before it is allowed to reach the blood. We retain about two hundred and fifty cubic inches of this resident air (which is the tempered reservoir whence the blood derives its oxygen), and gradually renew and change it by breathing. We inspire only some twenty-five to thirty cubic inches of fresh, cold air at each breath. This is a man's normal resting condition; of course, when strong exercise begins the blood demands more fresh air. The novice, or the uninstructed athlete, when exercise begins, commits the grave mistake of breathing out his resident air, to make room for a deeper inspiration. But the cold, fresh air is not allowed by nature to reach the air-cells: if it chances to get down too far it makes us cough ; it is too cold, and has too much oxygen. Therefore, a vacuum, or half-filled space, is created; the novice gets "out of breath;" and, if he cannot gradually recover what he has lost, he must come to a stop.
The properly trained man, on the contrary, endeavors to keep all the air he has got, and to add to it, by intruding on the complementary space. When he has regained the small quantity necessarily lost at starting the muscular action, and increased on it, he has got what is called his "second wind," and then he is able to go on while his muscular power holds out.
