BLOOD OF PORTAL VEIN. 79 



venous blood are sometimes not to be observed. If blood runs 

 very slowly from an artery, as from the bottom of a deep and 

 devious wound, it is often as dark as venous blood. In persons 

 nearly asphyxiated also, and sometimes, under the influence of 

 chloroform or ether, the arterial blood becomes like the venous. 

 In the foetus also both kinds of blood are dark. But, in all 

 these cases, the dark blood becomes bright on exposure to the 

 air. Bernard has shown that venous blood returning from a 

 gland in active secretion is almost as bright as arterial blood. 



b. General Composition. The chief differences between ar- 

 terial and ordinary venous blood are these. Arterial blood 

 contains rather more fibrin, and rather less albumen and fat. 

 It coagulates somewhat more quickly. Also, it contains more 

 oxygen, and less carbonic acid. According to Denis, the 

 fibrin of venous blood differs from arterial, in that when it is 

 fresh and has not been much exposed to the air, it may be 

 dissolved in a slightly heated solution of nitrate of potassium. 



Some of the veins, however, 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, that in 

 the gastric and mesenteric veins, which contains the soluble 

 elements of food absorbed from the stomach and intestines 

 during digestion, and that in the splenic vein ; it must, there- 

 fore, 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, dex- 

 trin, and other soluble matters which may have been absorbed 

 from the alimentary canal, this blood appears to be deficient 

 in solid matters, especially in red corpuscles, owing to dilution 

 by the quantity of water absorbed, to contain an excess of al- 

 bumen, though chiefly of a lower kind than usual, resulting 

 from the digestion of nitrogenized substances, and termed al- 

 buminose, and to yield a less tenacious kind of fibrin than that 

 of blood generally. 



The blood from the splenic vein is probably more definite in 

 composition, though also liable to alterations according to the 

 stage of the digestive process, and other circumstances. It 

 seems generally to be deficient in red corpuscles, and to con- 

 tain an unusually large proportion of albumen. The fibrin 

 seems to vary in relative amount, but to be almost always 

 above the average. The proportion of colorless corpuscles ap- 



