1819.] and on the Blood in general. 277 
With respect to the intimate nature of respiration, we are 
almost as ignorant as of that of the other steps of the assi- 
milating process. Is the carbonic acid given off as car- 
bonic acid by the blood, and an equal volume of oxygen 
gas absorbed; or is the carbon only given off, which, by 
combining with the oxygen of the atmosphere, forms the 
carbonic acid ? With respect to this important point, physiolo- 
gists have differed much in opinion. Some, as Hassenfratz, and 
agrange, supposed that the oxygen penetrates the delicate 
vessels of the lungs, remains in the arterial blood in a state of 
solution or loose combination, till it reaches the capillaries, 
where it passes into more intimate combination with carbon, 
and thus forms carbonic acid, in consequence of which, the 
blood passes into the venous state, and that this carbonic acid 
lies dormant in the venous blood till it reaches the lungs, where 
it escapes in a gaseous form, and a new portion of oxygen is 
absorbed. The most common opinien, however, is, that the 
carbonic acid gas is formed in the lungs by the union of the 
carbon of the blood with the oxygen of the atmosphere, though 
physiologists differ as to the precise mode in which this union 
.takes place, some supposing that the oxygen actually penetrates 
the delicate membrane lining the lungs, and forms the carbonic 
acid within the vessels; and others, especially Mr. Ellis, con- 
tending that the carbon escapes through the same membrane, 
and combines with the oxygen without the vessels.* As to the 
opinion, that the oxygen is not absorbed into the blood, but that 
the carbonic acid gas is formed in the lungs, it is certainly by 
far the most probable in the present state of our knowledge. We 
know, for example, that oxygen gas on being converted into 
carbonic acid gas, is not changed in volume; and, as before 
observed, the most accurate experiments on respiration appear to 
show, that during this function, a volume of oxygen ordinarily dis- 
appears, precisely equal to that of the carhonic acid gas formed + 
—a fact which itis extremely difficult to account for on any other 
supposition; for it is very unlikely that this coincidence in 
volume should so uniformly occur, if the phenomena were not 
more intimately connected as cause and effect than they would 
necessarily be, on the supposition that the carbonic acid is 
given off from the blood as carbonic acid, and the oxygen 
absorbed. 
With regard to the particular manner in which the carbonic 
acid is formed, whether internal or external to the vessels, I 
confess I have no decided opinion, It seems most probable 
that the carbon is excreted, perhaps in a state of solution in the 
watery vapour which is elicited from the blood, and that it com- 
bines with the oxygen of the air at the moment it escapes from 
the exhalents. For it is not easy to conceive, under the cireum- 
* See Ellis on Respiration. 7 
+ See Experiments on Respiration, by Messrs. Allan and Pepys, Phil. Trans. 
1868 and 1809. 
