THE CIRCULATION OF THE BLOOD. 



297 



The Circulation of the Blood. 



Now, let us follow the blood as it comes rushing along from 

 the various parts of the body, and pours itself into the right 

 auricle of the heart. From this chamber it is forced down into 

 the bottom chamber, on the same side, and is prevented from 

 rushing back by a wonderful 

 system of valves, which allow 

 it to rush down, but not back 

 again. From the ventricle, 

 the heart-muscles pump the 

 blood up into the lungs. 

 Here it spreads out into 

 smaller and smaller tubes, 

 until they are so small they 

 cannot be seen with the 

 naked eye. This blood, 

 which the heart has pumped 

 into the lungs, is called 

 Venous blood, because it is 

 full of poison gathered up 

 from all over the body. 

 The body is constantly 

 burning. The cells which 

 compose it are dying in mil- 

 lions every second of life. 

 Their dead bodies fall into 

 the blood and are carried 

 away to the heart and 

 pumped into the lungs. Now, 

 when we take in a breath 

 of fresh air, we breathe in 

 a quantity of gas, called 

 oxygen. 



The lungs contain mihions of little hollow bags, known as 

 sacs. Around these, the tiny blood vessels are embedded. They 

 are gorged with blood. Their walls are very thin, so that when 

 the air rushes into these tiny bags, the oxygen is sucked up into 

 the blood through the thin walls of the blood vessels. It in- 

 stantly mixes with the poisons in the blood, and a wonderful, 



Fig. 125. — From the artery the blood runs 

 into smaller and smaller channels, so 

 small that they can only be seen with a 

 strong microscope. They again run to- 

 gether and pour their contents into a 

 vein. {From Blackic's Physiology.) 



