90 HAND-BOOK OF PHYSIOLOGY, 



and in the atmosphere became equal. Other things would, of course, in- 

 fluence the absorption, such as the kind of gas employed, nature of the 

 liquid, and the temperature of both, but cceteris paribus, the amount of 

 a gas which a liquid absorbs depends upon the proportion of the gas the 

 so-called partial pressure of the gas in the atmosphere to which the 

 liquid is subjected. And conversely, if a liquid containing a gas in solu- 

 tion be exposed to an atmosphere containing none of the gas, the gas will 

 be given up to the atmosphere until its amount in the liquid and in the 

 atmosphere becomes equal. This condition is called a condition of equal 

 tensions. The condition may be understood by a simple illustration. A 

 large amount of carbonic acid gas is dissolved in a bottle of water by ex- 

 posing the liquid to extreme pressure of the gas, and a cork is placed in 

 the bottle and wired down. The gas exists in the water in a condition of 

 extreme tension, and therefore there is a tendency of the gas to escape 

 into the atmosphere, in order that the tension may be relieved; this causes 

 the violent expulsion of the cork when the wire is removed, and if the 

 water be placed in a glass the gas will continue to be evolved until it is 

 almost all got rid of, and the tension of the gas in the water approximates 

 to that of the atmosphere in which, it should be remembered, the carbon 

 dioxide is, naturally, in very small amount, viz., -04 per cent. Now the 

 oxygen of the blood does not obey this law of pressure. For if blood 

 which contains little or no oxygen be exposed to a succession of atmos- 

 pheres containing more and more of that gas, we find that the absorption 

 is at first very great, but soon becomes relatively very small, not being 

 therefore regularly in proportion to the increased amount (or tension) 

 of the oxygen of the atmospheres, and that conversely, if arterial blood be 

 submitted to regularly diminishing pressures of oxygen, at first very little 

 of the contained oxygen is given off to the atmosphere, then suddenly 

 the gas escapes with great rapidity, again disobeying the law of pres- 

 sures. 



Very little oxygen can be obtained from serum freed from blood cor- 

 puscles, even by the strongest mercurial air-pump, neither can serum be 

 made to absorb a large quantity of that gas; but the small quantity which 

 is so given up or so absorbed follows the laws of absorption according to 

 pressure. . , 



It must be, therefore, evident that the chief part of the oxygen is con- 

 tained in the corpuscles, and not in a state of simple solution. The chief 

 solid constituent of the colored corpuscles is haemoglobin, which consti- 

 tutes more than 90 per cent, of their bulk. This body has a very re- 

 markable affinity for oxygen, absorbing it to a very definite extent under 

 favorable circumstances, and giving it up when subjected to the action 

 of reducing agents, or to a sufficiently low oxygen pressure. From these 

 facts it is inferred that the oxygen of the blood is combined with haemo- 

 globin, and not simply dissolved; but inasmuch as it is comparatively easy 



