RESPIRATION 189 



which the catheter blocks diffusion from stationary air to tidal 

 is no longer allowed. At the same time, since the circulation is 

 not interfered with, the gases in the blood of the occluded lobe 

 of the lung are not in markedly different proportions from those 

 in the air-chambers of other parts. If at the end of a sufficient 

 interval the air of the occluded lobe is drawn off and its gases 

 measured, their tensions can be compared with the tensions of 

 gases in specimens of arterial and of venous blood. If from 

 10 c.c. of fluid 1 c.c. of gas can be removed by the air-pump, the 

 volume of gas dissolved is 10 per cent, of the volume of the fluid 

 which dissolved it. Commonly this is written "10 volumes per 

 cent." To ascertain experimentally the tension of a particular 

 gas in a particular fluid when dissolved to the amount of 

 10 volumes per cent, at the ordinary pressure of the atmosphere 

 and at the temperature of the body, it would be necessary to 

 place it in an open vessel in air containing a sufficient admixture 

 of the gas to prevent its escape from the fluid. Suppose that it 

 were found that, when the fluid containing the dissolved gas 

 was placed in air mixed with the same gas to the extent of 

 one-tenth of its volume, the fluid neither gave up gas nor 

 absorbed more gas, the tension of the gas would be equal 

 to one-tenth of an atmosphere. Since the pressure of the 

 atmosphere equals 760 millimetres of mercury, the tension 

 of the dissolved gas would be 76 millimetres. If more gas 

 were added to the air, more would dissolve in the fluid ; if 

 some of the gas were removed from the air, gas would escape 

 from the fluid. Gas passes from the medium in which its 

 tension is high to the medium in which its tension is low. 

 The tension of carbonic acid in tissues, particularly in muscles 

 and glands, is higher than in lymph ; in lymph higher than in 

 blood ; in blood higher than in air. Hence it passes by these 

 several stages from the tissues in which it is formed to the air 

 in the lungs. Much ingenuity has been devoted to perfecting 

 methods for the determination of the tension of carbonic acid 

 in lymph and in venous blood. Frequently results have been 

 obtained which seemed opposed to the doctrine that carbonic 

 acid progresses from one medium to another in accordance with 

 the law of pressures ; but such perplexing results were probably 

 due either to imperfections in method or to the establishment 

 of abnormal physiological conditions during the course of the 



