470 RESPIRATION. 



venous blood, in a bell-glass of oxygen. 1 In two hours a 

 quantity of oxygen had disappeared, and a large quantity of 

 carbonic acid had made its appearance. Dr. Rogers is fre- 

 quently referred to as the first to demonstrate the passage of 

 gases through animal membranes to and from the blood. 

 The credit of this is due to Mitchell, whose paper was pub- 

 lished in 1830, while the experiments of Rogers were pub- 

 lished in 1836. 



We have already seen that the blood is exposed to the air 

 in the lungs, separated from it only by a very delicate mem- 

 brane, over, an immense surface. The membrane, far from 

 interfering with the interchange of gases, actually favors it ; 

 and thus, in obedience to the laws which regulate endosmosis 

 between gases and liquids, the oxygen is continually passing 

 into the blood, and the free carbonic acid is exhaled. 



General Differences in ike Composition of Arterial and 

 Venous Stood. All observers agree that there are certain 

 marked differences in the composition of arterial and venous 

 blood, aside from their free gases. The arterial blood con- 

 tains less water, and is richer in organic, and most inorganic, 

 constituents than the venous blood. It also contains a greater 

 proportion of corpuscles, fibrin, and inorganic salts. It is 

 more coagulable, and offers a larger and firmer clot than 

 venous blood. Numerous analyses have failed to detect a 

 constant difference in the proportion of albumen ; sometimes 

 the proportion is greater in the venouFj, and sometimes in the 

 arterial blood. The only principles which are constantly 

 more abundant in venous blood are water and the alkaline 

 carbonates. 10,000 parts of venous blood contained 12*3 

 parts of carbonic acid combined, and the same quantity of 

 arterial blood contained but 8 '3 parts. 2 The deficiency of 

 water in the blood which comes from the lungs is readily ex- 

 plained by the escape of watery vapor in the expired air. 



1 Experiments on tlw Blood, etc. By ROBERT E. ROGERS, M.D., of Philadelphia. 

 American Journal of tlie Medical Sciences, August, 1836, p. 296. 

 * LONGET, op. cit. y tome L, p. 584. 



