develop and become powerful. The levatores of the ribs may also assist, and even the muscles of the neck and arms may help out, while in forced expiration the abdominal muscles are called into play. The glottis opens and closes rhythmically as the air enters and leaves the lungs, and the nostrils add their mite in the struggle for oxygen. Finally there may be scarcely a muscle in the body that is not striving to aid the respiration, and general convulsions may result, followed by exhaustion and death.
Air.—In ordinary breathing the lungs are not used to their full capacity and the air ordinarily used is known as tidal air. In forced inspiration the lungs are filled to their fullest extent and the air then taken in in excess of the tidal air is known as complemental air. In like manner, the difference between the air ordinarily breathed out and that breathed out in forced expiration is known as supplemental air. The sum of these three is the vital capacity of the lungs, while beyond this there is probably some air that is never expelled, the stationary or residual air.
Respiratory Sounds.—The entrance and exit of the air is accompanied by respiratory sounds or murmurs, which vary according to their position in the trachea, the bronchi, or the bronchioles and are modified in diseases of the lungs and bronchi, when they are often called râles.
Changes in Air in Lungs.—In passing through the nose and the rest of the respiratory tract the air is warmed to body temperature and saturated with moisture. After its entrance into the lungs various changes take place in it through the mingling of the tidal with the residual air. Thus, it gives up about 4 or 5 per cent. of its oxygen and acquires some 4 per cent. additional carbon dioxide, while the amount of nitrogen remains about the same. By its giving up more oxygen than it receives carbon dioxide, its volume is slightly diminished. Exhaled air also contains
