into the ventricle, and these structures so act as to prevent the 

 blood from being forced back into the caval veins, but they 

 allow it to flow into the ventricle. There are valves at the 

 opening of the ventricle into the aorta (that is, the great artery 

 that springs from it), and these act so as to allow the blood to 

 pass through the aorta when the ventricle contracts. Thus the 

 contractions and dilatations of the heart send the blood in one 

 direction only — from the caval veins into the aorta. 



From the latter vessel it goes to the gills, where the arteries 

 break up into networks of minute capillaries. Then the latter 

 vessels reunite until they form several large arteries, two of 



