192 CIRCULATION 



pressure and forcing do^vn the semilunar valves at the 

 mouths of the arteries; the blood from the ventricles enters 

 these arteries with a spiu-t. While this ventricle contrac- 

 tion is going on, the auricles begin to expand and the blood 

 flows from the veins into them. Before they finish ex-, 

 pandiug the ventricles also begin to expand, while the semi- 

 lunar valves of the arteries close to prevent back flow from 

 them. During this period of expansion the mitral and 

 tricuspid valves begin to open, and both auricles and ven- 

 tricles to fill with blood. This goes on tmtil the auricles 

 begin to contract again, and this cycle is repeated indefi- 

 nitely. The position of the valves may be summarized as 

 follows : 



(a) As the auricles contract, the mitral and tricuspid 

 valves are open ; the semilunar valves are closed. 



(b) As the ventricles contract, the mitral and tricuspid 

 valves are shut tightly. The semilunar valves are open and 

 blood is entering the arteries. 



(c) During the pause before the next contraction the 

 mitral and tricuspid valves are open to permit the filling of 

 ventricles, while the semilunars are closed to prevent 

 back flow from the arteries. 



The name of diastole has been given to the period when 

 the auricles and ventricles are expanding, while the period 

 of contraction of these organs is caUed the systole. Between 

 these two periods there comes a time when neither is con- 

 tracting or expanding. Using these terms the various 

 stages of a " cardiac cycle " or heart beat may be ex- 

 pressed as follows : 



First. An auricular systole. 



Second. A ventricular systole. 



Third. Heart pause (both organs in diastole). 



