N6 A Manual of Veterinary Physiology. 



ox}'gen and 79 volumes of nitrogen : what is the partial 

 pressure exercised by each gas in this mixture ? 



30X21 _ 6*3 inches of mercury, which is the partial 

 1 1 id pressure of the oxygen, 



and 



30x79 23*7 inches of mercury, which is the partial 

 ](J() pressure of the nitrogen. 



The term ' partial pressure ' occurs so constantly in the 

 following pages, that the above may make the matter 

 clearer. 



If a mixture of gases, say the atmosphere, be exposed 

 over a fluid already containing some of these gases dissolved 

 in it, it is found that if the proportion of gases dissolved in 

 the fluid is less than the proportion in the atmosphere 

 above it, the latter pass into the fluid until the amount of 

 gases in the fluid and that in the air above it are equal : 

 but, on the other hand, if the fluid contain more dissolved 

 gas than the atmosphere above it, gas will pass from the 

 fluid to the atmosphere until the amounts both in the fluid 

 and in the atmosphere are equal. This is really a process 

 of diffusion, and it is a most important physical law in 

 respiration, as it is the means by which the CO., passes from 

 the blood into the air-cells, and the oxygen from the air- 

 cells into the blood. 



If two different gases be placed on either side of a porous 

 partition, in a short time a complete mixture has occurred, 

 as both gases will pass through the porous diaphragm in 

 opposite directions until a complete and equal mixture has 

 occurred. This is termed the process of diffusion, and is 

 the chief means by which the air in the deeper part of the 

 lungs mixes with the fresh air introduced by breathing. 



Such are the physical laws which it is necessary to 

 understand before the processes involved in respiration can 

 be fully comprehended. 



The blood having been robbed of oxygen in the tissues 

 the ha'inoglobin makes its way back to the lungs in a partly 



