CIRCULATION 



71 



Hisly in one direction by valves. These are situated: 

 i) between the auricles and the ventricles, (b) between 

 e ventricles and the arteries and (c) throughout the 



eins. The valve between the auricles and the ventricles 



s like a swinging door that opens in one direction only. 



t opens in response to the pressure of the blood, allows 

 e blood to pass through, and then swings shut. The 



lood can not then pass 



>ack into the auricle. At 

 the outlet of the ventricle 

 there are three flat pockets 

 which allow the blood to 

 pass out of the ventricle, 

 but when it attempts to 

 return, they become filled 

 with blood and swell out 

 so that the opening is 

 closed (Fig. 47). Simi- 

 larly in the veins the 

 valves open to allow the 

 blood to pass toward the 

 heart but close against its return. Without the valves, 

 the flow in the veins would be backward, because of the 

 diminution of pressure. 



(3) The closed system. The system is closed; that is, 

 the irregularly branching tubes are so arranged that 

 when the blood has been carried from the heart to all 

 parts of the body it is gathered together and returned to 

 the heart. The vessels through which the blood leaves 

 the heart are tough, thick-walled, highly elastic tubes 

 called arteries. They divide and sub-divide into smaller 



Fie. 47.— Diagram of valves of veing. 

 A, valve opened by blood passing for- 

 ward toward heart ; B, valve closed 

 by attempted return of blood; C, vein 

 opened to show arrangement of 

 valves. (From Fitz.) 



