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14 The beatings of the pulse are due to the circulation of the blood. The pulse of a normal adult man beats some 75 times a minute. The pulses of children beat faster, while those of old men are slower.

The chief agency for keeping the blood pure is the air. When the blood returns to the lungs after one complete round over the body, it is impure and contains poisonous elements. The oxygen of the air which we inhale purifies this blood and is assimilated into it, while the nitrogen absorbs the poisonous matter and is breathed out. This process goes on incessantly. As the air has a very important function to perform in the body, we shall devote a separate chapter to a detailed consideration of the same. 



Of the three things that are indispensable for the subsistence of man,—namely, air, water and food—the first is the most important. Hence it is that God has created it in such large quantities as to make it available to all of us for nothing. Modern civilization, however, has rendered even fresh air somewhat costly, for, in order to breath fresh air, we have to go out of towns, and this means