Chap, ii.] RESPIRATION. 449 



that the oxygen in the blood was simply dissolved by the blood. 

 If this were so, then the amount of oxygen present in any 

 given quantity of blood exposed to any given atmosphere, 

 ought to rise and fall steadily and regularly as the partial 

 pressure of oxygen in that atmosphere is increased or dimin- 

 ished ; the absorption (or escape) of oxygen ought to follow 

 what is known as the Henry-Dalton law of pressures. But 

 this is found not to be the case. If we expose blood containing 

 little or no oxygen to a succession of atmospheres containing 

 increasing quantities of oxygen, we find that at first there is a 

 very rapid absorption of the available oxygen, and then this 

 somewhat suddenly ceases or becomes very small ; and if on the 

 other hand we submit arterial blood to successively diminishing 

 pressures, we find that for a long time very little oxygen is 

 given off, and then suddenly the escape becomes very rapid. 

 The absorption of oxygen by blood does not follow the general 

 law of absorption according to pressure. The phenomena on 

 the other hand suggest the idea that the oxygen in the blood is 

 in some particular combination with a substance or some sub- 

 stances present in the blood, the combination being of such a 

 kind that it holds good during a lowering of pressure down to 

 a certain limit, and that then dissociation readily occurs ; we 

 may add that this limit is very closely dependent on tempera- 

 ture. It is, however, not to be supposed that as the pressure 

 is lowered, no oxygen whatever is given off from the substance 

 until a certain point is reached, and that at that point the whole 

 store is in an instant dissociated, no more remaining to be given 

 off. The case is rather that while pressure is being lowered 

 down to a certain point, no appreciable dissociation takes place, 

 and that then having begun it increases rapidly with each 

 further lowering of pressure until the whole of the oxygen is 

 given off. During this narrow range, between the first begin- 

 ning to give off oxygen and the completion of the giving off, 

 the compound of the oxygen with the substance or substances 

 may be spoken of as partly, that is more or less, dissociated. 

 What is the substance or what are the substances with which 

 the oxygen is thus peculiarly combined? 



If serum, free from red corpuscles, be used in such absorption 

 experiments, it is found that, as compared with the entire blood, 

 very little oxygen is absorbed, about as much as would be 

 absorbed by the same quantity of water ; and such as is absorbed 

 does follow the law of pressures. In natural arterial blood the 

 quantity of oxygen which can be obtained from serum is exceed- 

 ingly small ; it does not amount to half a volume in one hundred 

 volumes of the entire blood to which the serum belonged. It 

 is evident that the oxygen which is present in blood is in some 

 way or other peculiarly connected with the red corpuscles. 

 Now the distinguishing feature of the red corpuscles is the 



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