There is, however, much left for us to inquire into in this process. In the first place, we have to ascertain, to what degree a mass of matter, of the same capacity with the body, and of equal bulk, would have been heated at the same temperature with that to which the individuals were exposed in the above experiments. We know, that when air is much heated, it is proportionably rarefied ; fewer particles of it, therefore, come into contact with the cold body, and hence the communication of heat will be much slower. Then, with respect to the animal functions taken in connexion with each other, we should examine what effect the respiration has, in these cases, upon the air taken into the lungs ? is there any carbonic acid form ed ? and, if there be, what quantity is generated ? \Vc should be led by analogy to suppose, that the amount of oxygen consumed must be very small, so that the usual supply of heat would be cut off. Our next object should be, to discover whether the quantity of aqueous vapour discharged from the lungs would be sufficient to counter act the operation of all the sources, either internal or ex ternal, by which the body acquires heat, and we should then be able to decide upon a point which has been much agitated, whether there be any specific function for the purpose of cooling the body, or rather, whether the eva poration of the cutaneous and pulmonary vapour be alone sufficient for that purpose. There has been much vague speculation employed by physiologists, on the power of the body in generating cold. No part of the animal eco nomy has been treated of in a more mysterious manner, and different metaphysical notions have been formed to account for an operation, the existence of which has not yet been satisfactorily proved. What, however, we do know upon the subject, seems to warrant us in pointing out this part of the animal system, as an additional exam ple of that beautiful adjustment of the functions to each other upon which we have already taken occasion to re mark ; for it appears, that not only have the lungs the power of evolving heat in greater or less quantity, in proportion to the demands of the system, but that the same organ, when necessary, can even produce the contrary effect, and generate cold.
It will be evident, that this account of animal tempera ture is a necessary step in our examination of the effects of respiration upon the living system ; for, if it should ap pear that the modern doctrines on this subject are in any degree well founded, we must conclude that the evolution of heat is one of the most important of the indirect effects which are brought about by the action of the lungs. The direct effects, as we have already seen, are the discharge of carbon and water, and these are the only changes of which we can be said to have any immediate evidence, either from experiment or observation; and it will now appear, that, among the secondary, or indirect effects, are the evolution of heat, by means of the discharge of car bon, and the regulation of the heat thus generated by the evaporation of water. But it is impossible to observe these operations without contemplating the farther effects which they may produce on the living system, more espe cially as, in the case of the discharge of carbon, it does not appear very obvious why it is so essential to life, or why so extensive and complicated an apparatus should be provided, for what we might he tempted to regard as a comparatively unimportant object. That there is some
farther purpose to be served than the mere discharge of carbon, for the purpose of producing animal heat, is evi dent from the consideration, that if an animal be plunged in warm water, so as to have its temperature kept at the natural standard, there is the same necessity for the regu lar supply of oxygen to carry off the carbon, as under or dinary circumstances. The sense of suffocation which takes place, when the supply of fresh air is not regularly preserved, seems to depend upon the stagnation of the blood in the ventricles of the heart, and this stagnation is owing to the muscles of the heart having lost their power of contraction, in consequence of their not receiving the due supply of arterial blood. The same loss of contrac tility occurs in other parts of the body under the same circumstances; and hence we draw the inference, that one important indirect effect is to support the contractility of the muscles.
Having now proceeded so far as to conclude, that one indirect use of respiration is to produce that change in the blood which enables it to preserve the muscles in a con tractile state, and that this object is attained by transmit ting to them a quantity of arterial blood, which is return ed from them in the venous state, we are led to inquire what is the nature of the operation that is here carried on, or in what way are the muscles rendered contractile by the abstraction of a portion of their carbon ? On this subject we can be guided only by analogy, and that of a vague and uncertain nature; but, proceeding upon such grounds as we possess, there would appear to be some connexion be tween the effects of respiration, and those changes by which animal matter is preserved from spontaneous decomposi tion. Now, the first step in the process of decomposition would appear to be, the discharge of a portion of carbon, which combines with the oxygen of the atmosphere, and forms carbonic acid ; and it may be inferred, that if the superabundant portion of carbon be continually removed from a body which has a tendency to putrefaction, its de composition may be obviated. We have already had oc casion to make the remark, and farther proofs of it will occur in the course of our investigations, that all the matter of which the body is composed, after it has perform ed its appropriate functions, seems to experience some change, which renders it no longer suitable for the pur pose ; that it is carried off, and its place supplied by fresh matter. It is this interchange of particles which preserves the living body in its healthy state ; and when it is sus pended by death, so that the effete matter is no longer duly removed, complete decomposition ensues. The blood ap pears to be the medium by which this change is immedi ately effected, the veins are the channels by which the matter is carried from the mass of fluids, and the lungs the outlet by which it is finally discharged. The chemi cal effect of respiration is therefore similar to the first step in the process of the putrefactive decomposition ; hence it appears that an important indirect effect of respiration is to carry off from the body the particles of matter which would produce a putrescent tendency in the blood, were they not removed from it as rapidly as they are deposited.