THE BLOOD 109 



The hemoglobin of the blood, as can be seen from what 

 has been said, is ever passing from one state to another, as 

 it alternately adds oxygen to itself and parts with it. 

 Evidently it can be described as existing in two varieties: 

 a form fully charged with detachable oxygen (oxyhemoglo- 

 bin), and a form from which this loosely engaged oxygen 

 has been removed (reduced hemoglobin) . The first is the 

 prevailing variety in arterial blood fresh from the lungs, 

 and it gives to such blood a bright scarlet color. The 

 venous blood, returning from the tissues, still contains a 

 goodly proportion of oxyhemoglobin, but mingled with it 

 there is now -a variable amount of the reduced compound. 

 Reduced hemoglobin is of a dark color, and venous blood, 

 in consequence, inclines toward a purple. The sharply 

 contrasting red and blue so commonly used in diagrams to 

 distinguish arteries from veins greatly exaggerate the 

 actual difference. 



The red corpuscle, it will now be clear, is a bearer of 

 oxygen. A service like this must be dependent on the 

 extent of surface exposed for the absorption and the dis- 

 charge of the gas. The disk is obviously more efficient 

 than a sphere would be, for it has more surface in pro- 

 portion to its mass. Small corpuscles likewise must be 

 quicker to load and to unload than large ones, and this 

 helps us to understand why the largest corpuscles in nature 

 are not found in large animals, but in those like the frog 

 and the fish, which do not have a very intense metabolism. 

 There is an additional reason why small corpuscles are 

 better fitted to meet the demand for a great oxygen supply: 

 the capillaries must be of a size to admit the corpuscles, 

 and an animal with large corpuscles cannot have the 

 closely woven capillary net which becomes a possibility 

 when the diameter of the corpuscles is reduced and which 

 brings the oxygen into closer relations with the cells which 

 require it. 



The red corpuscles originate, generally speaking, by the 

 transformation of cells in an unexpected locality. This is 

 the so-called "red marrow" which fills the small spaces that 



