NITROGEN OF THE BLOOD. 465 



illary gland of a dog, Bernard found 18-07 per cent, of car- 

 bonic acid during repose, and 10*14 per cent, during secre- 

 tion. The blood coming from the muscles is the darkest in 

 the body, and contains the greatest quantity of free carbonic 

 acid. 



The quantity of free carbonic acid is immensely increased 

 in the venous blood during digestion. It is owing to this 

 fact that the gas then exists in the arterial blood. During 

 the intervals of digestion, the quantity is so small that the 

 lungs are capable of completely eliminating it, and none 

 passes into the arteries ; but during digestion, the proportion 

 is so much greater, that for a time it cannot be entirely re- 

 moved, and a part finds its -way into the arterial system. 



These facts coincide with the views which are . now held 

 regarding the essential processes of respiration. . The blood 

 going to the lungs ordinarily contains carbonic acid, and no 

 oxygen ; for during the intervals of digestion, there is only 

 enough oxygen taken up by the blood to supply the wants of 

 the system. In the lungs, carbonic acid is given off, appear- 

 ing in the expired air, and the oxygen which disappears from 

 the air is carried away by the arterial blood. Under some 

 conditions, and particularly during the height of the digestive 

 process, the quantity of oxygen absorbed is largely increased, 

 and so much may exist in the arterial blood that a small por- 

 tion passes into the veins. At the same time the production of 

 carbonic acid is increased in activity, and it may exist in such 

 quantity in the venous blood, as temporarily to pass in small 

 quantity into the arteries. 



Nitrogen of the Blood. As far as is known, nitrogen has 

 no important office in the process of respiration. There is 

 generally a slight exhalation of this gas by the lungs, and 

 the analyses of Magnus and others have demonstrated its 

 existence in solution in the blood. Magnus found generally 

 a larger proportion in the arterial than in venous blood, 

 though in one instance there was a larger proportion in the 

 30 



