DR. STEVENS ON THE THEORY OF RESPIRATION. 351 



dead blood may be changed from venous to arterial, even through a dead mem- 

 brane. 



This power which gases possess of acting on each other is in some respects similar 

 to that which takes place in fluids, and which has been described by Dutrochet 

 under the name of endosmosis and exosmosis. In the experiments detailed by this 

 philosopher the intervening septum is supposed to contribute materially to the phe- 

 nomena. In the experiments with gases the intervening membrane does not prevent, 

 but it does not contribute to, the change. But independent of this, the existence of 

 this power in gases was not known to Dutrochet until after the fact had been fully 

 ascertained by others. Mr. Dalton many years ago proved that hydrogen possessed 

 the power of penetrating or mixing with carbonic acid in opposition to gravity ; that 

 oxygen possesses the same property, but in a higher degree, I ascertained in the 

 island of St. Thomas in 1 827. I afterwards made experiments on a larger scale, in 

 1830, at the high rock of Saratoga, where there is an atmosphere of natural carbonic 

 acid ; and the result was communicated to many physicians in America previously 

 to any of the American publications on the subject. 



It is now more than probable that the changes which Lavoisier believed to occur 

 in the lungs take place in reality in the extreme circulation. Some later writers 

 have assumed that the elements of carbonic acid exist in the blood, and that its for- 

 mation commenced in the large vessels as they leave the left side of the heart, and 

 was not finally completed until the blood arrives in the right auricle. But that this 

 opinion is erroneous, is evident from the fact, that the blood even in the smallest 

 arteries is as completely arterial as that in the left side of the heart, whilst the blood 

 in the smallest veins is equally venous. 



There is in the capillary system, over the whole body, an intermediate structure 

 which connects the arterial with the venous circulation, and it is in this structure 

 that the blood 4s changed from arterial to venous. When the arterial blood leaves 

 the minute arteries, it is no longer confined in actual vessels, but in cells that are 

 formed by the surrounding tissue. When this cellular structure is examined in 

 living animals, with the assistance of a good microscope, minute globules are seen to 

 leave the cells and penetrate the surrounding solids, whilst other globules are seen 

 to return and mix with the blood in the cells. Now as we know that it is in these 

 cells that the blood becomes dark and venous, from the addition of carbonic acid, 

 may we not suppose that the globules which leave the blood are minute particles of 

 oxygen, attracted perhaps from the arterial blood by the fixed carbon of the solids, 

 and that the globules which return are minute particles of carbonic acid ? This can- 

 not be easily proved ; but as carbon is a principal ingredient in the solids when these 

 are converted into fluids, previous to their removal by the lymphatics, it appears to 

 me not improbable that a part of their carbon may be liberated, perhaps for the pur- 

 pose of evolving heat. We have seen that the blood which receives oxygen in the 

 lungs passes unchanged through the arteries into the capillary system ; and it is ap- 



MDCCCXXXV. 2 z 



