RESPIRATION. 357 



function of distending the air-cells and alveolar passages, thus 

 maintaining the conditions essential to the free movement of 

 blood through the capillaries and to the exchanges of gases be- 

 tween the blood and alveolar air. As this air can not be dis- 

 placed by volitional effort, but resides permanently in the alveoli 

 and bronchial tubes though constantly undergoing renewal, it 

 was termed 



4. The residual volume, the amount of which is difficult of deter- 

 mination, but has been estimated by different observers at 914 

 c.c., 1562 c.c., 1980 c.c. 



The Vital Capacity of the Lungs. From foregoing statements 

 it is clear that the thorax and lungs are capable of a maximum degree 

 of expansion, at which moment the lungs contain their maximum 

 volume of air. This volume, whatever it may be, represents the 

 entire capacity of the lungs in the physiologic condition, and includes 

 the tidal, the complemental, the reserve, and the residual volumes. 

 Mr. Hutchinson, however, denned the vital or respiratory capacity 

 of the lungs as the amount of 

 air which can be expelled by 

 the most forcible expiration 

 after the most forcible inspi- 

 ration, and which therefore 

 excludes the residual volume. 

 The vital capacity was sup- 

 posed to be an indication of 

 an individual's respiratory 

 power, not only in physiologic FlG ' ^-- 

 but also in pathologic condi- 

 tions. Though averaging about 230 cubic inches (3593 c.c.) for an 

 individual 5 feet 7 inches in height, the vital capacity varies with a 

 number of conditions, the most important of which is stature. It 

 is found that between 5 and 6 feet the capacity increases 8 inches 

 (125 c.c.) for each inch increase in height. 



The volume changes of the thorax indicated by the volumes of 

 air entering and leaving the lungs can be not only determined but 

 graphically represented by means of an apparatus similar in principle 

 to the spirometer, devised by Gad and known as the pneumato graph or 

 aeroplethysmo graph (Fig. 175). This consists of a quadrangular box 

 with double walls, the space between which is filled with water. The 

 center of the box is an air chamber. A thin-walled mica box sinks 

 into the water. Posteriorly it is attached to and rotates around an 

 axis, which permits of an elevation or depression of the anterior portion. 

 It is also carefully counterpoised. A light lever attached to the mica 

 box records its movements. The interior of the box communicates 

 by a tube with a large reservoir into which the individual breathes, 



