384 RESPIRATION. 



monary vesicles we have next to ascertain the character of the 

 blood which is affected by this interchange of gases ; for without a 

 knowledge of its character, before and after this interchange, we 

 shall be unable to form an opinion of the principles which control 

 this most essential part of the process of respiration, that is to 

 say, the interchange of gases between the blood and the air in the 

 pulmonary vesicles. 



Two questions present themselves to our notice in entering 

 upon such considerations, regarding the manner in which, on the 

 confines of the air and the blood, as it were, this interchange is 

 effected between the carbonic acid and the oxygen : one of these 

 questions is, whence does the blood derive its carbonic acid, and in 

 what form does it convey this gas to the lungs ? The second question 

 is, in what physical or chemical relations does the oxygen stand to 

 the blood, or to this or that constituent, in its passage into the 

 blood ? In considering the first question as to the sources of 

 carbonic acid in the animal organism, we must, in the first place, 

 remember that all animal fluids contain gases, and especially 

 carbonic acid. We have already seen, in the second volume of 

 the present work, that carbonic acid, oxygen, and nitrogen, are 

 present not only in the blood, but also in the lymph, the trans- 

 udations, the parenchymatous juices of many organs, and even in 

 the urine. It is by means of these juices that all the animal 

 tissues, as well as the parenchyma of the organs, are permeated by 

 the gases in question, and there is not a single vital organ in the 

 whole animal body from which we might not, by means of the 

 air-pump, extract free carbonic acid, nitrogen, and some traces of 

 oxygen. An experiment which was begun long ago in my labo- 

 ratory placed this relation, as might readily have been conjectured 

 a priori, beyond all doubt, by affording a positive proof of the 

 quality of this gas ; we are, however, still deficient in the more 

 exact quantitative determinations for individual organs. 



But when we investigate the source of the carbonic acid in the 

 blood, and incline to the belief, after the most general examination 

 of the vegetative vital functions, that it must, in part at least, be 

 sought in the activity of the different organs themselves, we 

 cannot wholly refute the objection which might be offered, that 

 the carbonic acid may be formed in the blood itself, and be con- 

 veyed with the transudations of this fluid into the parenchyma of 

 the organs. It is, therefore, in the first place necessary to prove 

 the pre-existence of this carbonic acid in the fluids of the tissue. 

 The probability of such a pre-existence may readily be seen by 



