This section is from the book "The London Medical Dictionary", by Bartholomew Parr. Also available from Amazon: London Medical Dictionary.
The opinion of MM. Lavoisier and Seguin is more simple, but by no means meets so satisfactorily the phenomena, as the theory of Dr. Crawford. They consider respiration as a kind of combustion, in which pure is Converted into fixed air, and the heat separated as the cause of animal heat. This, however, establishes a focus of heat in the lungs. This part must be the warmest, and the extremities the coldest, in the body, while the heat of the intervening parts must vary in proportion to their distance from the centre of inflammation. They avoid this difficulty, however, by alleging the rapidity of the circulation; and they elude the consequence of extraordinary heat in the lungs, by its diminution in consequence of evaporation; but, if examined, neither would be found equal to the effects. Lavoisier, however, adopts part of the idea of Crawford; and, when we recollect that the work of this latter author appeared in 1779, and the improved system of Lavoisier in 1780, we shall not doubt to whom this addition is owing. On the whole, the system of Dr-crawford is apparently the true one. In the first edition there were some errors, both in the experiments and calculations; nor is the second, perhaps, though much more perfect, wholly free. Modern discoveries have, indeed, added to Dr. Crawford's system, and confirmed it; for, whether we consider the formation of carbonic acid gas from the addition of carbon, or that of water by the union of hydrogen, we shall find that in each change the vital air must lose a part of its specific heat. Yet it may be alleged, that Dr. Crawford, by ascertaining the capacity of aqueous vapour and of venous blood, has given a solution, though not so particular, equally satisfactory.
Various modifications of these opinions have been published. M. Girtanner, in the Journal de Physique for the year 1790, has suggested an opinion, that a part of the oxygen of the atmosphere unites with the arterial blood; a part with the carbon in the carbonated hydrogenous gas, which escapes from venous blood, forming carbonic acid gas; a part with the mucus, which is constantly decomposing; a part with the hydrogen gas of the blood to form water; and a part only remains in the blood to supply the animal heat. The effects of respiration will, therefore, be very numerous and different; but, when the products are examined, they will, he thinks, be found the same with those of combustion. If Dr. Crawford's system be considered with attention, it will not, we suspect, be found to require such a complicated process.
De la Grange adopts the opinion of Dr. Duguid Leslie, or rather of Dr. Duncan, putting it only into a modern dress; and Hassenfretz does not greatly differ.
Dr. Gren, in the Annales de Chimie, supposes that no oxygen is communicated to the blood; but that the change from the venal to the arterial is owing to the separation of carbon and hydrogen, with which the oxygen forms carbonic acid, and the water expired in respiration. M. Metheric, in the Journal de Physique, will admit only of the combination of oxygen as one cause of animal heat, recurring as assistants to muscular motion and fermentation. Respiration, he thinks, conducts the electrical fluid to the blood, as the air of an apartment in which a person has long breathed, is electrified negatively; but this proves nothing, as all our excrementitious fluids possess a negative electricity. Linnaeus hints at a similar cause of the heat of animals, when he observes, in his concise, energetic language, Flagrat electrico pulmonibus hausto.
Dr. Menzies' experiments are connected rather with the subject of respiration than with animal heat; and we need only remark, that he thinks all the heat observed in the animal system may be explained from the quantity of pure air vitiated in the lungs; thus referring the heat of animals, like Lavoisier, to a species of combustion. The conclusion is, however, more correct in a chemical than in a physiological view. It will undoubtedly explain the heat of the blood in the lungs; and if Mr. Hunter's experiments, formerly mentioned, be admitted, for a little increase of the heat in those organs; but it will not explain the nearly uniform temperature in different parts. Indeed, we know of no system which so readily meets all the physiological and pathological facts as that of Dr. Crawford, and it is, we believe, generally adopted. See Respiration.
To this system, however, one objection remains, viz. the heat, which the embryos of animals, and particularly of oviparous ones, possess, independent of the parent. As the blood, however, of the foetuses of viviparous animals passes regularly through the lungs of the mother, it may be supposed to convey sufficient heat for the embryo; and, in confirmation of this idea, the blood of pregnant women seems to be highly oxygenated. No blood from the mother, however, can reach the embryo inclosed in an egg; and, though nature has provided a reservoir of air at one end, it is too inconsiderable to supply the young animal with warmth. It is singular, however, that the nature of this air has not been examined; nor has it been ascertained, though the quantity is known to be diminished in the progress of incubation, whether it undergoes any chemical change. Vet, as the yolk by which the chick is nourished, and the albumen itself, contain oxygen, this may be gradually evolved and impart its caloric; nor is this change merely imaginary, for we know that the mild fluids of the egg are gradually changed to azotic ones, whose capacity for heat is of course diminished.
There are, however, many arguments which lead to Dr. Cullen's opinion, that the warmth of animals is connected with their life, and the effect of the principle which distinguishes them as living beings. It is certain, also, that vegetables which possess life, possess also some innate heat; though the change respecting the air, the inhale and exhale, is reversed; for they expire oxygen as an excrementitious fluid, while they draw in carbon at the radical fibres, and absorb hydrogen probably from the leaves. At present, however, we know too little of the vegetable economy to suffer a system, otherwise highly probable, to be disturbed by its apparent anomalies; and, while we thus put our readers in possession of all the facts, we shall leave the ultimate decision for the result of further investigation.
See Haller's Elementa Physiologiae; Hale's Statical Essays; Dr. Duguid Leslie's Philosophical Enquiry into the Cause of Animal Heat; Girtanner sur llrri-tabilite (Journal de Physique, 1790); Gren (Annales de Chimie, vol. xxiv.); Crawford's Experiments and Observations on Animal Heat, 2d edition, 1788; Me-moire sur la Chaleur, par MM. Lavoisier and De la Place; Memoires de lacademie, 1790; Menzies on Respiration, 1796.