The interchange between the nitrogen and the other gases at the lungs is very small in the normal condition of the respiration, but there is every reason to believe that this is regulated by circumstances similar to those which determine the interchange of the oxygen and carbonic acid. The nitrogen is much less soluble in the blood than the oxygen and carbonic acid, and we presume that its power of permeating moist animal membranes is much inferior to these gases, and that the smaller quantity of it held in solution in the blood may be in this manner explained. We have already pointed out that, in the experi ments made to determine whether nitrogen is absorbed or exhaled at the lungs, opposite results have been obtained, but that the evi dence preponderates in favour of the opinion that a small quantity of this gas is evolved from the blood during respiration. By an alteration of the usual relation between the quantities of nitrogen present in the air and in a free state in the blood, the evolution of nitrogen from the blood may be increased or suspended, or it may be absorbed by the blood instead of being evolved by it. In a previous part of this article we have referred to ex periments which prove that when animals breathe oxygen or hydrogen gases, or a mix ture of both, azote is evolved in greater quantity than usual from the blood in the lungs; and that when they breathe azote alone, part of this gas is absorbed at the lungs.
The exact condition in which the whole of the oxygen absorbed at the lungs exists in the blood, notwithstanding the light thrown upon this point by recent researches, is still not free from considerable difficulties. Pre vious to the experiments of Magnus upon the gases of the blood, already referred to, the opinion of Le Grange and Hassenfratz, that the greater part of the oxygen gas absorbed at the lungs is dissolved in the blood and carried along with it in that condition to the systemic capillaries, was considered untenable by many celebrated physiologists, the more especially as the attempts to detect free oxygen in the arterial blood had failed in all the more trust-worthy experiments. Different opinions as to the kind of chemical combination formed by the oxygen in the arterial blood have been entertained by those who believe that the portion of this gas that disappears from the inpired air does not unite with car bon in the lungs to form carbonic acid, and that little or none of it is simply dissolved in the arterial blood. In the greater number of these hypotheses, however, the oxygen is supposed to unite itself in whole or in part to the red corpuscles, and especially to the iron contained in these: and as the exact state in which the metal exists in the red corpuscles is still undetermined, this has given rise to very different notions regarding the changes effected upon it by the oxygen.
According to other views, the oxygen in whole or in part is united chemically to some of the other constituent parts of the arterial blood, and from these it is again separated in passing through the systemic capillaries, and unites with carbon to form carbonic acid.* The presence of a larger quantity of free oxygen gas in the arterial blood than what is sufficient to form the carbonic acid gas evolved at the lungs, amounting in some cases to rather more tharr 10 per cent. or the vo lume of the blood in the experiments of Magnus, naturally leads to the conclusion that the greater part, at least, of the absorbed oxygen is not chemically combined in the arterial blood, and is simply held in solution by it. We are not, however, quite prepared to concur in the opinion of Nlagnus, that the whole of the absorbed oxygen is held in solu tion in the arterial blood, and that an inter change between part of the free carbonic acid of the venous blood, and part of the oxygen of the atmospheric air, embraces the entire changes in the blood as it passes from the venous to the arterial condition : for, if the opinion be correct that the elaboration of the materials of the chyle into blood is completed in the lungs, and that certain marked differ ences in the fibrin of the two kinds of blood, noticed above, really exist, something more than this is probably necessary. Though the experiments of Marchand appear to prove that the absorbed oxygen does not enter into any chemical combination with the consti tuent parts of the arterial blood in the lungs, by which carbonic acid gas is formed ; yet, while the greater part of the absorbed gas is held in solution in the arterial blood, a small portion of it may enter into chemical combi nation in a manner hitherto not definitely ascertained.* It is almost universally believed that the free carbonic acid gas in the blood is formed by the combination of the absorbed oxygen with carbon in the blood, chiefly if not en tirely in the course of its circulation through the systemic capillaries ; but this opinion, however plausible it may appear, and though it apparently accounts for the evolution of animal caloric in a satisfactory manner, does not rest upon any direct evidence. There are no facts that militate against the exist ence of such a combination, and there can be no doubt that in the present state of our knowledge it affords the readiest and most complete interpretation of the phenomena referred to it, but still it is quite possible that the carbonic acid may be formed during the process of nutrition differently from what is generally supposed.