CHAP, ii.] RESPIRATION. 333 



steadily and regularly as the partial pressure of oxygen in that 

 atmosphere is increased or diminished. 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 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 dissociation readily occurs at certain pressures and certain 

 temperatures. What is that substance or what are those sub- 

 stances ? 



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; but 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 exceedingly 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 (p. 26) 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. 



Hcemoglobin; its properties and derivatives. 



When separated from the other constituents of the serum, 

 haemoglobin appears as a substance, either amorphous or crystal- 

 line, readily soluble in water (especially in warm water) and in 

 serum. 



Since haemoglobin is soluble in serum, and since the identity of the 

 crystals observed occasionally within the corpuscles with those obtained 

 in other ways shews that the haemoglobin as it exists in the corpuscle is 

 the same thing as that which is artificially prepared from blood, it is 

 evident that some peculiar relationship between the stroma and the 

 haemoglobin must, in natural blood, keep the latter from being dissolved 

 by the serum. Hence in preparing haemoglobin it is necessary first of 



