CIRCULATION. 



85 



artery is provided with half-moon-shaped folds of membrane, 

 the semi-lunar valves. 



The blood, having traversed the lungs, collects in the pul- 

 monary veins, and is carried to the left side of the heart, 

 where it is received into the left auricle. This circuit, from 

 the right ventricle through the lungs and back to the left 

 auricle, is called the short or pulmonary circulation. From 

 the left auricle the blood is driven on through an opening in 



FlQ. 27. Cross section of the Heart, showing: A, tricuspid valve; 

 B, mitral valve; C, semi-lunar valves of the pulmonary artery; D, 

 semi-lunar valves of the aorta. 



another partition, into the left ventricle. The opening in this 

 partition is also provided with a valve, called the mitral, 

 because it is shaped like a bishop's mitre. 



The walls of the left ventricle are nearly three times as 

 thick as those of the right. When the left ventricle con- 

 tracts, the blood is sent into the first artery, the aorta, with 

 sufficient force to carry it to every part of the system. Its 

 return through the various veins, to the right side of the 

 heart, completes the long or body circulation. Semi-lunar 



