VARIATIONS OF BLOOD. 93 



single venesection, the portion of blood last drawn has often 

 a less specific gravity than that of the blood that flowed first 

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

 fluid from the tissues of the body. The physiological im- 

 port 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. It indicates that urgent need which always 

 exists for a certain quantity of blood, irrespective, within 

 certain bounds, of its quality. The benefit which, at least 

 temporarily, results from the injection of warm water into 

 the veins of those dying from haemorrhage or cholera, 

 illustrates the same thing very well. 



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 com- 

 position of the blood for more than a very short time, the 

 loss of the other constituents, including the pale corpuscles, 

 being very 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 com- 

 position and general characters are uniform throughout 

 the whole course of the systemic arteries, they are not 

 so throughout 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 



