DIGESTION AND USES OF FOOD. 165 



race or a small canal with a swinging gate that will open in only 

 one direction. When the water rushes against it one way it 

 opens the gate and passes on ; if it rushes back it shuts the 

 gate and thereby stops itself. So in the heart, the valves open 

 only in one direction, and the tubes of the heart have valves 

 that allow the blood to flow in only one direction. Thus by 

 means of these automatic or self-closing little gates the course 

 of the heart's blood is controlled and the circulation is always 

 properly directed. 



Now let us very briefly follow the course of the blood. It 

 comes from all parts of the body into the right auricle through 

 two veins, whose valves open only towards the heart. The 

 heart contracts and the blood flows into the right ventricle 

 through the opening, whose valve opens only towards that 

 ventricle. From the right ventricle it goes by an artery to the 

 lungs, where it gets a supply of fresh oxygen from the air and 

 where it gives up its load of carbonic acid gas to be breathed 

 out into the atmosphere. Thus purified it comes back by the 

 veins to the left auricle. Then it passes to the left ventricle. 

 From the left ventricle it is forced out of the heart through 

 the arteries and is carried to all parts of the body. These 

 arteries divide and sub-divide until they become a network of 

 fine tubes called the capillaries. These capillaries uniting 

 again form the veins which carry the blood back again to the 

 right auricle. Thus every beat of the heart sends fresh blood 

 out to all parts of the body, and the old blood comes back to 

 be purified before being sent out again through the arteries. 

 The veins are the tubes that carry the old blood to the heart ; 

 the arteries are the tubes that carry the fresh blood from the 

 heart. We see, therefore, why the cutting of an artery is much 

 more dangerous than the cutting of a vein. In cutting an 

 artery we open up the flow direct from the heart the sluice- 

 gate is opened for the free flow of the blood. 



With tha stomach, and especially with the intestines, are 



