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.