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TEXT-BOOK OF PHYSIOLOGY. 



Volumes of Air Breathed. The volumes of air which enter 

 and leave the lungs with each inspiration and expiration naturally 

 vary with the extent of the movement, though four at least may be 

 determined: (i) that of an ordinary inspiration; (2) that of an ordi- 

 nary expiration ; (3) that of a forced inspiration; (4) that of a forced 

 expiration. 



The apparatus employed for the determination of these different 

 volumes is the spirometer, a modification of the gasometer. The 



form introduced by Jonathan Hutchinson 

 (Fig. 174) consists of two metallic cylin- 

 ders, one (a) containing water, the other 

 (b) containing air, the latter being inserted 

 into the former. The air cylinder is bal- 

 anced by weights so accurately that it re- 

 mains stationary in any position. A tube, 

 penetrating the base of the water cylinder, 

 is continued upward through and above 

 the level of the water. The air-space 

 above is thus placed in free communica- 

 tion with the external air. A stopcock at 

 the outer end of this tube prevents 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 at- 

 tached, 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 

 one of the side supports, graduated in 

 cubic inches or centimeters, indicates the 

 volume of air inspired or expired. 



With this apparatus Hutchinson, from 



a long series of observations, defined 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 (312 to 468 c.c.). 



2. The complemental volume, that which flows into the lungs, in addi- 



tion to the tidal volume, as a result of a forcible inspiration, and 

 which amounts to about no cubic inches (1748 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 (1562 c.c.). 



After the expulsion of the reserve volume there yet remains 

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



FIG. 174. SPIROMETER. 

 (Hutchinson.) 



