THE BLOOD. 165 



being of somewhat higher specific gravity, from its containing a rela- 

 tively larger quantity of red corpuscles. 



2. Pregnancy. The blood of pregnant women is rather lower than 

 the average specific gravity. The quantity of the colorless corpuscles is 

 increased in the latter months, especially inprimiparae; it is also claimed 

 that the fibrin is increased in amount. 



3. Age. The blood of the foetus is very rich in solid matter, and 

 especially in colored corpuscles; and this condition, gradually diminish- 

 ing, continues for some weeks after birth. The quantity of solid matter 

 then falls during childhood below the average, rises during adult life, 

 and in old age falls again. 



4. Temperament. There appears to be a relatively large quantity of 

 solid matter in those of a plethoric or sanguineous temperament. 



5. Diet. Such differences in the composition of the blood as are due 

 to the temporary presence of various matters absorbed with the food and 

 drink, as well as the more lasting changes which must result from gen- 

 erous or poor diet respectively, need be here only referred to. 



6. Effects of Bleeding. The result of bleeding is to diminish the 

 specific gravity of the blood; and so quickly, that in a single venesection, 

 the portion of blood last drawn has often a less specific gravity than that 

 of the blood that flowed first. 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, dia- 

 betes, and the like. 



For some little time after bleeding the want of colored corpuscles is 

 well marked, but with this exception, no considerable alteration seems 

 to be produced in the composition of the blood for more tjian a very 

 short time; the loss of the other constituents, including the colorless 

 corpuscles, being very quickly repaired. 



Variations 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. 



Differences between Arterial and Venous Blood. The differences be- 

 tween arterial and venous blood are these : 



(a.) Arterial blood is bright red, from the fact that almost all its 

 haemoglobin is combined with oyxgen (Oxy-haemoglobin, or scarlet haa- 

 moglobin), while the purple tint of venous blood is due to the deoxida- 



