150 



to the blood-vessels in its entire state, and to be dis- 

 solved and afterwards suffer decomposition during 

 the circulation of that fluid : while these changes are 

 going on, a part of the oxygen combines with the 

 carbon of the blood, and forms carbonic acid, which 

 is liberated through the moist coats of the cells and 

 vessels, when the blood returns to the lungs. In 

 all these hypotheses, it is taken for granted, that 

 the carbonic acid is in some way formed by the union 

 of the carbon of the blood with the oxygen gas of the 

 inspired air ; and the chief difference arises from 

 this union being held by some to take place only in 

 the capillaries of the lungs, and by others, through 

 the whole course of the vascular system. But if, as 

 we contend, no part of the inspired air gains ad- 

 mission into the blood-vessels of the lungs, neither 

 of these opinions can be any longer maintained ; 

 for no oxygen gas can, on such grounds, be held to 

 enter into the blood to unite with its carbon, neither 

 could the acid, which it is supposed to form, be af- 

 terwards expelled from that fluid, 



12 K The objection stated above, applies, how r 

 ever, in part only to the opinion of Mr Aberncthy, 

 who considers the carbonic acid of respiration to be 

 derived, not from the oxygenous part of the inspired 

 air, but to be simply exhaled from the pulmonary 

 vessels *. As this opinion rests on the belief that 

 the quantity of oxygen gas that disappears in respi- 

 ration is not sufficient to account for the bulk of car- 

 bonic acid produced, it can no longer be maintained. 



Essays, Surgical, &c. p. 146. 



