ARTERIAL AND VENOUS BLOOD. 95 



are not, perhaps, in a position to deny altogether the 

 possible influence of mechanical conditions of the red 

 corpuscles on the colour of arterial and venous blood 

 respectively ; but it is probable that this cause alone would 

 be quite insufficient to explain the differences in the colour 

 of the two kinds of blood, and therefore if it be an element 

 at all in the change, it must be allowed to take only a 

 subordinate position. 



The greater part of the cruorin in both arterial and 

 venous blood probably exists in the scarlet or more highly 

 oxidized condition, and only a small part is de-oxidized and 

 made purple in its passage from the arteries into the veins. 



The differences in regard to colour between arterial and 

 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 some- 

 times, 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 

 arterial 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 potash. 



Some of the veins, however, contain blood which differs 

 from the ordinary standard considerably. These are the 

 portal, the hepatic, and the splenic veins. 



