62 BIOLOGY. [BOOK i. 



borrowed by the respiratory organs, as we shall see when speak- 

 ing of respiration. It is oxygen which, combining with the 

 globules or hsematia, gives them the vermilion tint which they 

 have in the arterial blood. But the globules do not keep their 

 oxygen long. Elaborated in the fine circulatory vessels, where 

 they are in almost immediate contact with the anatomical ele- 

 ments, they surrender to them their vivifying oxygen, indispensable 

 to the chemical reactions of nutrition. In exchange they take 

 back the gaseous residuum of the oxydation of the tissues, the 

 carbonic acid, which gives them a blackish tinge, that of the 

 venous blood. It must be observed that the sanguineous globules 

 never completely despoil themselves of one of these gases to 

 impregnate themselves completely with the other. They retain 

 them simultaneously, oxygen predominating in the arterial blood, 

 carbonic acid in the venous blood. It suffices, besides, for the 

 gases to be in equal quantity in the blood for the globules to 

 become blackish. The change in the proportion of the two gases 

 is not effected suddenly, but by degrees in proportion as the 

 arterial blood goes away from the lungs and the heart to approach 

 the tissues, the oxygen gradually yields the place to the car- 

 bonic acid. 



The water of the blood is not normally free ; it is found com- 

 bined with albuminoidal matters. This is why it cannot nitrate 

 mechanically through the vascular walls. Almost in totality it 

 comes from the aliments, for it is doubtful whether any notable 

 quantity thereof is formed in the organism. 



The immediate saline principles of the first class which are 

 contained in the blood are the chlorures, the chlorohydrates, the 

 sulphates, the carbonates, the phosphates, and so on. Of all the 

 salts, the chlorure of sodium is by far the most abundant in 

 the blood of man. The proportion of the salts varies, besides, 

 according to the animal species. 



The phosphates predominate in the blood of the carnivora, but 

 yield the superiority to the carbonates of soda and of potash in 

 the herbivora. This is, after all, as we have remarked in refer- 



