EGESTA IN THE HUMAN BODY. 937 The exact form in which the compounds which are destined to produce animal heat exist in the blood is unknown, but there can be no doubt that the permanently alkaline condition of the blood (which Liebig attributes to trisodic phosphate in the carnivora, and Lehmann to sodic carbonate in the herbivora), is concerned in a very important manner in this oxidating process: for it is uniformly observed that compounds of organic origin are much more readily oxidized in the presence of alkalies than they are when in a neutral or in an acid condition; doubtless because the products of oxidation being commonly acid, the presence of a base with which those acids can combine facilitates the formation of the acid, whether it be carbonic or any other acid compound. (1724) Animal Heat.-The temperature of the human body, whatever may be the temperature of the atmosphere, is uniformly maintained at 98° or 100° (36°.6 or 37°7 C.), that of birds at from 106° to 108° (41° to 42° C.), and that of warm-blooded reptiles at about 85° (29°4 C.). But as the temperature of the air is many degrees, in this climate often 40° or more, below that of the human body, it is clear, since the animal frame is subject to cooling at the same rate as other natural objects, that a constant supply of heat is necessary to the maintenance of this steady temperature. In order to diminish the quantity of heat required to be generated, animals are furnished with coats of hair, of feathers, of wool, and of other light, porous, non-conducting materials, which greatly retard their rate of cooling; and the nitrogen found by analysis in the food taken, and that furnished by the solid and liquid excretions. This mode of calculation necessarily involves these numbers in some uncertainty, since all the errors of experiment fall upon them. It was found that the total quantity of oxygen absorbed amounted to fully one-third of the entire weight of the solid and liquid constituents of the food. The proportion of nitrogen to that of carbon contained in the food taken was as I : 128. The excess of nitrogen in the food over that of the solid and liquid excretions varied from one-third to one-half; and this excess must have passed off through the skin and lungs; but this estimate is probably too high, although it does not attain to much more than 1 per cent. upon the volume of the carbonic anhydride. The proportion of nitrogen found in the solid excreta did not amount to one-fifth of that contained in the urine. The proportion of carbonic anhydride exhaled in winter is much greater than in summer, and the quantity of water is also a little greater in winter. The latter circumstance is explained by the fact, that more air passes through the lungs in winter than in summer, and it is less loaded with moisture on entering them; but in both cases the air quits them nearly saturated with aqueous vapour at the temperature of the body, consequently it will carry off more moisture in the winter than in summer. According to the observations of E. Smith, the respiratory process, cæteris paribus, is most active in spring, and least so in autumn; but the proportion of carbonic acid decreases as the summer advances, and the frequency of respiration and the amount of air inspired are about one-third less in August than in April. thickness of these coats varies in the same animal with the season of the year, and the average temperature to which the animal is exposed. Man supplies himself with clothing, which experience teaches him to adapt to the varying circumstances under which he is placed. This constant evolution of heat in the living body is occasioned by the gradual combustion of the carbon and of the hydrogen supplied in the food; the combustion being effected by the agency of the respired oxygen. All bodies, when they enter into combination with others, whether quickly or slowly, evolve heat, and when the products resulting from combination are the same, the amount of heat which is developed for the same weight of the compound products is also identical (199). Carbon and hydrogen are already combined with each other in the body, and in the act of their combination have already evolved a certain amount of heat; the combustion of a hydrocarbon, therefore, does not produce the same amount of heat as the combustion of an equal quantity of carbon and hydrogen not previously in a state of combination; still a very large quantity of heat is developed during their oxidation, and if this oxidation be effected within the body, the heat thus liberated will necessarily contribute towards maintaining the temperature of the body. According to the experiments of Despretz, 1 ounce of pure carbon during combustion evolves heat enough to raise the temperature of 14,200 times its weight of water through 1o of F., or enough heat to convert into vapour 12.63 times its weight of water at 100°, or the temperature of the body; the heat developed by the combustion of 8.5 ounces of carbon should, therefore, suffice to evaporate 107.7 ounces of water daily, or nearly 675 lb. This quantity of water is more than double the average amount of that which is actually exhaled from the entire body in the twenty-four hours, the average not exceeding, if the air from the lungs be assumed to be saturated, 1lb. from the lungs and 1lb. from the skin.* The surplus heat from the carbon, as well as that from the hydrogen, the amount of which is not so easily estimated, is expended in keeping up the temperature of the body, and in generating the muscular force which is exerted by the individual. Upon the principle of the conservation of force, it follows of necessity that in climbing, the * Dr. E. Smith, however, in his experiments, found that the air which passed from the lungs was never fully charged with moisture. During a working day the average amount of water exhaled by him from the lungs was 5 oz. whilst fasting; under ordinary circumstances it varied from 7:4 to 84 oz. in the same time. DEMAND FOR FOOD VARIES WITH THE TEMPERATURE. 939 calculated quantity of heat expended is less than that which is really evolved, part being actually consumed in the effort of lifting; while in the descent from an elevation the reverse must hold good. Experiments have shown that animals exhale more carbonic anhydride in proportion to their weight, as their temperature is higher. It has been estimated that birds evolve of carbonic anhydride half as much again as the mammalia do; but Regnault finds that, in some cases, the disproportion is much more considerable. The smaller the animal, the larger is the proportionate extent of its cooling surface, and consequently the larger is the quantity of carbon which must be oxidized within it in order to maintain a temperature equal to that of the larger animal; linnets, for instance, evolve 10 times as much carbonic anhydride in proportion to their weight, as the domestic fowl. Small animals consequently consume a much larger quantity of food in proportion to their weight than large animals, and this may account for the greater proportionate activity of many small animals when compared with larger ones. Since the diffusion of animal heat over different parts of the body is tolerably uniform, the source of heat should be diffused likewise. Provision appears to have been made for this essential condition, in the circumstance that the principal action of oxygen upon the constituents of the body takes place in the capillary vessels, which are distributed throughout the whole organized structure; and therefore the heat resulting from this action is also equally distributed through the different parts of the body. A certain amount of chemical action doubtless takes place in the lungs, where the blood first comes into contact with the air, and this increased chemical action would be needed to supply the heat carried off by the vapour which passes off at each expiration; but the main oxidizing actions occur deep in the structures of the body itself. (1725) Demand for Food varies with the Temperature.