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§ 1. Of the efficient cause of Animal Heat.

The body is surrounded by an atmosphere, which is frequently 40, 50, or even a greater number of degrees colder than itself, so that heat must be rapidly abstracted from it, yet it possesses the power of continually supplying the loss thus occasioned. This faculty appeared to the ancients so far beyond the reach of all physical agency, that they did not even attempt to assign any cause for it, but regarded it as an innate quality of the body, or something essentially connected with life, so that in speculating upon the subject of animal temperature, they thought it necessary to direct their inquiries rather to the modes by which the heat of the body might be reduced to the proper standard, than be maintained above that of the surrounding medium. Without entering into any minute detail of opinions, which can be interesting only from their antiquity, it may be sufficient to remark that Galen and the ancients generally conceived animal heat to be an innate or primary quality of the body, and that it is contemporary with life.3 Its origin or focus was supposed to be in the heart, from which, by means of the blood, it was distributed to all parts of the system. A principal office of the circulation was therefore supposed to be the convey

3 For the opinions of the ancients see Boerhaave, Prælect. § 169, 202. cum notis; Haller, El. Phys. vi. 3. S; we have an account of the doctrine maintained by Hippocrates on this subject in his Treatise de Corde, Op. t. i. p. 269; also in Le Clerc, Hist. de la Medecine, p. 125.

ance of this heat to the various organs, while one great use of the respiration was to cool the blood, or to prevent its heat from exceeding the degree which was consistent with the well being of the animal. After the revival of letters, when the causes of the phenomena of the living body began to form an object of investigation, physiologists still very generally regarded the blood as the source of animal heat, and according to the peculiar theory which they had adopted respecting the operations of the system, they explained the production of heat either upon chemical or mechanical principles. The chemists*

◄ See Haller, El. Phys. vi. 3. 8, 10, for a summary of these opinions. As a specimen of the opinions and mode of reasoning that were adopted on this subject by the chemical physiologists, I may refer to the Treatise of Willis, "de Accensione Sanguinis," which was probably written in the year 1670. The author supposes that there is a proper combustion in the blood, which, according to his general principles, depends upon the fermentation excited by the combination between different chemical substances. He strenuously maintains the doctrine of the life of the blood, which he conceives to consist in its property of producing heat. In the middle of the last century we had a very learned dissertation by Stevenson; Ed. Med. Essays, v. v. pt. 2. p. 806, et seq. He observes that four opinions were then prevalent; 1. That animal heat depends upon attrition between the arteries and the blood; 2. That the lungs are the fountain of this heat; 3. That the attrition of the parts of the solids upon each other produce it; and lastly, the process by which our aliments and juices are constantly undergoing some alteration. With respect to the idea of the heat being derived from the lungs, the author adduces various arguments to prove that the blood is rather cooled than warmed in passing through the pulmonary vessels. This final conclusion is in favour of the

ascribed it to fermentation in the blood which took place in the heart, while the mechanicians accounted for it by the friction of the particles of the blood against the sides of the vessels, and consequently, supposed that it was produced during the course of the circulation."

Although we meet with occasional expressions, which may appear to indicate a more correct opinion, yet perhaps the first clear intimation of a regular hypothesis to account for the generation of animal heat, upon what we should, now consider as a legitimate mode of reasoning, is to be met with in the writings of Mayow. After he had made his interesting discoveries on the constitution of the atmosphere, and the share which it has in the extrication of heat in combustion, he was led to extend the analogy to animal heat, and concluded, contrary to the opinion almost universally embraced at that time, that the

last supposition; he remarks concerning it, that heat is frequently excited by the chemical change which results from the mixture of fluids with each other, and that these changes proceed either from fermentation or putrefaction; the case in question he decides to be one of fermentation, or rather something intermediate between the two. He supposes that the process is principally carried on in the veins, and less in the lungs than in the other parts of the body.

s For specimens of the mechanical mode of reasoning, see Boerhaave, Aphor. cum notis Sweiten; § 382. 675. Perhaps the last attempt to form a mechanical theory of animal heat (at least in this country) is that of Douglas, published in 1747; he lays down the theorem that, "animal heat is generated by the friction of the globules of blood in the extreme capillaries," p. 47.

The following are among the eminent physiologists of

use of the lungs is not to cool the heart, but to generate heat, an effect which is brought about by the absorption of the nitro-aereal spirit of the air. This, mixing with the sulphureous particles of the blood, excites a species of fermentation by which heat is produced, in a way precisely similar to that in which heat is excited by the combustion of inflammable matter generally," Mayow therefore explicitly advanced the two positions, that the effect of respiration is not to cool the blood, but to generate heat, and that it does this by an operation in every respect analogous to combustion. His hypothesis was however imperfect in consequence of the erroneous opinions which he entertained respecting the nature of combustion generally, or the mode by which combustion generates heat, as well as respecting the nature of the combustible matter which is discharged from the blood. Perhaps, however, this latter should be regarded as a verbal, rather than as a real inaccuracy, the word sulphureous being a general term applied to any

that period, who supposed that the effect of respiration is to cool the blood. Sylvius, Disp. Med. cap.7; Fabricius de Respiratione, lib. 1. cap. 6; Bartholine, Anat. p. 430; Harvey de Motu Cordis, Ex. 2. p. 194, 5. and Ex. 3. p. 232; Swammerdam, de Respiratione, sect. 1. c. 1. § 9. and c. 3. § 4; Descartes supposes that the fermentation of the blood in the heart produces heat, which is carried from this organ over the body; De Homine, p. 197. Boyle remarks that "divers of the new philosophers, Cartesians, and others, think the chief, if not the sole use of respiration, to be the cooling and tempering of the heat in the ' heart and blood, which otherwise would be immoderate." Works, v. i. p. 103. Haller discusses the question in El. Phys. viii. 5, 6. Tract. p. 151 et seq.; 296, 7.

kind of inflammable matter, not as is now the case, confined to one species. Mayow's doctrine respecting the connexion between respiration and animal heat does not appear to have made any considerable impression upon the minds of the contemporary physiologists, for we find that the old hypotheses still continued to prevail with little alteration, or only with slight and immaterial modifications, until the middle of the last century. During this period the question was warmly discussed, whether the lungs were the agents for generating heat, or for reducing the temperature of the blood by bringing it into proximity with the cold air, and the sentiments of the most eminent physiologists appear to have been, for the most part, in favour of the latter opinion. And it must be remarked, that even most of those who adopted the former opinion, that the lungs serve to generate heat, supposed that the effect was produced entirely by friction or some other mechanical means."

It was not until after Black had completed his valuable train of experiments on fixed air, that more

7 See Boerhaave, Prælect. § 202. 220. cum notis; Boerhaave, Aphor. 382. cum Comment. Sweiten; Haller, prim. lin. § 303; Martine and Stevenson ubi supra; Cullen displays his accustomed caution in examining into the cause of animal heat, Physiol. § 262; he perceived the connexion which there seemed to be between the lungs and the production of heat, but he could not account for the method in which they act in producing it. Haller, as usual, gives us an account of the opinions that had been entertained on both sides of the question, and inclines to the negative; El. Phys. vi. 3. 13. sub finem. Perhaps Morozzo is the latest author of any considerable respectability, who defends the opinion that the effect of respiration is to cool the blood; Jour. Phys. t. xxv. p. 120.

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