of considerable intricacy, but by a series of well conceived experiments, which were varied and modified in such a manner as to meet the difficulties which successively presented themselves, the general conclusion seems to be well established, that the evolution of heat always bears a relation to the consumption of oxygen, and that the variations are not greater than might be naturally expected, from the complicated nature of the operations which are always going forwards in the animal economy.

མ

As Legallois' experiments are many of them novel and ingenious, and lead to many curious results, it may be proper to give a brief account of some of the most important of them. After establishing the four positions that are mentioned above, he informs us that laying an animal on its back lowers its temperature, and he examined whether in this case the consumption of oxygen was diminished. The experiment was performed on rabbits and cats. For the purpose of comparison he first operated upon the animal while it was at liberty, and afterwards when confined on its back; the results are not quite uniform, but, for the most part, there was considerably more oxygen consumed when the animals were at liberty. Upon repeating the experiment, with certain variations, it appeared that when the respiration was in any way constrained, by the animal being tied on its back, or by there being a deficiency of air, less oxygen is consumed. It was found, however, that in certain cases, the cooling was more rapid than ordinary, even when more oxygen is consumed, owing, as the author con

jectures, to the struggles which are made, carrying off a portion of the heat. In order, therefore, to render the experiment still more decisive, he rendered the respiration so difficult, that no voluntary effort of the animal could enable it to consume the usual quantity of oxygen; this was accomplished either by placing it in air that was much rarefied, or by mixing with it a large proportion of nitrogen." The experiment was then performed in this way upon dogs, cats, rabbits, and guinea-pigs, when the greatest cooling effect was always found to correspond with the least consumption of oxygen. We are informed that the quantity of carbonic acid formed is always less than that of the oxygen which disappears. When a quantity of carbonic acid is mixed with the air, this is in part removed, so as to prove that carbonic acid is absorbed by the lungs. The author's final decision is, that the nervous system affects the temperature, but that it is by an indirect operation, so far as it contributes to bring the air into contact with the blood. We may therefore conclude upon the whole, that although there are many difficulties which attach to particular parts of the subject, the source of animal heat is in the action of the air upon the blood, and that it ultimately depends upon the abstraction of carbon from this fluid, and the conversion of oxygen into carbonic acid.

As to the question, why the union of oxygen and carbon produces heat, I have already remarked, that

6 Ann. Chim. et Phys. t. iv. p. 21. et seq.

it is one which belongs rather to chemistry than to physiology; it is sufficient for our purpose to state, that no greater difficulty occurs in one case than in the other. With respect to the second position of the theory, the mode in which the heat is distributed through the various parts of the body, it is admitted, that we are still unable to form a decisive conclusion. After attentively perusing the experiments of Crawford, and comparing them with those that have been performed with a contrary result, I confess that the balance of evidence appears to me to be greatly in favour of the former, but I acknowledge, that they are of so delicate a nature, as not to be entitled to implicit confidence, and that it would be extremely desirable to have them carefully repeated.

If we consider the subject upon general grounds, we shall find that there are many circumstances which afford a strong presumption in favour of the chemical doctrine, by tending to establish an intimate connexion between the functions of respiration and calorification. In the first place, all animals that have a temperature, much superior to that of the medium in which they are immersed, have their lungs constructed in the most elaborate manner. What are styled the warm-blooded animals, have the organs of a large size, and so formed as to permit the air and the blood to come into the most extensive proximity, and thus to exercise the most powerful influence upon each other. In the amphibia, the lungs are furnished with fewer vessels and with larger air-cells, at the same time that only a part of

the blood passes through them at each circulation, and their temperature is consequently found to be proportionally low. In fishes, the quantity of blood which passes through the gills to receive the action of the air, and the chemical effect produced upon it, is still smaller, and it is found that the temperature of this class of animals differs but a degree or two from that of the water in which they are immersed. It appears, in short, that if we compare the different classes of animals with each other, we, in all cases, perceive a strict relation between their temperature, and the quantity of oxygen which they consume. It is farther observed, that in the same class of animals, or even in the same individual, whatever quickens or impedes the passage of the blood through the lungs, or whatever promotes or retards the action of the air upon the blood, in the same proportion increases or diminishes the temperature of the animal, so to afford a strong argument in favour of the opinion, that these operations bear to each other the relation of cause and effect.

I shall enumerate a few circumstances, principally taken from Dr. Edwards, which, although of rather a miscellaneous nature, may be properly classed together, as they all bear indirectly upon the question of the connexion between animal temperature and the consumption of oxygen by the lungs. The phenomena of hybernation, as they were related in the last chapter, p. 197, confirm this connexion, as we find in all cases, that exactly in proportion to the diminution "of the chemical effect upon the air, so is the decrease

of animal heat; a fact which is abundantly proved by the references that were made to Hunter, Spallanzani, Carlisle, and others. These authors all agree, that the temperature of the external parts of the body is nearly reduced to that of the surrounding medium, while the internal parts, as well as the blood and the vital organs, are not more than 1° or 2o higher. But we have an experiment of De Saissy's, related by Dr. Edwards, which is peculiarly illustrative of this point, for he found that he was unable to bring an hybernating animal into the torpid state, by the reduction of temperature alone, without also constraining the respiration. Dr. Edwards informs us, that such of the mammalia as are born with the ductus arteriosus large and open, have less power of producing heat, but that in proportion as the duct closes, their power of generating heat is increased; and that when individuals of the same species have the ducts more closed than usual, their temperature is more stationary. He found by experiment, that after making due allowance for their bulk, young animals consume less oxygen than adults, and that they have a less power of generating heat, the production of heat and the consumption of oxygen being in all cases proportionate." I have already adverted to the observation of Buffon, that a newly-born animal can live for a certain length of time under water; the fact was confirmed by Dr. Edwards, but he found that this power belonged only to those animals which, while

8

7 De l'Influence, &c. p. 152. Ibid. p. 618. 9 Ibid. p. 190..3.