VARIATIONS OF BLOOD. 85 



the venous system, the blood contained in some veins 

 differing remarkably from that in others. 



I. Differences between arterial and venous blood. These 

 may be arranged under two heads, differences in colour, 

 and in general composition. 



a. Colour. Concerning the cause of the difference in 

 colour between arterial and venous blood, there has been 

 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, however, 

 that, at least for the present, this vexed question has, by 

 the results of investigations undertaken by Professor Stokes 

 and others, been now set at rest. 



The colouring matter of the blood, or hemoglobin (p. 74), 

 is capable of existing in two different states of oxidation, 

 and the respective colours of arterial and venous blood are 

 caused by differences in tint between these two varieties 

 oxidised or scarlet haemoglobin and de-oxidised or purple 

 haemoglobin. The change of colour produced by the passage 

 of the blood through the lungs, and its consequent exposure 

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

 and its conversion into scarlet haemoglobin; while the 

 readiness with which the latter is de-oxidised offers a 

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

 arterial into venous blood, the transformation being 

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



