Main veins 



Main arteries 



Open; 

 Closed 



^Semiiuna^ 

 valves If 



icuspid 

 valve 



THE HEART A DOUBLE ORGAN 



The two auricles receive blood at the same time from veins. Blood passes from the 

 auricles to the ventricles, through valves that prevent flow in the opposite direction. 

 The two ventricles discharge blood at the same time into the main arteries, through 

 the semilunar valves, which keep blood from returning when the ventricles expand 



ing all parts of the body. The smallest branches, the capillaries, form a 

 network and combine into larger and larger tubes — the veins. The capil- 

 laries thus carry the blood over from the arteries to the veins. The capillaries 

 were first seen by the Italian Marcello Malpighi (1628-1694), who was 

 born in the very year that Harvey published his book on the circulation of 

 the blood, and who solved Harvey's puzzle — How does the blood complete 

 its circuit? 



Among warm-blooded animals (birds and mammals) the heart is a 

 double organ. Each half of the heart consists of an auricle, or receiving 

 chamber, and a ventricle, or pumping chamber (see illustration above). 

 Blood cannot pass directly from either side to the other. 



The lejt heart is somewhat larger and stronger than the right heart. Its 

 ventricle contracts at fairly regular intervals, forcing the contained blood 

 into the largest artery of the body, the aorta. Branches of the aorta carry 

 the blood on to the various organs and tissues of the whole body. The 

 auricle of the left heart receives blood from a large vein into which blood 

 gathers from the capillaries oj the lungs. A set of valves between the auricle 

 and the ventricle keeps the blood from flowing back when the ventricle 

 contracts. Another set of valves prevents the blood from flowing back from 

 the aorta when the ventricle expands again. 



The left heart thus pumps blood received from the capillaries of the 

 lungs into arteries reaching to all parts of the body. 



The auricle of the right heart receives blood from two large veins, and 



190 



