554. If a thermometer be placed under the tongue of a healthy person, in all climates and seasons the temperature will be found nearly the same. Sir Charles Blagden, “while in the heated room, breathed on a thermometer, and the mercury sank several degrees; and when he expired forcibly, the air felt cool as it passed through the nostrils, though it was scorching hot when it entered them in inspiration.”
Observation. Did not the human body possess within itself the power of generating and removing heat, so as to maintain nearly an equality of temperature, the most fatal consequences would ensue. In northern latitudes, especially, in severe weather of winter, the blood would be converted into a solid 255 mass, and on the other hand, the fatty secretion, when subjected to equatorial heat, would become fluid, and life would be extinguished.
What is related of Chantrey? Of Chaubert? Of Sir Charles Blagden? 553. Give Sir Charles’s own statement. 554. What is said of the temperature of the human tongue? Mention the experiment by Sir Charles Blagden. What would be the effect if the human system did not maintain an equality of temperature?
555. To enable man, and other warm-blooded animals, to maintain this equilibrium of temperature under such extremes of heat and cold, naturally suggests two inquiries: 1st. By what organs is animal heat generated? 2d. By what means is its uniformity maintained?
556. The ancients had no well-arranged theory on the subject of animal heat. They believed that the chief object of respiration was to cool the blood, and that the heart was the great furnace where all the heat was generated. At a later period, Mayow, from his discoveries respecting respiration, asserted that the object of respiration was to produce heat, and denied that the blood was cooled in the lungs.
557. When it was discovered that, both in combustion and respiration, carbonic acid was produced and oxygen absorbed, it led Dr. Black to conclude that breathing was a kind of combustion by which all the heat of the body was produced. This theory was objected to, because, if all the heat was generated in the lungs, like those parts of a stove in contact with the fuel, they would be at a higher temperature than those parts at a distance, which was known not to exist.
558. The next theory, and one which received the sanction of the scientific men of Europe, was proposed by Dr. Crawford. He agreed with Dr. Black that heat not only was generated in the lungs, but that the arterial blood had a greater capacity for heat than the venous, and that this increase of capacity takes place in the lungs. At the moment heat is generated, a portion of it, under the name of latent heat, is absorbed and conveyed to the different parts of the body 256 Wherever arterial blood is converted into venous, this latent heat is given out. But, unfortunately for this theory, Dr. Davy proved the capacity of both, for heat, to be nearly the same.
555. What inquiries are naturally suggested? 556. What was the theory of the ancients? What did Mayow assert at a later period? 557. What was the theory of Dr. Black? The objection? 558. What was the theory of Dr Crawford?
559. No one can doubt that respiration and animal heat are closely connected. Those animals whose respiratory apparatus is the most extended, have the highest temperature. An example is seen in birds, whose organs of respiration extend over a large part of the body, and their temperature is 12° above man; while the respiratory apparatus of cold-blooded animals, as some kinds of fish, is imperfect, and only a small quantity of blood is subjected, at any time, to the effects of respiration.
560. To understand the process by which heat is generated in the human system and in animals, it will be necessary to state: 1st. That the apparent heat of a body, as perceived by the touch, or as indicated by a thermometer, is not the measurement of heat contained in the body, or its capacity for heat.