SECTION III.
USE OF FAT IN THE ANIMAL ECONOMY.
As fat forms so large a portion of the body, it is evident that the part it plays in the animal economy must be a most important one. The general opinion which prevails amongst scientific men as to its physiological functions was originated by the celebrated Liebig. According to his theory, the food of animals includes two distinct kinds of substances—plastic[!--4--][4] and non-plastic. The plastic materials are composed of carbon, hydrogen, oxygen, nitrogen, and a little sulphur and phosphorus. Albumen, fibrine, and casein are plastic elements of nutrition; they form the lean flesh, or muscles, the membranes, and cartilages, the gelatine of the bones, the skin, the hair, and, in short, every part of the body which contains nitrogen. The non-plastic elements of nutrition include fat, oil, starch, sugar, gum, and certain constituents of fruits, such as pectine.
All non-plastic substances—and of each kind there are numerous varieties—are capable of conversion, in the animal mechanism, into fat and oil. The non-plastic food substances do not contain nitrogen, hence they are commonly termed non-nitrogenous elements. The oily and fatty matters contain a large proportion of carbon, their next most abundant component is hydrogen, and they contain but little oxygen. Unlike the plastic elements, they are—except the fats of the brain and nervous tissue—altogether destitute of sulphur and phosphorus. The starchy, saccharine, and gummy substances are composed of the same elements as the fatty bodies, but they contain a higher proportion of oxygen. According to Liebig, fat is used in the animal economy as a source of internal heat. We all know that it is a most combustible body, and that during its inflammation the most intense heat is developed. It is less evident, but not less true, that heat is evolved during its slow oxidation, or decay.
The more rapidly a body burns, the greater is the amount of heat evolved by it in a given time; but the total amount of heat developed by a specific weight of the body is the same, whether the combustion takes place rapidly or slowly. An experiment performed with phosphorus illustrates the case perfectly. If we burned two pieces of equal weight, the one in oxygen, the other in atmospheric air, we should find that the former would emit a light five times as brilliant as that evolved by the latter, for the simple reason that its combustion would be five times as rapid. The white, vapor-like matter into which phosphorus is converted by its combustion, is termed phosphoric acid. It is composed of phosphorus and oxygen. In forming an ounce of this compound, by the direct oxidation, or combustion of phosphorus, the amount of force, either as heat, or as heat and light, evolved is precisely the same, whether the time expended in the process be a minute or a month.[!--5--][5] If, in the experiment I have described, we were to substitute two pieces of fat for the fragments of phosphorus, the results would be precisely similar. The fat burned in oxygen gas would emit intense light and heat; but the total amount of these forces evolved would be neither greater nor less than that developed during the slower and therefore less brilliant combustion of the fat in ordinary atmospheric air. Now, as we can demonstrate that an ounce of fat will emit a certain amount of heat, if burned within a minute of time, and that neither a larger nor a smaller amount will be developed if the combustion of the fat extend over a period of five minutes, I think we may fairly assume that the amount of heat evolved by the complete oxidation of a specific quantity of fat is constant under all conditions, except, as I have already explained, at high temperatures, when a portion of the heat is converted into light.
In the animal organism fat is burned. The process of combustion no doubt is a very slow one, but still the total amount of heat evolved is just the same as if the fat were consumed in a furnace. When the fat constituting a candle is burned, what becomes of it? Its elements, carbon and hydrogen (we may disregard its small amount of oxygen) combine with the oxygen of the air, and form carbonic acid gas and water. What becomes of the fat consumed within the animal body? It also is converted into carbonic acid gas and water. It is not difficult to prove these statements to be facts. A candle will not burn in atmospheric air which has been deprived of its oxygen, because there is no substance present with which the elements of the taper can combine, consequently the process of combustion cannot go on. Now, a man may in one respect be compared with this taper. He is partly made up of fat; that fat is consumed by the oxygen of the air, and the heat developed thereby keeps the body warm. In the process of respiration oxygen is introduced into the lungs, and from thence, by means of the blood vessels, is conveyed throughout every part of the body. In some way, at present not thoroughly understood, the elements of the fat combine with the oxygen, and are converted into carbonic acid gas and water, which are exhaled from the lungs and from the surface of the body.
Fat is a constituent of both animals and plants. The animal derives a portion of its fat directly from the vegetable; but it possesses the power of forming this substance from other organic bodies, such, for example, as starch. Plants elaborate fat directly from the minerals—carbonic acid gas, and water.
I have already explained that the growth of plants is, cæteris paribus, directly proportionate to the amount of sunlight to which they are exposed. Not less certainly is the force which constitutes the sun-beam expended in grouping mineral atoms into organic forms, than is the heat which converts water into steam. But in neither case is the force destroyed. When the vaporous steam is condensed into the liquid water, all the heat is restored, and becomes palpable. By the ultimate decomposition of vegetable substances all the force expended on their production is liberated, and, in some form, becomes manifest.
When the fat formed in the mechanisms of plants is decomposed in the animal organism, two results follow:—The atoms of the fat are re-converted to their original mineral, or statical conditions of carbonic acid gas and water; and the force which maintained them in their organic state is set free as heat, and its equivalent, motive power.
One of the most useful instruments which the ingenuity of man has devised, is the Thermometer. It is so familiarly known that I need not describe it. This instrument does not enable us to estimate the actual quantity of heat contained in a substance, but it indicates the proportion of that subtile element which is sensible—that is recognisable by the sense of touch. The dusky Hindu, clad in his single cotton garment, and the Laplander in his suit of fur, are placed under the most opposite conditions in relation to the heat of the sun—the Indian is exposed during the whole year to Sol's most ardent beams, whilst but a scant share of its genial rays goes to warm the body of the Laplander. Now, if we placed the bulb of a thermometer beneath the tongue of a Hindu, we would find the mercury to stand at 98 degrees on Fahrenheit's scale, and if we repeated the experiment on a Laplander, we would obtain an identical result. Numerous experiments of this nature have been made on individuals in most parts of the world, and the results have proved that the temperature of the blood of man is 98 degrees Fahrenheit, whether he be in India or at Nova Zembla, on the steppes of Russia, or the elevated plateaus of America. This invariability[!--6--][6] of the temperature of the bodies of men and of all other warm-blooded animals, appears the more wonderful when it it is considered that the range of the temperature of the medium in which they exist exceeds 200 degrees Fahrenheit. In India, the mercury in the thermometer has been observed to stand at 145 degrees in the direct sunlight, and at 120 degrees in the shade. In high latitudes the temperature is sometimes so low as 100 degrees below zero. A Russian army, in an expedition to China, in 1839, was exposed for several successive days to a temperature of 42 degrees below zero, and suffered severely in consequence.