306 FUNCTIONS OF NUTRITION. 



consumed elsewhere ; and accordingly it changes its color from dark 

 purple to bright red while passing through the pulmonary capillaries. 



Both the simpler and the more important phenomena of the circu- 

 lation vary therefore at different times and in different organs. The 

 blood has a different composition as it returns from different parts, 

 or has been employed in different functions. In the parotid gland it 

 yields the ingredients of the saliva ; in the kidneys those of the urine. 

 In the portal vein it contains the products of intestinal digestion ; and 

 in the hepatic vein it has suffered a further alteration by passing 

 through the capillaries of the liver. In the lungs it changes from blue 

 to red, and in the greater part of the general system, from red to 

 blue ; and even its temperature varies in different veins, according to 

 the special nutritive changes in the organs from which they come. 



