The Philosophy of Health; Volume 2 (of 2) / or, an exposition of the physical and mental constitution of man

by Southwood Smith

3. If the body be weighed at different periods, an accurate account being taken of the ingesta and the egesta, it is found to undergo a loss of weight sensibly greater than can be attributed to any of the visible discharges: this loss must be owing to the transmission of a quantity of matter out of the body, under the form of invisible vapour.

872. The matters excreted under the form of perspiration are separated from the blood by a true and proper secretion, like the other secretions of the body. The process by which this is effected is called transudation. The matter of transudation deposited on the surface of the skin by a vital function is removed from the body by evaporation, a physical process which consists of the conversion of a liquid into a vapour by the addition of heat. Consequently the process of perspiration is a cooling process, and it is chiefly by the increase of the perspiration that the body is enabled to bear the intense degrees of heat which it has been shown (491, et seq.) to be capable of sustaining. Sitting one day in repose in the shade during the intense heat of an American summer’s day, the skin freely perspiring at every pore, Dr. Franklin happened to examine the temperature of his body with a thermometer. He found that the temperature of his body was several degrees lower than that of the surrounding air. The physiologists who exposed themselves in heated chambers, for the sake of ascertaining the greatest degree of heat which the human body is capable of enduring, perspired profusely during the experiment ([495]). The artisans who carry on their daily occupations in elevated temperatures perspire most profusely (884, et seq.). Under such circumstances, caloric is communicated to the human body just as freely as to inorganic matter yet it does not injure the body, because it does not accumulate in the system, but is immediately expended in supplying the heat necessary to convert the water, which is poured out upon the skin, into vapour. In this manner that surface of the body at which, under ordinary circumstances, a large portion of its animal heat is generated, is the very surface at which, under extraordinary circumstances, cold is generated, and the heat of the system positively reduced.

873. The physical process of evaporation would go on to a certain extent, though the vital function of transudation did not exist, and does go on in the dead body when the vital function is at an end. An organic tissue enclosing a liquid may not be porous enough to give passage to a single drop of liquid, and yet sufficiently porous to admit air. In this case the air in contact with the tissue dissolves the liquid in its interior, and carries it off in the form of invisible vapour; hence liquids contained in organic bodies in contact with the air diminish in quantity by evaporation. But if an animal be placed in air saturated with moisture, and of the same temperature as its own, the air can no longer deprive that animal of a single particle of its moisture: evaporation from the body, in such a condition of the air, is suppressed. On the other hand, when an animal is placed in air saturated with moisture, and of the same temperature as its own, so far is transudation from being suppressed, that the sweat streams from every part of the external surface of the body. By modifying the condition of the air, in regard to its hygrometrical state and its temperature, the result of the physical process and of the vital function may thus be separated from each other, and the amount of each may be ascertained with perfect exactness. Now, by numerous experiments on the cold-blooded vertebrata, placed under such conditions of the air, it is found that, in these animals, perspiration by evaporation is to that by transudation as 6 to 1. But since the human body presents to the air an immense extent of surface over which is constantly flowing a large proportion of the whole quantity of blood contained in the system, the loss by the physical process compared with that by the vital function must be still greater in man than in the cold-blooded animal.

874. Taking together the average quantity of matter removed from the human body by both processes, or the whole loss of weight sustained from perspiration, on the comparison of the results of many observations, it is estimated to vary from twenty ounces in the twenty-four hours of the colder, to forty ounces in the warmer climates of Europe. Keill estimated it at thirty-one ounces. In the climate of Paris it is stated to be thirty ounces.

875. By the delicate tests of modern chemistry, various substances are found to be contained in the aqueous fluid which constitutes the great proportion of the matter of perspiration, namely, an acid, probably the lactic, a small proportion of animal matter, some alkaline and earthy salts, an oily or fatty substance, probably derived from the sebaceous follicles. All these matters are so analogous to the constituents of the serum of the blood as to leave little ground for doubt that they are merely separated from this part of the blood as it is flowing through the complex net-work of vessels spread over the surface of the cutis ([861]).

876. The skin, when in contact with the air, also separates a portion of carbon from the blood, and to the extent in which it does this it is auxiliary to the lungs; but the quantity of carbonic acid excreted by the skin is small and variable in amount. The primary office of the skin as an organ of excretion is to relieve the blood of its superabundant watery particles, that is, to remove from the system its superfluous hydrogen.

