MECHANISM OF THE INTERCHANGE OF GASES. 469 



Mechanism of the Interchange of Gases between the Blood 

 and the Air, in the Lungs. The gases from the air pass into 

 the blood, and the gases of the blood are exhaled through 

 the delicate membrane which separates these two fluids, in 

 accordance with laws which are now well understood. The 

 first to point out the power of gases thus to penetrate and 

 pass through membranes was the late Dr. J. 3L Mitchell, of 

 Philadelphia. 1 His attention was first directed to this subject 

 by noticing the escape of gas from gum-elastic balloons filled 

 with hydrogen. In order to satisfy himself that the gas 

 passed through the membrane independently of pressure, he 

 put different gases in wide-mouthed bottles covered with gum- 

 elastic, and by a series of ingenious experiments, which have 

 become so common that it is unnecessary to describe them 

 in detail, extended Dutrochet's law of endosmosis and exos- 

 mosis to the gases. He demonstrated the same phenomena 

 when he used thin animal membranes instead of the gum- 

 elastic, and found that the more recent the membrane, the 

 more rapid was the action. The rapidity of transmission was 

 found to be very great in living animals. Observations on 

 the lungs of the snapping turtle, filled with air and placed in 

 an atmosphere of carbonic acid or nitrous oxide, showed a 

 very rapid passage of gas from the exterior to the interior. 

 Dr. Mitchell recognized the passage of gases through mem- 

 branes into liquids, and the exhalation of gases which were 

 in solution in these liquids. He noted this action in the ab- 

 sorption of oxygen and the exhalation of carbonic acid in the 

 lungs ; though he fell into the error of supposing that there 

 was no carbonic acid in solution in the blood, and that it was 

 exhaled as soon as formed. 2 A few years later, Dr. Rogers, 

 of Philadelphia, enclosed a fresh pig's bladder, filled with 



1 On the Penetrativeness of Fluids. By J. K. MITCHELL, M.D., Lecturer on 

 Medical Chemistry in the Philadelphia Medical Institute. American Journal of 

 the Medical Sciences, Nov., 1830, p. 36. 



9 Ibid., p. 56. 



