NUTRITION 685 the components of its excretion from antecedent forms which it obtains from the blood. In other words, is the kidney a simple separating or straining mechanism, or are the constituents of the urine, like the constituents of bile or saliva, elaborated in the course of metabolic changes going on within the secreting epithelium ? This question has already been touched in discussing the metabolism of liver-cells. There is little doubt that the kidney exhibits some metabolic activity. Inasmuch as the blood of herbivorous animals contains no trace of liippuric acid, this must presumably be formed within the kidney. If blood containing sodium benzoate and glyco- <"in be passed through the vessels of a fresh kidney, hippuric acid arises. The kidney, therefore, must be assumed to be capable of elaborating hippuric acid out of simpler antecedent forms. But it is not certain that it has any further power. With regard to urea, it appears cer tain that the activity of the kidney is confined to strain ing it off from the blood. Normal blood contains about 1 part of urea in 4000, while the blood of the renal vein contains less than this. Urea, therefore, is separated from the blood in the kidney. If the kidneys be extirpated, or if their blood-vessels be ligatured so as to exclude them from the circulation, or if the ureters be tied (the effect of which is speedily to unfit the kidney-epithelium for the work of excretion), then the amount of urea increases in the blood up to 1 part in 300 or 400, while much urea is voided in the fluids ejected from the stomach and intestines during such experiments. Similar experiments in birds have led to similar results in the case of uric acid. All these facts point to the conclusion that urea and the allied bodies in urine are derived proximately from the blood rather than elaborated in the kidney itself. We are there fore justified in regarding the kidneys as almost exclusively an apparatus for purifying the blood from the injurious pro ducts of the cell-metabolism of other organs and tissues. Excretions of the Sweat-Glands,. The second great ex cretory system is that of the skin, which supplements in important particulars the excretory functions of the kidneys. This function of the skin is effected in great measure by certain glands, called sweat-glands, opening on the sur face of the skin. There are, indeed, other glands besides the sweat-glands connected with the skin, viz., the sebaceous glands, which open chiefly into the sacs of hair -follicles and secrete an oily material which keeps the surface of the skin supple and water-tight. The sebaceous secretion resembles in its formation the secretion of milk. Inasmuch as it is not reabsorbed, it is a true excretion ; but there is reason to believe that the material removed from the blood is elaborated out of complex fat -yielding molecules con tained in the blood very much as the milk is secreted. We know very little either of the nature of the bodies ex creted or of the processes of their formation. The chief excretory products of the skin are furnished by the sweat- glands, and constitute sweat. In addition, however, there is constantly being thrown off from the skin a certain quantity of carbonic acid. Nature of Sweat. It is impossible to collect sweat for analysis under perfectly normal conditions ; either the body must be subjected to great heat to adduce a copious flow, or a part of the body must be enclosed in an air tight bag of india-rubber. In both cases the conditions are abnormal. So far as can be ascertained, sweat is a colourless clear fluid of acid reaction and characteristic odour. The odour varies with the part of the skin from which the sweat is obtained. It consists of water con taining 1 81 per cent, of solids. The solids are (1) sodium chloride and other inorganic salts, (2) urea and other nitrogenous bodies, (3) fats and cholesterin (which are not altogether due to contamination with sebaceous matter), (4) fatty acids (formic, acetic, butyric, &c., but not lactic), (">) a trace of pigment. In addition to these bodies the skin excretes a small amount of carbon dioxide. Although the excretion of the skin is small in amount (if we except the water), if the escape of it be prevented by varnishing the skin death very quickly ensues. This is probably due to the retention of some poisonous substance, the nature and production of which are very little understood. In concluding these remarks on excretory organs it may be pointed out that the lungs (looked at as excretory organs), the kidneys, and the skin are all engaged in the great task of ridding the system of its superfluous matters, and that each supplements the action of the others. The lungs are the great excretors of carbonic acid, which is the chief oxidation product of the body, though they share with the kidneys and skin the task of getting rid of water. The kidneys have thrown upon them the task of removing from the system nearly the whole of the nitrogenous waste products and the superabundant salts, besides being . the greatest excretors of water. The skin, on the other hand, looked upon as an excretory organ, is second in import ance to the kidneys as a remover of water, and comes next to the lungs in separating carbonic acid. The skin, it must be remembered, however, has many functions besides those of an excretory organ, for, besides being an organ of sense, it takes the chief part in regulating the temperature of the animal body. V. TRANSFORMATIONS OF ENERGY ASSOCIATED WITH THE EXCHANGE OF THE MATTERS OF THE BODY. The chemical changes which occur in all the tissues and organs of the body are, it has been stated, in the main, processes of oxidation, in which energy that was potential in the organic compounds and the oxygen that takes part in them become in great part kinetic. This energy takes the form of mechanical work and heat ; the mechanical work is in part expended within the body itself and ultimately takes the form of heat ; in part, however, it is expended upon the objects of the external world, and, though even then ultimately transformed into heat, this is not heat which is available for the purposes of the body. There can be no doubt, however, that a large portion of the total heat evolved in the body is the immediate result of chemical operations, and has not in the first instance taken the form of mechanical work. In the article DIETETICS (q.v.) attention has been drawn to the amount of energy Avhich is stored up in the organic matters constituting the food of animals, and which can approximately be estimated by determining the amount of heat which the organic matters evolve when burned in a calorimeter. An approximate estimate is thus formed of the energy which is at the disposal of the animal or man whose diet is subjected to study. Thus it has been calculated that the available energy derived from the oxida tion of the organic matters of the food of a well-fed man amounts to about 2,700,000 units of heat, the unit chosen being the amount required to heat 1 gramme of water 1 C. When the man is doing no external work this energy is dis sipated from the body almost entirely as heat, and, accord ing to the calculations of Helmholtz, the losses of heat are approximately distributed as follows : ,, ., , Percentage of Employed in raising the temperature of H efc ^JdS! matters introduced into the alimentary canal 70,157 Employed in warming inspired air 70, 032 Employed in bringing about the vaporiza tion of water in the lungs 397,536 147 Lost by radiation, conduction, and evapora tion from the skin 2,162,275 801 2,700,000 100-0
