398 TEXT-BOOK OF PHYSIOLOGY. 



the escape of the air when this is not desirable. To the free end of the tube 

 a rubber tube provided with a suitable mouthpiece is attached, through which 

 air can be breathed into or out of the air cylinder. With each inspiration 

 the air cylinder descends; with each expiration it ascends. A scale, on the 

 side support, graduated in cubic inches or centimeters, indicates the volume 

 of air inspired or expired. 



With an apparatus of this character Hutchinson, from a long series of 

 observations, denned and determined the above-mentioned four volumes as 

 follows : 



1. The tidal volume, that which flows into and out of the lungs with each 



inspiration and expiration, which varies from 20 to 30 cubic inches (330 

 to 500 c.c.). 



2. The complemented volume, that which flows into the lungs, in addition to 



the tidal volume, as a result of & forcible inspiration, and which amounts 

 to about no cubic inches (1800 c.c.). 



3. The reserve volume, that which flows out of the lungs, in addition to the 



tidal volume, as a result of a forcible expiration, and which amounts to 

 about 100 cubic inches (1650 c.c.). 



After the expulsion of the reserve volume there yet remains in the 

 lungs an unknown volume of air which serves the mechanic function of 

 distending the air-cells and alveolar passages, thus maintaining the 

 conditions essential to the free movement of blood through the capil- 

 laries and to the exchanges of gases between the blood and alveolar air. 

 As this volume of air cannot be displaced 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 determination, 



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 expan- 

 sion, at which moment the lungs contain their maximum volume of air. 

 This volume 4 , 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 inspira- 

 tion, this therefore excludes the residual volume. The vital capacity 

 was supposed to be an indication of an individual's respiratory power, not 

 only in physiologic but also in pathologic conditions. Though averaging 

 about 230 cubic inches (3770 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 in- 

 creases 8 inches (130 c.c.) for each inch increase in height. 



The Total Volume of Air Breathed Daily. For the solution of certain 

 problems connected with ventilation it is necessary to determine the total 

 volume of air taken into the lungs in the course of 24 hours. This can 

 be determined approximately if the two factors, the average volume of air 

 taken into the lungs at each inspiration, and the average number of res- 