877. A full account has been given (359, et seq.) of the primary office of the lungs, which, it has been shown, is to decarbonize the blood. The details of the calculations have been stated ([457]), from which it is estimated that 10 ounces and 116 grains of carbon are daily exhaled by the lungs under the form of carbonic acid; and the reasons have been assigned which favour the conclusion that the carbonic acid expired is not formed immediately in the lungs by the combination of the oxygen of the atmospheric air with the carbon of the blood; but in the system, where the oxygen taken into the blood at the lungs unites with carbon, the carbonic acid resulting from the combination passing as soon as formed into the capillary veins. The blood contained in these vessels, thus become venous, returns to the lungs, where it gives off the carbonic acid accumulated in it, and by that depuration again assumes its arterial character.

878. Some interesting experiments performed by Dr. Stevens appear to show that there exists a powerful attraction between oxygen and carbonic acid, and that the venous blood, as it is flowing through the lungs, is freed from its carbonic acid by virtue of that attraction. Chemists were so universally agreed that the carbon in carbonic acid is united with its maximum dose of oxygen, that the idea of an attraction between carbonic acid and oxygen appeared highly improbable. The evidence of the fact, however, is decisive. If a receiver, filled with carbonic acid, and closed by a piece of bladder, firmly tied over it, be exposed to the atmospheric air, the carbonic acid, notwithstanding its superior specific gravity, rapidly escapes, and does so without the exchange of an equivalent portion of atmospheric air; the bladder is consequently forcibly depressed into the receiver. If the converse of this experiment be tried, and the receiver, containing atmospheric air, be tied over with a piece of bladder or thin leather, and then be immersed in carbonic acid, this gas will so abundantly penetrate the membrane and enter the receiver as to endanger its bursting.

879. Dr. Stevens had repeated opportunities of verifying these facts, during a stay which he made at Saratoga, in the United States, the springs at which place liberate a large quantity of carbonic acid. In the high rocks it often collects in considerable quantity and purity, and experiments on dogs and rabbits are often made for the entertainment of strangers, as at the Grotto del Cano, near Naples. This rock stands by itself in a low valley, through which there run two currents of water, the one fresh and superficial, the other beneath and charged with salts and carbonic acid. A current of this water rises to some height in a cavity of the high rock, which appears to have been formed by a deposition of earthy salts from the water. It has a conical figure, the base of which is below the surface of the ground, and is about nine feet in diameter. It rises about five feet from the ground, where it is truncated, and presents an aperture a foot in diameter. The water rises in general only about two feet above the ground, and in the three feet above the surface of the water the liberated carbonic acid collects. By luting a large funnel over the aperture, carbonic acid may be collected at the mouth of the funnel in indefinite quantities, of which Dr. Stevens availed himself to multiply and vary his experiments, the result of which appears to be the complete establishment of the fact that there exists a powerful attraction between carbonic acid and oxygen.

880. The application of this fact to the explanation of the phenomena of respiration is highly interesting. By virtue of this mutual attraction, two currents are established, which flow in opposite directions, through the membranous matter of the air-vesicles of the lungs and the pulmonary blood-vessels spread out upon their surface; the oxygen of the air flows to the blood attracted by its carbonic acid, and the carbonic acid of the blood flows to the air attracted by its oxygen. According to Dr. Stevens, the moment the blood parts with its carbonic acid it loses its dark colour, and becomes of a bright vermilion colour, for the following reason: all acids impart a dark colour to the blood. With respect to most acids, this colour remains, although the added acid be afterwards saturated. Carbonic acid forms an exception, for on the removal of this aërial acid the blood resumes its bright and arterial colour. Alkalies, like acids, darken the colour of the blood, but salts produce a bright and vermilion colour when added to the colouring matter of the blood. When the blood loses its carbonic acid, the salts contained in the blood produce upon its colouring matter the vermilion tint natural to the combination when the influence of the salts is not counteracted by the presence of a redundant acid. At the moment the venous blood gives up its carbonic acid it receives in exchange a portion of the inspired air, which is chiefly at the expense of the oxygen. It retains somewhat more oxygen than it yields back in the shape of carbonic acid. The reddened and oxygenated blood, having returned to the heart, is diffused over the system, where it parts with its oxygen and combines with carbon, forming by the union carbonic acid; the necessary result of this combination is the generation of animal heat in the exact proportion to the quantity of the carbonic acid which is produced. The venous blood, which receives the carbonic acid as it is formed in the system, is darkened by its presence, which counteracts the effects of the salts of the blood upon its colouring matter.