—Since, then, the combustion of the hydrocarbonous portions of the blood are those best adapted to the production of animal heat, it might be expected that, as the demand for heat varies at different seasons and in different climates, the quantity and quality of the food demanded should also vary accordingly. Experience proves that such is the case; the appetite is keener in winter than in summer, and the aliments which we are then in the habit of consuming partake more of a fatty and carbonaceous character: the inhabitants of the polar regions maintain the necessary supply 940 RELATIONS OF THE LIVER AND LUNGS. of heat by an abundant consumption of blubber and train-oil,* while those who live in tropical climates content themselves with a lighter and more succulent vegetable diet. The liver secretes a fluid rich in carbon, and in certain cases may act as one outlet of hydrocarbonous matters to the system. Respiration in a hot climate takes place with diminished frequency. It must also be remembered, that equal volumes of air admitted to the lungs by respiration at high and at low temperatures contain very different weights of oxygen; air at 40° contains one-tenth more by weight of oxygen than an equal bulk of air at 90°. Less carbon, therefore, is thrown off by the lungs in equal intervals of time in a hot climate than in a cold one, by which means the too rapid production of animal heat is avoided; but the excess of carbon carried into the system must, nevertheless, be got rid of, and this must be effected in a manner that shall not produce heat. Owing to the beautiful compensating system upon which our bodies are constructed, one organ, if necessary, can relieve another from its burden, and the liver carries off this excess of carbon in an unburned form; provided the quantity be only moderate, this vicarious action may be effected without inconvenience, but if it be excessive, enlargement and congestion of the gland ensue, and disease, more or less serious, is the result. In a dry state of the atmosphere, however, less duty falls upon the liver, inasmuch as free evaporation goes on from the surface both of the lungs and of the skin, and thus the temperature of the body is reduced. In dry weather, therefore, carbon may be emitted by the lungs in larger quantity without inconvenience; but when the temperature of the air is high, and at the same time loaded with moisture, this source of relief is cut off, and an oppressive sensation of languor is experienced. There is still a great want of accurate direct experiments on the production of carbonic anhydride under great differences of temperature, and a correct series of observations on this subject, in arctic and in tropical regions, would be highly valuable both to the physician and the physiologist.† * It is, however, difficult to say whence the whale and the seal in their icy climate derive the means of accumulating their stores of blubber and oil. + Upon this point the experiments of Lehmann (Lehrbuch der Physiologischen Chemie, vol. iii. page 304) may be cited. He found that 1000 grammes weight of pigeons yielded in dry air 6055 grammes of carbonic anhydride per hour at 750, and 4'69 grammes at 100°; the same animals in moist air yielded 6769 grammes at 73°, and 7.76 grammes at 100°. And 1000 grammes weight of rabbits exhaled in dry air o ̊451 grammes of carbonic anhydride per hour at 160°, and as much as 0677 grammes in a moist atmosphere at the same temperature. RELATIONS OF FOOD TO ANIMAL HEAT. 941 When the saccharine and oleaginous portions of our food are given in excess, they are thrown off by the liver and the kidneys: Tiedemann and Gmelin found, when they fed animals on butter and starch, that the urine was loaded with fatty matter, and bilious diarrhoea was an almost constant attendant. The azotised principles of the food go to form the azotised principles of the blood, and are necessary for the reproduction of the muscular and other tissues; and hence the importance of a mixed diet for the due support of the vital functions. The nutritive value of different articles of food is, therefore, entirely relative; and is dependent upon the proportion in which each is mingled with other bodies in which the four staminal principles are combined together in due proportion. It must not be overlooked, however, that the most advantageous mixture of nutritive materials will vary with the circumstances under which the animal is placed. In the human species, the milk is adapted to the wants of the new-born infant, which will be wrapped in clothes, and shielded from the weather; and the materials consist of about 10 parts of plastic matter, 10 of fat, 20 of sugar, and o6 of salts. In the case of the cow, where the young animal is more exposed to the vicissitudes of the weather, the proportion of these constituents is altered, the milk containing to every 10 parts of plastic material, 11'11 of fat, and 14 of sugar. The composition of the milk at different periods of lactation also varies in an important manner, in order to meet the altering wants of the young animal in the successive stages of its growth. A few animals are entirely carnivorous; but it must be remembered that all flesh, even that which is usually considered to be lean, contains, in addition to azotised matter, a certain quantity of the oleaginous or fatty principle. The herbivora consume with their food large quantities of saccharine and amylaceous compounds, which, by the abstraction of oxygen, are converted into fat; such animals are capable of being fattened in a remarkable manner, whereas the carnivora seldom acquire any great store of fat, in consequence of a deficient supply of the material best adapted for conversion into that substance. It has been already remarked, that when increased demands are made upon the strength of animals, as in taking exercise, the number of respirations is increased, and the circulation quickened, more oxygen is absorbed, and more carbonic anhydride evolved; and there can be no doubt that this increased demand upon the muscular effort is, for the time, supported by the increased stimulus resulting from the more rapid chemical action upon the blood |