This section is from the book "Human Physiology For The Use Of Elementary Schools", by Charles Alfred Lee. Also available from Amazon: Human Physiology, for the Use of Elementary Schools.
1. Calorification, is a function of animal bodies, not yet thoroughly understood. We see certain phenomena, but the causes are hidden from our view. It is very doubtful whether we shall ever be able to penetrate the veil which conceals the wonderful operations of vital chemistry ; and perhaps it would lead to no useful result if we could ; should we ever attain to the knowledge of all the natural laws of life, we shall then be assured that it is only in consequence of their violation that man pays, by suffering, sickness and premature death, the penalty of their transgression.
2. What causes the temperature of the body to be maintained at an average of ninety eight degrees, and this, too, under all climates, and at all seasons ? In the first place, respiration is much concerned in the production of animal heat. It was once indeed believed, that the chief office of respiration was to cool the blood; and that the heart was the great furnace of the system, where all the heat was produced.
3. When it was discovered that both in combustion and respiration, carbonic acid was produced, and oxygen ab sorbed, it was at once surmised, that breathing must be a kind of combustion by which all the heat of the body is generated. But it was objected to this, that if the heat was all produced in the lungs, why were not the lungs hotter than the other parts of the body, as those parts of a stove in contact with the fuel, are hotter than those at a distance.
4. This objection to Mr. Black's hypothesis, led Mr. Crawford to propose the following solution. Although animal heat is produced in the lungs by the process of respiration, yet as arterial blood has a greater capacity for caloric fchan venous, that is, requires more caloric to preserve it at the same temperature, the heat becomes latent in saturating this increased capacity of arterial blood ; and is gradually given off in every part of the body, as the blood assumes the venous character. But unfortunately for this theory, it has been ascertained that there is no difference, and that arterial and venous blood have an equal capacity for heat.
5. But however it may be explained, no one can doubt, that calorification is closely connected with respiration. If the latter is increased by any cause, the heat of the body is also increased. When it is impeded, as in asthma, fainting, breathing deleterious gases, and suffocation, the animal heat is sensibly diminished. Those animals whose respiratory apparatus is the most perfect and the best developed, have the highest temperature, as we see in birds, whose bodies are several degrees warmer than the human species.
6. On the contrary, if we look at the cold blooded animals, we shall find that a large proportion of them live in water, where the supply of oxygen is but scanty, and that their respiration is very imperfect; while in animals that lie torpid during the winter, and are quite cold, respiration is almost if not quite suspended. According to Majendie, respiration produces four fifths of the heat in herbivorous animals; three fourths in carniverous, and the same in birds.
7. It is found by experiment, that arterial blood is warmer than venous. The blood is found to acquire one degree of heat in passing through the lungs, and as the whole mass of the blood passes through the lungs twenty times an hour, it follows, that the system receives from respiration twenty degrees of heat in an hour, or two hundred and forty degrees every twelve hours. Respiration, then, is one of the chief sources of animal heat.
8. Another theory in relation to animal heat is, that it is produced by, or depends on, nervous influence. This is supported by a few experiments of Mr. Brodie, who kept up artificial respiration in animals after cutting off their heads. He found that, notwithstanding, the usual changes took place in the blood, and in the air introduced into the lungs, yet the temperature fell even faster than in another animal killed at the same time, in which respiration was not kept up. This experiment, however, is not conclusive, as an animal may be cooled by forcing too much air into the lungs; and if less be introduced the heat may be preserved for some time.
9. Another theory is, that animal heat is generated in the capillary system. This is supported by the fact, that some parts of the body are often hotter than others, as in inflammation ; that it is always proportioned to the energetic action of any organ, as the head becomes hotter by hard study ; and a glass of spirits excites a feeling of warmth in the stomach.
10. It may be laid down as an axiom, or admitted truth, that all movements among the particles of bodies which cause a change of state, are attended with change of temperature. This is the case in combustion, which is a union of oxygen with the inflammable body, and the production of carbonic acid ; and so also when two liquids are mixed together, which chemically unite, heat is always evolved. Now, in all the processes of nutrition, secretion, digestion, respiration, etc, such changes are constantly going on. In respiration, a part of the blood is thrown off in a gaseous form, while a quantity of oxygen unites with the remaining portion. This process, according to all the known laws of caloric, must be attended with an elevation of temperature. And this proves to be the fact.
11. By the function of nutrition, fluids are changed into solids ; and by absorption, solids are changed into fluids ; by secretion, new chemical compounds are formed out of a few simple elements contained in the blood; and by digestion, a new fluid, chyle, is formed out of the solids, which constitute the food. By all these processes, animal heat must necessarily be generated.
12. But in order to the production of animal heat, by the action of the capillary vessels, two conditions are necessary. One is, the presence of arterial blood; the other, the action of the nervous system. That arterial blood is necessary, is shown by the operation of tying the vessels which supply a limb with blood. The consequence always is, that the temperature immediately falls, and such limbs have to be wrapped in cotton, and other means used to preserve a comfortable degree of warmth. That nervous influence is also productive of animal heat, is shown by dividing, in like manner, the nerves which go to any part. The temperature of a paralytic limb is always lower than that of the sound one.