BY H. S. HALrRO WARHLAW. 



793 



The manner in which the composition of alveolar air changes 



when the breath is held, as expressed by the above figures, is 



more clearly shown by the accompanying diagram ('J'ext-fig.l). 



In the diagram, times are plotted as abscissae, the corresponding 



tensions of carbon dioxide and of oxygen as ordinates. The 



points shown in the diagram represent the average values given 



in the Table. 



Diagram i. 



Curve A in the diagram represents the variation of the 

 alveolar tension of carbon dioxide with the period of holding the 

 breath. This curve shows that, as the breath is held, the 

 alveolar tension of carbon dioxide rises at a continually decreas- 

 ing rate for about 30 seconds. During the first 5 seconds of 

 holding the breath, the alveolar tension of carbon dioxide rises 

 from 38-0 to 41-6, 3-3 mm.Hg. Between the 2oth and 30th 

 seconds, the tension rises from 47*5 to 48-0, 0-5 mm.Hg, that is, 

 at only about one-seventh of the initial rate. After the 30th 

 second, the rate of increase of the alveolar tension of carbon 

 dioxide ceases to fall, and begins to rise again. This increase 

 in steepness continues to the 35th second, beyond which 

 the subject was unable to hold the breath and still retain the 

 power of making the forced expiration necessary to obtain a 

 sample of alveolar air. The increase in the rate at which the 

 alveolar tension of carbon dioxide rises, after holding the breath 

 for about 30 seconds, is due to the fact that the subject then 

 begins to make involuntary movements of the diaphragm and of 

 the muscles of the chest, which grow in intensity as the holding 



