RESPIRATION, CIRCULATION, DIGESTION DIAGRAM 1. 13 



this reason, when we are about to descend into a well, or mine, or 

 a cistern, where no one has been for a long time, we must let 

 down a lighted candle by a cord; if it is not extinguished, 

 oxygen is present, and we can descend without fear ; but if 

 the candle goes out, there is no oxygen left, and a man would 

 die there. 



Nitrogen is a gas like oxygen, but it can neither support 

 combustion, nor sustain life. 



Carbonic acid is the gas which causes the froth of bottled beer, 

 seltzer-water, cider, or of sparkling w r ine. Carbonic acid, like 

 nitrogen, is neither fitted to support combustion nor to sus- 

 tain life. 



Air contains about one part of oxygen to three parts of 

 nitrogen, with a very small quantity of carbonic acid. During 

 respiration, the air which enters the lungs leaves behind a certain 

 quantity of oxygen, and returns charged with a larger proportion 

 of carbonic acid. Therefore, if a man is shut up in a room where 

 the air cannot be renewed, he gradually exhausts all the oxygen, 

 and at last dies. He dies very quickly, under water, because the 

 oxygen no longer reaches his lungs, and he can no longer 

 breathe ; and this also happens when the neck is squeezed 

 sufficiently to compress the windpipe. 



The air expelled from the lungs during respiration, contains 

 some aqueous vapour, as well as a large quantity of carbonic 

 acid ; this forms the moisture of the breath, and we can thus 

 perceive if a patient still breathes, by holding a glass to his 

 mouth. 



CIRCULATION. The body contains a great number of vessels 

 which proceed from the heart and return to the heart. The 

 first are the arteries, and the second are the veins. These 

 vessels, which grow finer and finer the further they extend from 

 the heart, and larger and larger according as they approach it, 

 nre all filled with blood. But it is not the same colour in the 

 veins and in the arteries ; and it has no longer the same 

 quality. It is often believed that venous blood is blue, and it 



