BY H. S. HALCRO VTARDLAW. 803 



This diagram shows that, when the above values are assumed 

 for the final tensions, P, the points obtained for the value of log 

 ( P - p) fall upon straight lines. The curves through these depart 

 visibly from straight lines wlien the values chosen foi' P lie 

 outside of tlie following limits: 48-5J:l-0 mm.Hg for curve A; 

 60-5il2mm.Hg for curve C; 550±5 mm.Hg for curve H: 0"*:]0 

 mm.Hg for curve D. 



The figures for the variations of the alveolar tensions of carbon 

 dioxide and of oxygen after the stoppage of normal breathing, 

 according to the above relations between them, may have the 

 following interpretation. 



When the breath is held, the alveolar tension of carbon dioxide 

 rises, during the first 25 seconds, from its initial value of 38-0 

 mm.Hg at such a logarithmically decreasing rate that, if tlie 

 rise were to continue in the same manner, a final tension of 48-5 

 mm.Hg would be approached closely. This final tension is 

 actually passed during the period of the experiment The 

 effective diflerence of tension driving carbon dioxide from the 

 blood into the alveolar air, when the holding of the breath 

 begins, is thus lOS mm.Hg in the present subject. This final 

 tension of carbon dioxide lies within the range of values found 

 by Christiansen, Douglas, and Haldane {he. cit.) for the tension 

 of carbon dioxide in venous blood by their aerotonometric 

 method. It is also within 2 mm.Hg of the value calculated by 

 Boothby (1915) from the consumption of oxygen, the fiow of 

 blood through the lungs, and the respiratory quotient. 



With regard to the variation of the alveolar tension of oxygen 

 when the breath is held, the results of the present investigation 

 show that the tension falls at a logarithmically decreasing rate 

 such that, starting from the initial value of 116-4 mm.Hg, a final 

 value of 55 mm.Hg would be approximated to if the fall continued 

 in the same way. The difference of tension driving oxygen from 

 the alveolar air into the blood is thus about 61 mm.Hg, when 

 the holding of the breath begins. At the end of the experiment, 

 the alveolar tension of oxygen is still 26 mm.Hg above this final 

 value. 



When the air in the lungs is breathed into and out of an 



