CHAP. IL] RESPIRATION. 343 



is according to some observers actually greater than that which is 

 present in an equal volume of blood, i.e. an equal volume of mixed 

 corpuscles and serum. 



When serum is subjected to the mercurial vacuum, 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, all the carbonic acid is given off, 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 

 serum over that present in 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 blood 

 is less than that in an equal volume of serum, admit that the 

 tension of the carbonic acid in blood is greater than in serum. 



If these statements be accepted it seems probable that the 

 carbonic acid exists associated with some substance or substances 

 in the serum, 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 corpuscles. It is further probable that 

 the association of the carbonic acid in the serum is with sodium as 

 sodium bicarbonate, and it is even possible 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 tension of the entire blood. Other observers however 

 maintain that the serum does not hold this exclusive possession of 

 the carbonic acid, but that a considerable quantity of this gas is in 

 some way associated with the red corpuscles. Indeed further 

 investigations are necessary before the matter can be said to 

 have been placed on a satisfactory footing. 



The relations of the Nitrogen in the Blood. 



The small quantity of this gas which is present in both arterial 

 and venous blood seems to exist in a state of simple solution. 



