HEAT, in the animal economy, known by the several names of natural heat, vital heat, innate heat, and animal heat, is commonly supposed to be that generated by the attrition of the parts of the blood, occasioned by its circulatory motion, especially in the arteries.
To what organs or operations, the heat of the human body, and other animal bodies, is owing, is hitherto extremely doubtful. The opinions that at present prevail are, 1. That the heat of animal-bodies is owing to the attrition betwixt 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 on one another produce it. 4. That it is owing to the mechanical attrition of the particles of our fluids. To which opinions Dr Stevenson of Edinburgh added a 5th, viz. That whole process by which our aliment and juices are constantly undergoing some alteration.
The reasonings in favour of these several opinions may be seen at large, as laid down by the above-mentioned author in an essay on the cause of animal-heat, in the Medical Essays, vol. vi. The chief arguments in favour of the first opinion, are, that if an artery is tied, or cut, the part to which it goes turns cold; and on the ceasing of the pulsation of the arteries, cold and death follow. An increase of heat attends a brisk circulation, and a languid circulation is accompanied with a small heat. One who burns in a fever, or is hot with exercise, has a full and frequent pulse. In cold faintings, chlorosis, &c. the pulse is small and slow. To these they add, that the thermometer shews the arterial blood to be a little hotter than that of the veins.
This is accounted for from the conical figure of the arteries, from their fluxes and branches into exquisitely small capillaries; whence the resistance, and consequently the attrition, must be great, from the number, strength, and elasticity of their coats, from the propelling power of the heart, and their strong resistance. From all these it is inferred, that the particles of blood perpetually getting new motions, directions, and rotations, are attenuated, condensed, have their angles grinded off, and are made homogeneous: hence, it is said, follows the fluidity, red colour, and heat of the mass, which is here perfected.
The second opinion is, that the lungs are the fountain of heat in the human body. All that has been said for the blood's being heated in the arteries is advanced to prove this hypothesis, with considerable additions, viz. that in the lungs the blood-vessels every where attend, divide, and subdivide, along with the ramifications of the wind-pipe; and as these are perpetually changing their situation and form, becoming longer or shorter, making more acute or more obtuse angles,
angles, so must the concomitant blood-vessels every moment make new angles, and give the blood new directions; that at last it enters into an exquisitely fine net-work, spreads every where on the vastly thin air-vesicles, where these air bladders are perpetually changing their angles, points of contact, their form, volume, interstices, and so forth. From these and the elasticity of the air, and weight of the atmosphere, the blood is said to be churned, pressed backward and forward, broken and kneaded together, dissolved and condensed, made red and hot in respiration.
The third opinion is, that the cause of the animal heat is owing to the action of the solid parts upon one another. The reason in support of this opinion, is, that the heart and arteries move most; thence that it is natural to think, that the heat should be owing to this motion.
The fourth opinion is, the mechanical attrition of the particles of the fluids upon one another. Dr Stevenson observes, that those who support this hypothesis, must not only suppose that mechanical attrition begets heat, but begets itself without diminution; that they must not only shew what sets this attrition a-going, but what maintains it, because all mechanical force perpetually decreases in a resisting medium; in short, that they must shew the possibility of a perpetuum mobile, the impossibility of which they themselves demonstrate.
The fifth opinion is, what Dr Stevenson calls the animal process, or that process by which our aliment and fluids are perpetually undergoing some alteration. This process, according to that writer, may be one sui generis, somewhat of a middle nature betwixt fermentation and putrefaction; and he thinks it comes so near to the latter, that he chuses to call it by that name. In putrefaction, which is a most powerful dissolvent of bodies, the intestine action of their minute particles creates, collects, or some way or other is the cause or means of heat. The doctor thinks it probable that this process is constantly carried on in all our juices, especially where there is blood; and this is chiefly in the veins, so that the blood is both the fountain of heat and the first spring and motion.
The late Dr Mortimer, in the Philos. Trans. no 476. gives it as his opinion, that the heat of animals is explicable from the phosphorus and air they contain. Phosphorus exists, at least in a dormant state, in animal fluids; and it is also known, that they all contain air: it is therefore only necessary to bring the phosphoreal and aerial particles into contact, and heat must of consequence be generated.