600 METH^MOGLOBIN. [BOOK n. 



present not so well understood as those which determine the 

 presence of oxygen. The carbonic acid is not simply dissolved in 

 the blood; its absorption by blood does not follow the law of 

 pressures. It exists in association with some substance or sub- 

 stances in the blood, and its escape from the blood is a process of 

 dissociation. We cannot however speak of it in the same decided 

 way as we can of the oxygen, as being associated with the haemo- 

 globin of the red corpuscles. 



According to most observers, so far from the red corpuscles con- 

 taining the great mass of the carbonic acid, the quantity of this gas 

 which is present in a volume of serum is actually greater than that 

 which is present in an equal volume of blood, i.e. an equal volume 

 of mixed corpuscles and serum ; that is to say, the carbonic acid is 

 much more largely associated with the serum (or, in the living 

 blood, with the plasma) than with the red corpuscles. When 

 serum is subjected to the action of the mercurial pump, by far 

 the greater part of the carbonic acid is given off; but a small 

 additional quantity (2 to 5 vols. per cent.) may be extracted by 

 the subsequent addition of an acid. This latter portion may 

 be spoken of as ' fixed ' carbonic acid in distinction to the larger 

 ' loose ' portion which is given off to the vacuum. When however 

 the whole blood is subjected to the vacuum until the carbonic 

 acid ceases to be given off, the subsequent addition of acid is 

 said not to set free any further quantity ; so that when serum is 

 mixed with corpuscles all the carbonic acid may be spoken of 

 as 'loose;' and it is stated that the excess of carbonic acid in 

 a quantity of serum over that present in the same bulk of entire 

 blood, corresponds to the fixed portion in serum which has to 

 be driven off by an acid. Moreover, even those who maintain 

 that the quantity of carbonic acid in entire blood is less than 

 that in an equal volume of serum, admit that the carbonic 

 acid exists in some way or other at a higher pressure in, and 

 is more readily given off from entire blood than from serum. 

 If these statements be accepted it seems probable that the 

 carbonic acid exists associated with some substance or substances 

 in the serum, or rather plasma, but that the conditions of its 

 association (and therefore of its dissociation) are determined by 

 the action of some substance or substances present in the cor- 

 puscles. It has been suggested that the association of the 

 carbonic acid in the plasma is with one or other of the proteids 

 of the plasma ; but it has also been suggested that the association 

 is one with sodium as sodium bicarbonate, and further that the 

 haemoglobin of the corpuscles plays a part in promoting the 

 dissociation of the sodium bicarbonate or even the carbonate, and 

 thus keeping up the carbonic acid of the entire blood. 



Other observers however maintain that the plasma does not 

 hold this exclusive possession of the carbonic acid, but that at least 

 a considerable quantity of this gas is in some way directly associated 



