RESPIRATION. 95 



they suffice to confine the air and the blood in distinct 

 cavities; but like the other organic tissues, they have the 

 property of allowing themselves to be penetrated by endos- 

 mosis and exosmosis. The oxygen of the air therefore passes 

 through them in order to combine with the blood; while those 

 gases contained in this fluid, which should be eliminated, 

 separate from it and mingle with the air, which carries them 

 away with it during expiration. It is an interchange of gases 

 between the air and the blood, the air giving up oxygen 

 to the blood, and receiving from it other gaseous fluids, 

 among which carbonic acid gas predominates in volume. 

 This being in excess in the venous blood is exhaled from 

 the lungs, while the oxygen of the air combines with the 

 blood which is carried to the heart by the veins, which has 

 been deprived of a part of its nutritive elements, and has 

 become unfit to support life. On coming in contact with 

 the oxygen, the venous blood loses its dark colour, becomes 

 a brilliant red, and returns to the heart transformed into arte- 

 rial blood. This group of phenomena is called sanguifi- 

 cation. 



On the one hand the oxygen of the atmosphere burns 

 carbon in the lung, on the other the lung exhales carbonic acid 

 gas, nitrogen, and the vapour of water. From whence are 

 these gases and this water derived? The carbonic acid gas 

 is not produced in the lungs alone. The venous blood 

 reaches the organ of respiration poor in oxygen, and charged 

 relatively with the carbonic acid gas which it has received in 

 its course from all the tissues ; everywhere this acid has been 

 produced by the combination of the carbon with the oxygen, 

 which in the lung is borrowed from the air, but everywhere 

 else it comes from the arterial blood. In a word, the oxygen 

 combined with the blood by respiration, is separated from it 

 little by little in the capillaries throughout the whole body in 

 order to produce numerous products, among others car- 

 bonic acid gas. On leaving the heart and in the arteries, 

 the blood contains 24 parts of oxygen per 1000, in the veins 

 it contains only n per 1000. As for the nitrogen and the 

 vapour of water, one is disengaged, and the other produced 

 during this same process of nutrition, and both are drawn 



