If we have found it difficult to arrive at any certain con clusion on the first point that we proposed to discuss, we shall probably find it still more so with respect to the second—By what means is the uniformity of the animal temperature preserved ? because any speculation which we may form upon this subject must, in a great measure, depend upon our ideas of the means by which animal heat is produced. With the general fact we are well acquaint ed, that, in warm-blooded animals, each species has a temperature that is natural to itself, from which it deviates very little while in its healthy state. The temperature of the internal parts of the human body is between 98° and 99'; and this temperature is preserved with as much re gularity by the Greenlander as by the African. There are, no doubt, many circumstances in tneir modes of life by which the inhabitants of these different regions en deavour to counteract the extremes of heat and cold to which they are exposed ; but after making allowance for all these circumstances, some system of adjustment of the functions will be necessary, in order to preserve that uni formity of temperature which is so essential to life.
We have already referred to the experiments of Priest ley and Crawford, which first threw some light upon this intricate subject, and which were repeated and consider ably extended by Lavoisier. They led to the same con clusion, that the union of oxygen and carbon in the lungs is influenced by the temperature of the inspired air; the lower the temperature the more tendency there is to their union, there is a greater consumption of oxygen, and a more rapid genaration of carbonic acid. Hence, accord ing to our usual notions upon this subject, there must be a greater evolution of animal heat ; and this will naturally have the effect of counteracting the lower temperature in which the body, in this case, is conceived to be immersed. According to Crawford's theory, the greater quantity of carbon is removed from the blood, the more perfectly is it converted from the venous to the arterial state, the more is its capacity for heat increased, and the more will it re quire to supply this increased capacity, which will be afterwards liberated, during the course of the circulation, to maintain the due temperature of the body. Although this explanation proceeds in part upon the principles of Crawford's theory, and may be so far considered as of doubtful authority, it depends, to a certain extent, upon the direct results of experiments that were performed without any view to this hypothesis, and which appear to be entitled to our confidence; while it must be admitted, that the admirable manner in which they explain the phe nomena affords at least some presumption of their truth. But although the experiments that have been performed are favourable to the hypothesis—that the formation of carbonic acid in the lungs is so regulated by the tempera ture of the air as to produce heat according to the demand for it in the system—still they are not sufficiently nume rous or decisive to amount to a demonstration of its truth.
Should it be confirmed by subsequent facts and experi ments, it must he admitted to be one of the most beautiful examples of the adaptation of means to ends that is to be met with in any part of the animal economy.
It was an opinion generally received among the older writers, and it was maintained even by Boerhaavc, that life cannot exist in a temperature higher than that which is natural to the body ; but many facts have been lately brought to light which completely disprove this position. The first of them, which rested upon good authority, was communicated by 'Fillet and Duhamel. They gave an ac count of some young women, in the service of a baker, in one of the provincial towns in France, who were accus tomed to enter the hot ovens for the purpose of turning the loaves ; and this, it was said, was done without any ap parent inconvenience, provided they were careful not to touch the heated surface. The narrative was scarcely cre dited at the time, but subsequent facts have fully establish ed its credibility. A set of experiments were performed in London, by Blagden and Fordyce, in which a chamber was heated to a temperature higher than that of boiling water, and these gentlemen found that they could easily remain in it for an indefinite length of time. It is, how ever, to be regretted, that they almost exclusively direct ed their attention to the effects of the heated air upon the various substances in the room, and unfortunately neglect ed to observe its action upon the living body itself. We are, indeed, informed, that they perspired very copiously, but we have no information respecting the most important point, whether their temperature was actually raised ; or, at least, what we are told on this subject is too vague to allow us to place much confidence in the statement. Some experiments have been lately performed by M. De la Roche, which give us some insight into this intricate subject. He found that the body was capable of remaining in a tempe rature considerably higher than that which is natural to it, as long as there was a free access to the surrounding air ; but that, when the animal was confined in a small space, an uneasy sensation was produced, and the temperature was elevated. Hence it may be conjectured, that the eva poration of aqueous vapour from the lungs, and perhaps also from the surface of the body, is the means by which the superabundant heat is carried off in these cases, so as to form a kind of balance to that operation, whatever it be, by which heat is generated under ordinary circumstances.