94 THE BLOOD. 



much, doubt, not to say confusion. For while the scarlet 

 colour of the arterial blood has been supposed by some 

 observers, and for some reasons, to be due to the chemical 

 action of oxygen, and the purple tint of that in the veins 

 to the action of carbonic acid, there are facts which made 

 it seem probable that the cause was a mechanical one 

 rather than a chemical, and that it depended on a difference 

 in the shape of the red corpuscles, by which their power of 

 transmitting and reflecting light was altered. Thus, car- 

 bonic acid was thought to make the blood dark by causing 

 the red cells to assume a bi-convex outline, and oxygen 

 was supposed to reverse the effect by contracting them 

 and rendering them bi-concave. We may believe, how- 

 ever, that Prof. Stokes has, at least for the present, set 

 this vexed question at rest. 



From the results of spectrum analysis, he has been led 

 to the conclusion that the colouring matter of the blood, 

 or cruorin, is capable of existing in two different states 

 of oxidation, and that the respective colours of arterial 

 and venous blood are caused by differences in tint be- 

 tween these two varieties scarlet cruorin and purple 

 cruorin. The change of colour produced by passage of 

 the blood through the lungs, and its consequent exposure 

 to oxygen, is due, probably, to the oxidation of purple 

 cruorin, and its conversion into scarlet cruorin ; while the 

 readiness with which, the Latter is de-oxidized offers a 

 reasonable explanation of the change, in regard to tint, of 

 arterial into venous blood, the transformation being 

 effected, probably, by the delivering up of oxygen to the 

 tissues, by the scarlet cruorin, during the blood's passage 

 through the capillaries. The changes of colour are more 

 probably due to this cause, namely, a varying quantity 

 of oxygen chemically combined with the cruorin, than to 

 any mechanical effect of this gas, or to the influence of 

 carbonic acid, either chemically, on the colouring matter, 

 or mechanically, on the corpuscles which contain it. We 



