62 Sir Everard Home’s examination into the 
more distinctly defined outline to all the parts, than he 
thought it possible to produce, when objects are so highly 
magnified. 
From this account of the anatomical structure of the lungs, 
it is evident they are calculated not only to receive supplies 
from the atmosphere, but to convey a part of them, with the 
greatest rapidity, as well as facility, to the heart ; since the 
momentary interruption to the passage of the blood from the 
arteries to the veins, and the numerous valves in the absorb- 
ents, as well as those in the veins, are admirably fitted for 
that purpose ; which is at variance with the theory of decar- 
bonising the blood. 
As carbonic acid gas has been occasionally detected by 
Professor Brande, both in the urine and perspirable matter, 
it must have been formed in the blood circulating through 
the arteries ; and the supply of oxygen which enables this to 
take place, is now shown to be derived from the lungs. 
This mode of decarbonising the arterial blood, by a por- 
tion of its oxygen uniting with the carbon, and carrying it off 
in the form of carbonic acid gas by the urine and perspirable 
matter, is extremely simple, and appears to require no elabo- 
rate chemical process. 
Having ascertained upon a former occasion, that soon after 
digestion has been begun, the oxygen employed in that pro- 
cess unites with carbon, and the quantity of carbonic acid 
gas, met with in the venous blood, is greater than at any 
other time ; this will sufficiently account for the blood in the 
branches of the pulmonary artery always containing a suf- 
ficient quantity of this gas to replace the oxygen that is 
removed from the cells of the lungs, and carried to the heart ; 
