BY H. S. HALCRO WARDLAW. 81 1 



seconds. The figures in Table ii. show that, between the 20th 

 and 25th seconds of breathing into the bag, the alveolar carbon 

 dioxide is rising at the rate of 005% per second. Curve C, in 

 Fig.l, shows that, during tliis period, the rise is practically uni- 

 form. From the percentages of carbon dioxide found, therefore, 

 005 has been subtracted for each second for which the bi'eath 

 was held longer than 20 seconds. 



The above results may be divided into two groups, (1) those 

 obtained when the breath is held under pressures numerically 

 less than - 10 mm.Hg, (2) those obtained under pressures numer- 

 ically greater than -10 mm.Hg. The alveolar percentages of 

 carbon dioxide shown in the first group of results vary with the 

 pressure under which the breath is held The lower the negative 

 pressure is, the higher the percentage of carbon dioxide is. In 

 the second group of results, however, the percentages of carbon 

 dioxide found are, with one exception, practically constant and 

 independent of variations of the pressure under which the breath 

 is held. The mean alveolar percentage of carbon dioxide reached, 

 when the breath is held for 20 seconds under negative pressures 

 numerically greater than - 10 mm.Hg, is 051 higher than that 

 readied in an equal period under normal pressure. The average 

 deviation of these results from the mean is j:0-12. Practically 

 the same increase of the rate of gaseous exchange is produced, 

 therefore, by holding the breath under pressures more than 

 10 mm.Hg below that of the atmosphere, as by performing 

 the movements of breathing into a closed bag. This fact is 

 additional evidence that the increased respiratory exchange, 

 caused by the movements of breathing, is not brought about by 

 a quickening of the circulation. The respiratory exchange is 

 increased during breathing owing to the existence of negative 

 pressure in the chest during the act of inspiration. The figures 

 indicate that the pressure in the lungs, during inspiration, must 

 fall at least as low as - 10 mm.Hg. 



In these experiments, the alveolar percentages of oxygen have 

 not been estimated, as a knowledge of the variations in the per- 

 centages of carbon dioxide alone is sufficient to lead to the recosr 

 nition of differences in the rates of gaseous exchange. The 



