64 APPLIED PHYSIOLOGY 



tightly by two curtains. A hole in its side opens into an 

 artery. 



When the ventricle is full of blood, all at once it becomes 

 smaller and squeezes the blood. The pressure closes the 

 opening to the auricle and keeps the blood from going out 

 that way, and so it passes into the artery. Three little 

 curtains keep it from running back into the ventricles. 

 The heart keeps the arteries full of blood, and presses 

 it through their branches and through the capillaries and 

 veins. It finally reaches the heart again and flows into 

 the auricles. Then it flows into the ventricles again, ready 

 for another round of the body. 



The sides of the auricles are thin, for they are made 

 only to hold the returning blood while the ventricles are 

 squeezing blood into the arteries. 



118. Why the heart is double. The left part of the 

 heart sends blood to all parts of the body to nourish its 

 cells. The right half of the heart sends the blood only 

 to the lungs. There the blood gets air, which it carries to 

 the cells. Food is gathered from the intestine by blood 

 which is pumped by the left half of the heart, but in man 

 only the right half of the heart sends blood to get air. 

 A fish has only one auricle and one ventricle, which send 

 blood to get both air and food. 



119. Course of the circulation. The left half of the heart 

 sends blood through the arteries to all parts of the body. 

 They empty it into the capillaries. Here some of the 

 plasma and air go out to feed the cells. The rest of 

 the blood then enters the veins, which bring it back to the 

 heart, where it enters the right auricle and ventricle. 

 They send it to the lungs to get air. Then it returns to 

 the left auricle and ventricle ready for another round of 

 the body. It takes twenty seconds to send a drop of 



