CHAP. IL] RESPIRATION. 559 



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 absorpt:' 

 according to pressure. The phenomena on the other hand su[ 

 the idea that the oxygen in the blood is in some parti 

 combination with a substance or some substances present in 

 blood, the combination being of such a kind that it holds 

 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 temperature. 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 beginning 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 presence of 

 haemoglobin. We have already seen ( 24) that this constitutes 

 90 per cent, of the dried red corpuscles. There can be d priori 

 little doubt that this must be the substance with which the 

 oxygen is associated ; and to the properties of this body we must 

 therefore direct our attention. 



344. Hcemoglobin. When separated from the other con- 

 stituents of the serum, haemoglobin appears as a substance, either 

 amorphous or crystalline, readily soluble in water (especially in 

 warm water) and in serum. 



