BY H. S. IIALC'HO WAHnLAAV. 787 



taking a deep inspiration. He found that the percentage of 

 carbon dioxide in the expired air rose at a continually decreasing 

 rate, and, towards the end of the period of holding the breath, 

 seemed almost to have reached a constant value. 



By shutting off one lobe of the lung of an animal from the 

 exchange of gases with the inspired air, and drawing off samples 

 of the contents througli a catheter, Wolffberg (1871) attempted 

 to measure the final tension of carbon dioxide in this portion of 

 the lung after a state of equilibrium with the venous blood had 

 been reached. 



Loewy and von Schrotter (1905) carried out similar experi- 

 ments upon human beings. They found that the alveolar ten- 

 sions of carbon dioxide and of oxygen eventually reached con- 

 stant values, the former gas sooner than the latter. These values 

 they regarded as the venous tensions of the gases. 



Hill and Flack (1908) observed the length of time for which 

 the breath could be held, under normal conditions, after breath- 

 ing oxygen, and after muscular exercise. They measured the 

 tensions of carbon dioxide and of oxygen in the alveolar air 

 when the breaking-point was reached. They also measured the 

 final alveolar percentages after breathing as long as possible from 

 an anaesthetic bag, tilled, in one case, with expired air, in another 

 case, with oxygen. In each case, the time of holding the breath 

 was longer, and the final tensions of carbon dioxide were higher 

 than when similar gaseous mixtures were simply held in the 

 luno-s. The final tensions of carbon dioxide reached were also 

 higher when oxygen was present in excess. These investigators 

 made experiments to determine the alveolar percentages of 

 carbon dioxide and of oxygen after holding the breath for 

 various periods, and found that the percentage of oxygen fell 

 more rapidly than that of carbon dioxide. They concluded 

 that it was the alveolar percentage of oxygen, and not that of 

 carbon dioxide, which determined the period for which the breath 

 could be held. From their experiments on rebreathing the same 

 air from a bag, they concluded that holding the breath obstiucted 

 the circulation and so hindered the exchange of gases between^__.^^ 

 the alveolar air and the blood, /vN^ ^^/\ 



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