THE BLOOD AND ITS CIRCULATION 



116. Veins. The capillaries come together again and 

 form tubes called veins. Each vein is a tube much like 

 an artery. It carries blood away from the cells. It has 

 valves which permit blood to flow toward the heart but 

 keep it from flowing back to the cells. Each vein runs by 

 the side of the artery which brought the blood to it. So 

 the blood tubes are like 



a net hanging from two 

 ropes. One rope is an 

 artery, the net is the 

 capillaries, and the other 

 rope is the vein. The 

 blood is always moving 

 in the tubes. We must 

 find out what moves it. 



117. The heart. If 

 we follow the arteries 

 back, we shall see that 

 they unite and grow larger 

 until we reach a single 

 tube nearly an inch 



through. This artery is c closed valve to keep blood from flowing 

 . , back into the auricle. 



called the aorta. At the 4 ventricle. 

 beginning of the aorta is e arterv - 



/ valve to keep blood from returning to the 



a pump called the heart. ventricle. 

 The heart lies just to the 



left of the middle of the body, and just below the level 

 of the armpits. It is a bag about the size of the fist. 

 Its inside is divided so as to make two bags. Each bag 

 is a complete pump. The lower part of each side of the 

 heart is called a ventricle, and the upper part an auricle. 

 There are two openings in each ventricle. A hole in 

 its upper part leads to the auricle. It can be closed 



Diagram of the heart while it is beating. 



a vein entering the auricle. 

 b auricle. 



