There is no doubt that the analogy is very striking between the changes which _appear_ to be wrought in respiration, and those which take place in ordinary combustion. A very little consideration, however, shows that the idea that respiration is the cause of animal heat, or that it is due to any other change of oxygen merely, is not only an assumption, but in the highest degree doubtful. In the first place, the carbonic acid thrown out when we expire, is certainly _not_ made by the immediate union of the oxygen inspired with the charcoal expired; secondly, nothing is so obvious that in respiration there is an immense quantity of heat _thrown out_ of the body. But as it is very desirable that the subject of this paper of Abernethy's on the Skin and Lungs should be understood, we will give the reader a simple view of the nature of these important organs; and as one (functionally considered) is as much a breathing organ as the other, we will say a few words first of the lungs.
In all animals[19], the blood, or other fluid in which the elements of nutrition are sent to all parts, is exposed to the action of the air; and this is what we call breathing or respiration; and the exposing of the blood to air is so arranged that both fluids are in _more or less_ rapid motion. The staple constituents of the air, so to speak, are about one-fifth oxygen and four-fifths nitrogen gases, with about two parts perhaps in a thousand of carbonic acid; and although, as we too well know, the air is occasionally polluted by many _additions_, yet, whether we take air from the top of Mont Blanc, or a cellar in London, the _staple_ principles of oxygen and nitrogen have their proportions unchanged. The air breathed by animals who live in the water is somewhat differently constituted; the proportion of _oxygen_ is considerably _greater_, probably about as much as one-third or thirty-two parts in one hundred; so that fish breathe a more _highly oxygenated_ air than we do.
Now it is found that, when we inhale the air of the atmosphere (that is to say, one-fifth oxygen and four-fifths nitrogen), we expire some oxygen, some carbonic acid, and some nitrogen also; and to ascertain the _actual changes_ which took place, was the object of Abernethy's inquiry.