RESPIRATION 6l 



gas are directly combined as such with the molecules of haemo- 

 globin, just as molecules of water are combined with molecules of 

 a salt or other substance to form hydrate molecules. In methaemo- 

 globin, on the other hand, the atoms in the molecules of oxygen 

 which enter into combination are separately combined just as in 

 ordinary chemical compounds containing oxygen. When the 

 oxidation of haemoglobin to methaemoglobin occurs the new 

 molecule formed loses its capacity for forming the molecular 

 compounds oxyhaemoglobin and carboxyhaemoglobin. In conse- 

 quence of this the molecular oxygen and carbon monoxide are 

 liberated from oxyhaemoglobin or carboxyhaemoglobin by the 

 action of ferricyanide, and can be measured with the greatest ac- 

 curacy in the gaseous form by a simple method which I described 

 in 1900 (see Appendix).^ 



The ferricyanide method afforded a ready means of measuring 

 directly the gas combined in the molecular form with haemo- 

 globin, and for this purpose replaced the complicated procedure 

 and involved calculations required when the mercurial pump 

 was used. One of the first discoveries made with the new method 

 was that the coloring power of haemoglobin or any one of its 

 molecular compounds with gases varies exactly as its capacity for 

 combining with gas. Hence the "oxygen capacity" of the haemo- 

 globin in blood — in other words its power of fulfilling its physio- 

 logical function of carrying oxygen — can be measured easily by 

 means of a reliable colorimetric method.^ The following table 

 (p. 62) shows the results we obtained on this point. 



That oxygen capacity and depth of color run parallel also in 

 various anaemias and other pathological conditions was shown 

 by Morawitz ;* and Douglas^ showed that even during the rapid 

 regeneration of haemoglobin after loss of blood this also holds. 



At the time when the ferricyanide method was introduced there 

 existed several well-known forms of "haemoglobinometer." Of 

 these the apparatus of the late Sir William Gowers was by far the 

 most convenient. In his method 20 cubic millimeters of blood, 

 obtained from a prick of the skin, are introduced into a small 

 graduated tube and diluted with water until the depth of color 

 is the same as that of a standard solution of picrocarmine in an- 

 other similar tube. The depth of color of the picrocarmine solution 



' Haldane, Journ. of Physiol., XXV, p. 295, 1900. 



* Haldane and Lorrain Smith, Journ. of Physiol., XXV, p. 331, xgoo. 



* Moiawitz and Rohmer, Deutsch. Arch. f. klin. Med., XCIII, p. 223, 1908. 

 'Douglas, Journ. of Physiol., XXXIX, p. 453, 1910. 



