Crawford, taking Black's doctrine of latent heat as the basis of his inquiries, proceeded to examine the relative capacities for heat of all the substances concerned in the process of respiration. He found that the capacity of the air, before it had been received into the lungs, was greater than after it had been expired, in this way explaining the liberation of heat under these circumstances. In the next place, he examined the respective capacities of arterial and venous blood, and, after a long and careful investiga tion of this point, he announced that the specific heat of arterial was greater than that of venous blood, in the pro portion of about 11 to 10. The conclusion appeared ob vious and important, that when the blood is converted from the venous to the arterial state, it absorbs and renders latent a part of the heat which would otherwise have been extricated by the union of the oxygen and carbon, and these operations would appear to be so nicely adjusted to each other, that the actual temperature of the blood, as it passes through the lungs, is not elevated; a part of the heat produced by the formation of the carbonic acid being required for warming the inspired air, and for evaporating the aqueous vapour which is mixed with the expired air, while the remainder is employed in supplying the arterial blood with a sufficient quantity of heat to compensate for its increased capacity. In conformity with that nice ad justment of the different operations to each other, which forms so remarkable a feature in the animal economy, it will follow that the more carbon is consumed, and the more heat consequently extricated, the more perfectly is the blood arterialized, the more is its capacity necessarily in creased, and the greater proportion of caloric will it re quire to maintain its temperature. According to Craw ford's hypothesis, the blood is not warmed in passing through the lungs, but during the course of the circula tion, when it is converted into the venous state. In portion as this effect is brought about, the blood loses its increased capacity ; its heat, which was before latent, then becomes sensible, and is thus diffused over the body, pre serving every part at its proper temperature.
This theory has the merit of being consistent in all its parts, and of easily explaining the phenomena; and it was announced as being supported in every part by direct ex periments. It rests principally upon the three following data : that the temperature of the blood is nearly uniform in all parts of the body ; that the capacity for heat is greater in the arterial than in venous blood ; and that the capacity of oxygen is greater than carbonic acid. Every one of these points has, however, been called in question ; and some of them are contradicted by experiments, the results of which are precisely the reverse of Crawford's. Thus we are informed, by writers of respectability, that the arte rial blood is always about one or two degrees warmer than the venous blood. With respect to their relative capa cities, the experiments of Dr. John Davy do not indicate that difference which was announced by Crawford ; while, in the third place, we learn from M. M. De la Roche and Berard, that the capacities of oxygen and carbonic acid are at least much less different than they had been pre viously supposed to be. It is not easy to decide upon points where opposite facts are brought forward by in dividuals, whose authority is so respectable as that of Crawford and his opponents. In the present state of the question, we conceive it the most prudent plan to suspend our judgment until the experiments have been repeated, or some new ones adduced which may be more unexcep tionable.
We must not, however, pass by without noticing a train of experiments which have been performed by Al r. Brodie; the results of which have been supposed to be directly opposed to the chemical doctrine of animal heat in all its parts. He opened the chest of an animal soon after death, and kept the lungs inflated with atmospheric air, when he observed that the blood underwent the usual change from purple to scarlet, and that carbonic acid was produced in the same manner as during life ; yet, notwithstanding these changes, the heat of the body diminished as rapidly as if it had been left at rest. The experiments appear to
have been performed with considerable address, and are certainly entitled to much attention ; but there is one im portant circumstance which seems to have been overlook ed, that no more air should admitted into the lungs than they have the power of acting upon, otherwise the directly cooling effect of the air will more than counteract the operation of any chemical change which it may expe rience. Dr. Philip accordingly found, that, when the air was admitted into the lungs, in very small quantities only, the cooling of the body after death was sensibly retarded. And although, in Mr. Brodie's experiments, the blood experienced the same change of colour as in the act of respiration, and carbonic acid was produced, still many circumstances were wanting to render the state of the animal exactly similar to what it is during lite, particularly with respect to the manner in which the blood is after wards disposed of through the body, and the re-conversion of it from the arterial to the venous state.
Independent of direct experiment, there are many cir cumstances connected with animal temperature which would lead us to conclude, that it is intimately connected with the function of respiration. In the first place, all animals that have a temperature much superior to that in which they are immersed, have their lungs constructed in the most perfect manner, and possessing the most elabo rate organization ; while there is an obvious relation be tween the quantity of oxygen which they consume and the heat which they evolve. Thus, what are styled the warm blooded animals, have lungs of a large size, and so formed as to permit the blood and the air to exercise the most ex tensive influence over each other. In amphibia, the pul monary vessels of the lungs are much more scanty ; while the circulation is so arranged, that only a part of the blood passes through them during each circulation. The tem perature of these animals is proportionably low ; and in fish, where there is only a small quantity of blood to receive the action of the air, and that in a less direct man ner, the temperature is only a degree or two above that of the medium in which they live. In the second place, it is observed, that, in the same species of animals, or even in the same individual under different circumstances, what ever quickens the circulation raises the temperature; and that, when the respiration is impeded, either from disease or from an original mal-conformation of the organs, the temperature is proportionably lowered. Lastly, it may be urged in favour of the chemical theory of animal heat, that oxygen is actually united to carbon ; and that, accord ing to the ordinary effect of this union, caloric must be liberated, so that it would be difficult to explain how it is disposed of, if it be not employed in raising the tempera ture of the body. There is also a farther circumstance to be held in view, that, if we reject the hypothesis of the lungs being the source of animal heat, we have no other adequate cause for its production ; for, although some writers have supposed that the stomach, and others, that the nervous system is concerned in this function, yet these have been thrown out as mere conjectures, without being digested into any regular system, so as to point out, in either case, in what manner the effect follows the supposed cause. Upon the whole, therefore, we think ourselves warranted in concluding, in the present state of our know ledge upon the subject, that animal heat is derived, in the first instance, from the union of oxygen and carbon, which takes place in the lungs during the process of re spiration.