After ordinary expiration, such as that which expels the breathing air, a certain quantity of air remains in the lungs which may be expelled by a forcible and deeper expiration; this is termed reserve air. But even after the most violent expiratory effort, the lungs are not completely emptied; a certain quantity of air remains in them, over which there is no voluntary control, which may be called residual air. Its amount depends, in great measure, on the absolute size of the chest, and has been variously estimated at from 40 to 200 cubic inches. Power of Inspiratory Power of Expiratory Muscles. Muscles. 1.5 inches. weak 2.0 inches. 2.0   ,,    ordinary 2.5   ,,    2.5   ,,    strong 3.5   ,,    3.5   ,,    very strong 4.5   ,,    4.5   ,,    remarkable 5.8   ,,    5.5   ,,    very remarkable 7.0   ,,    6.0   ,,    extraordinary 8.5   ,,    7.0   ,,    very extraordinary 10.0   ,,    The blood as it moves through the respiratory organs is exposed to the air that alternately moves into and out of the air-cells and minute bronchial tubes. The blood is propelled from the right ventricle through the pulmonary capillaries in steady streams, and slowly enough to permit every minute portion of it to be for a few seconds exposed to the air, with only the thin walls of the capillary vessels and air-cells intervening. The atmosphere we breathe has in every situation in which it has been examined in its natural state a nearly uniform composition. It is a mixture of oxygen and nitrogen, carbonic acid, and watery vapor, with traces of other gases, as ammonia, sulphuretta, hydrogen, etc. Of every 100 volumes of pure atmospheric air, 79 volumes consist of nitrogen and 21 of oxygen, about. The proportion of carbonic acid is extremely small: 10,000 volumes of atmospheric air contains only about 4 or 5 of carbonic acid. The average quantity of watery vapor in the atmosphere in this country is about 1.40 per cent. The changes produced by respiration on the atmosphere are that: 1. It is warmed; 2. Its carbonic acid is increased; 3. Its oxygen is diminished; 4. Its watery vapor is increased; 5. A minute amount of organic matter and of free ammonia is added to it. 1. The expired air is hotter than the inspired air. The temperature varies from 97° to 99 1/2°.