THE LUNGS AND RESPIRATION. 129 



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 glot- 

 tis 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 res- 

 piration, 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 complemen- 

 ted 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 rales. 



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 nitro- 

 gen remains about the same. By its giving up more 

 oxygen than it receives carbon dioxide, its volume 

 is slightly diminished. Exhaled air also contains 



