90 HANDBOOK OF PHYSIOLOGY. 



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

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

 ous 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, diabetes, 

 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 than a very short 

 time ; the loss of the other constituents, including the colored 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 

 composition 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 

 between arterial and venous blood are these: 



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

 haemoglobin is combined with oxygen (Oxy-hsemoglobin, or scarlet 

 haemoglobin), while the purple tint of venous blood is due to the deoxi- 

 dation of a certain quantity of its oxy-hsemoglobin, and its consequent 

 reduction to the purple variety (Deoxidized, or purple haemoglobin). 



(b.) Arterial blood coagulates somewhat more quickly. 



(c.) Arterial blood contains more oxygen than venous, and less car- 

 Tronic acid. 



Some of the veins contain blood which differs from the ordinary 

 standard considerably. These are the Portal, the Hepatic, and the 

 Splenic veins. 



Portal vein. The* blood which the portal vein conveys to the liver 

 is supplied from two chief sources; namely, from the gastric and mesen- 

 teric veins, which contains the soluble elements of food absorbed from 

 the stomach and intestines during digestion, and from the splenic vein; 

 it must, therefore, combine the qualities of the blood from each of these 

 sources. 



The blood in the gastric and mesenteric veins will vary much according 

 to the stage of digestion and the nature of the food taken, and can 

 therefore be seldom exactly the same. Speaking generally, and without 

 considering the sugar, and other soluble matters which may have been 



