RESPIRATION 77 



O2 in another way. In presence of 40 mm. of CO2 his blood be- 

 comes half -saturated with CO (in the absence of oxygen) at a 

 pressure of .017 per cent of an atmosphere of CO, as shown in 

 Figure 21, and half-saturated with O2 (in the absence of CO) at 

 a pressure of 4.0 per cent of an atmosphere, as shown in Figure 

 20. These pressures are in the ratio of I 1235, which is nearly the 

 same ratio as when the relative affinities are estimated by the pre- 

 vious method. 



As already seen, we may be able to account for varying dis- 

 sociation curves of the oxyhaemoglobin in whole blood by the 

 varying composition and concentration of the salts contained in 

 the red corpuscles, and by varying alkalinity; but we cannot so 

 account for the varying relative affinities of different specimens 

 of haemoglobin for CO and O2, since the curves in Figure 22 are 

 not affected by varying concentration of salts or degrees of alka- 

 linity. There seems to be no escape from the conclusion that in 

 different individuals of the same species, as well as in different 

 species, the haemoglobin molecules are different. Whether the 

 haemoglobin in each individual is made up of homogeneous mole- 

 cules, or is a mixture in some definite proportion of two or more 

 different kinds of haemoglobin, we do not as yet know. What 

 seems pretty certain, however, is that each individual has a specific 

 kind of haemoglobin just as he has a specific shape of nose. At 

 whatever time we have .investigated my own and Dr. Douglas's 

 haemoglobin their specific differential characters have appeared 

 to be sensibly the same. It seems pretty certain that, since the ratio 

 of oxygen capacity to both the coloring power and amount of 

 iron in haemoglobin is constant, the difference in the haemoglobin 

 molecule in different kinds of blood is due to the protein and not 

 the haemochromogen fraction of the molecule; but as yet there 

 are no data to indicate more specifically the nature of the differ- 

 ence. It is of considerable biological significance to have found, 

 however, that, looking at living organisms from a purely chemical 

 standpoint, individual differences express themselves, not merely 

 in the relative amounts of the different molecules which can be 

 separated from different parts of the body, but also in their chemi- 

 cal constitution. 



Since the dissociation curve of CO-haemoglobin in presence of 

 a constant pressure of oxygen and varying pressure of CO, or in 

 presence of a constant pressure of CO and varying pressure of 

 oxygen, is a rectangular hyperbola, provided that the gases are 

 present at sufficient pressure to saturate the haemoglobin, it is 



