CHANGES OF THE BLOOD IN RESPIRATION. 453 



vein, during digestion, it contains materials absorbed from 

 the alimentary canal ; and finally, there is every reason to 

 suppose that parts which require different materials for their 

 nutrition, and produce different excrementitious principles, 

 exert different influences on the constitution of the blood 

 which passes through them. After this mixture of different 

 kinds of blood has been collected in the right side of the 

 heart and passed through the lungs, it is returned to the left 

 side, and sent to the system, thoroughly changed and reno- 

 vated, and, as arterial blood, has a uniform composition, as 

 far as can be ascertained, in all parts of the system. This 

 fact has been proven by the direct experiments of Beclard, 

 who analyzed blood from the abdominal aorta, the carotid, 

 temporal, occipital, crural, and epigastric arteries, in the 

 same animal during life, and found the composition identical 

 in all the specimens. His experiments were performed on 

 horses and dogs, and care was taken to draw but a small 

 quantity from each vessel, so as not to change the constitu- 

 tion of the fluid. 1 The change, therefore, which the blood 

 undergoes in its passage through the lungs, is the transfor- 

 mation of the mixture of venous blood from all parts of the 

 organism into a fluid of uniform character, which is capable 

 of nourishing and sustaining the function of every tissue and 

 organ of the body. 



The capital phenomena of respiration, as regards the air 

 in the lungs, are loss of oxygen and gain of carbonic acid; 

 the other phenomena being accessory and comparatively un- 

 important. As the blood is capable of holding gases in solu- 

 tion, in studying the essential changes which this fluid un- 

 dergoes in respiration, we look for them in connection with 

 the proportions of oxygen and carbonic acid before and after 

 it has passed through the lungs. In respiration, the most 

 marked effect on the venous blood is change in color. 



1 Archives Generates de Medecine, 4me serie, tome xviii., p. 123 ; and BERABD, 

 Physiologic, tome iii., p. 369. 



