386 PHYSIOLOGY CHAP. 



serum, dependent to a much greater extent on the partial pressure 

 of the C0 2 . According to Bohr, this fact depends on the capacity 

 of the haemoglobin to unite chemically not only with the oxygen, 

 but also with the carbonic acid. He further showed that this last 

 combination is in no way obstructed by the simultaneous combina- 

 tion with oxygen ; which leads us to suppose that the two gases 

 are fixed in two different portions of the haemoglobin molecule, i.e. 

 the oxygen in the iron-containing portion of the colouring matter, 

 the carbonic acid in the protein residue (Fig. 169). 



According to Fredericq the non -coagulated venous blood of the 

 horse is capable of absorbing 71 '4 vols. per cent of carbonic acid, 

 while the mass of corpuscles from the same animal only absorbs 



FIG. 169. Curve of absorption of CO 2 , by 1-76 per cent solution of haemoglobin (dotted line), and 

 by one of 3'8 per cent (unbroken line) in relation to progressive increase of pressure. (Bohr.) The 

 pressure (in mm. Hg) is recorded along the axis of the abscissae ; the amount of CO2 (in c.c.) 

 absorbed by 1 grm. haemoglobin, along the axis of the ordinates. 



49 - 6 vols. per cent. It follows that the amount of CO 2 fixed by the 

 plasma is greatly in excess of that fixed by the corpuscles. 



The carbonic acid of the serum, according to the unanimous 

 results of Fredericq, Zuntz, and Alex. Schmidt, is about 86 per 

 cent of that contained in the whole of the blood. It is possible, 

 however, that in the process of defibrination, part of the carbonic 

 acid of the corpuscles may pass into the serum, and that under 

 normal conditions the gas content of the blood is divided in 

 different proportions between the corpuscles and the plasma. 

 Certain experiments of Hamburger show, indeed, that by merely 

 changing the amount of gases in the blood, some individual sub- 

 stances may pass from the plasma to the corpuscles, and from 

 the corpuscles to the plasma: 



Another notable fact is, that by means of the Torricellian 

 vacuum, it is possible to extract from the blood the whole of the 

 carbonic acid which it holds in combination (Setschenow). From 



