In fact, for want of due caution, or it may be of a sufficient range of information, the assistance which Chemistry has hitherto rendered to Physiology has been attended with so many assumptions, that it is extremely difficult to say on which side the balance lies—of advantage or error. We are aware that at this moment there is a contrary feeling—a kind of furore for chemical solutions of physiological phenomena. We believe the caution we venture on suggesting was never more necessary.
The discovery of oxygen gas by Priestley, not only gave a great impetus to chemical inquiries, but affected Physiology in a very remarkable manner; when it was found that the more obvious phenomena of all cases of ordinary burning—lamps, candles, and fires of every kind—consisted mainly of the chemical union of charcoal and oxygen (carbonic acid); and again, when it was discovered that animals, in breathing, somehow or other produced a similar change, one may conceive how ready every one was to cry, "I have found it. The heat of animals is nothing more than combustion! We inhale oxygen; we breathe out carbonic acid; the thing is plain. This is the cause of animal heat!"
It has always struck us as a curious thing that chemists should have attached such a dominant influence, in the production of heat in animals, to the union of carbon and oxygen; because nobody is necessarily so familiar as they are with the fact that the evolution of heat is not at all peculiar to the union of these bodies, but is a circumstance common to all changes of every kind, in all forms of matter—there always being either the absorption or the evolution of heat.
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.
The subject is one of great interest to the public; and, in justice to Abernethy, we should remark (that which perhaps a few more years may render it more important to record), that this essay was written more than half a century ago—1793.
Thousands die every year of affections of the lungs; and many diseases of these organs, if not in their nature incurable, have too generally in practice proved to be so. There are not wanting, however, many persons who ascribe these mournful results, not so much to the abstract difficulty of the case, as to imperfect and erroneous views of the functions and relations of these important organs; and who entertain the opinion that the investigation of the subject has been, either from preconceived notions, from a too limited view of the phenomena, or from some other cause, so infelicitously conducted, that the conclusions arrived at have been either merely assumptions, extremely doubtful, or absolutely erroneous.
It is sufficiently obvious that if we are ignorant of the use of any part of a machine, it must be the most unlikely thing in the world that we should know how to set about repairing it when it is out of order; and the matter must be still worse, if we should happen to ascribe to certain parts of it purposes different or contrary to that which they as really fulfil. So, in an animal, if we are ignorant of the use and relations of any organ, it is very improbable that we can understand the nature of its disorders, or treat them in any case successfully, except by the merest accident, which, though it may waken us up to a sense of our ignorance, leaves us so blind to the causes of our success that we have no power of repeating it.