VAEIATIONS IN COMPOSITION. 77 



(J. Davy and Polli). This is, of course, due to absorption of 

 fluid from the tissues of the body. The physiological import 

 of this fact, namely, the instant absorption of liquid from the 

 tissues, is the same as that of the intense thirst which is so 

 common after either loss of blood, or the abstraction from it 

 of watery fluid, as in cholera, diabetes, and the like. 



For some little time after bleeding, the want of red blood- 

 cells is well marked ; but with this exception, no considerable 

 alteration seems to be produced in the composition of the blood 

 for more than a very short time, the loss of the other constitu- 

 ents, including the pale corpuscles, being v.ery quickly repaired. 



Variations in the Composition of the Blood, in different Parts of 

 the Body. 



The composition of the blood, as might be expected, is found 

 to vary in different parts of the body. Thus, arterial blood 

 differs from venous ; and although its composition and general 

 characters are uniform throughout the whole course of the 

 systemic arteries, they are not so throughout the venous sys- 

 tem the blood contained in some veins differing remarkably 

 from that in others. 



1. Differences between Arterial and Venous Blood. These 

 maybe arranged under two heads, differences in color, and in 

 general composition. 



a. Color. Concerning the cause of the difference in color 

 between arterial and venous blood, there has been much doubt, 

 not to say confusion. For while the scarlet color of the ar- 

 terial blood has been supposed by so me 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 de- 

 pended on a difference in the shape of the red corpuscles, by 

 which their power of transmitting and reflecting light was al- 

 tered. Thus, carbonic acid was thought to make the blood 

 dark by causing the red cells to assume a biconvex outline, 

 and oxygen was supposed to reverse the effect by contracting 

 them and rendering them biconcave. We may believe, how- 

 ever, 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 coloring matter of the blood, or haemoglobin (p. 69), is 

 capable of existing in two different states of oxidation, and the 

 respective colors of arterial and venous blood are caused by 

 differences in tint between these two varieties oxidized or scar- 



