"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. ... The air which we inhale is taken into the lungs through the windpipe, and the blood is purified by it. It is of the utmost importance to breathe through the nose, instead of through the mouth. (Chapter II, The Human Body)"