THE RESPIRATORY CHANGES IN THE BLOOD. 351 



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 combi- 

 nation with a substance or some substances present in the blood, the combi- 

 nation 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 temperature. It is, how- 

 ever, not to be supposed that as the pressure is lowered, no oxygen what- 

 ever 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 re- 

 maining 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 the 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 pecu- 

 liarly 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 pressure. 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 ( 24) 

 that this constitutes 90 per cent, of the dried red corpuscles. There can be 

 a 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. 



287. Haemoglobin. When separated from the other constituents of the 

 serum, haemoglobin appears as a substance, either amorphous or crystalline, 

 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 shows 

 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 

 all to break up this connection and to set the haemoglobin free from the corpuscles. 

 This maybe done by the addition of water, of ether, of chloroform, and of bile-salts, 

 or by repeatedly freezing and thawing ; blood so treated becomes " laky " (cf. I 24). 

 It is also of advantage previously to remove the alkaline serum as much as possible, 

 so as to operate only on the red corpuscles. The stroma and haemoglobin berns thus 

 separated, a solution of haemoglobin is the result. The alkalinity of the solution, 

 when present, being reduced by the cautious addition of dilute acetic acid, and the 

 solvent power of the aqueous medium being diminished by the addition of one- 

 fourth its bulk of alcohol, the mixture, set aside in a temperature of C. in order 

 still further to reduce the solubility of the haemoglobin, readily crystallizes, when the 

 blood used is that of the dog, cat, horse, rat, guinea-pig, etc. In the case of the 

 dog, indeed, it is simply sufficient to add ether carefully to the blood until it just 

 becomes "laky," and then to let it stand in a cool place ; the mixture soon becomes 



