difficulties stand in the way of obtaining minutely accurate results, and the difference in different constitutions, and in the same person at different times, is so great, that we must be satisfied with an approximation to the truth. Sanctorius, who carefully weighed himself, his food, and his excretions, in a balance every day for thirty years, came to the conclusion that five out of every eight pounds of substances taken into the system passed out of it again by the skin, leaving only three to pass off by the bowels, the lungs, and the kidneys. This is certainly an exaggeration, but Lavoisier and Seguin, who conducted their experiments with still more caution against error, estimate the highest amount at thirty-two grains per minute, or five pounds a-day, and the smallest amount at eleven grains per minute, or one pound eleven ounces and four drachms per day; while the greater number of observers agree that the cutaneous exhalation is more abundant than the united excretions of both bowels and kidneys; and that, according as the weather becomes warmer or colder, the skin and kidneys alternate in the proportions of work which they severally perform; most passing off by the skin in warm weather, and by the kidneys in cold, and vice versa. The quantity exhaled increases, after meals, during sleep, in dry warm weather, and by friction or whatever stimulates the skin; and diminishes when digestion is impaired, and in a moist atmosphere. According to Thenard, the cutaneous exhalation is composed of a large quantity of water, a small portion of acetic acid, of muriate of soda and potass, of an earthy phosphate, a little oxide of iron, and some animal matter; but Berzelius considers the acid as lactic, and not the acetic. There is besides some carbonic acid and oily matter excreted. But the composition of the perspiration probably varies both at different ages and on different parts of the skin, as is presumable from the peculiarity of odour which it exhales in some situations. The arm-pits, the groins, the forehead, the hands and the feet, perspire most readily, in consequence of their receiving a proportionally larger supply of blood. Every thing tends to show that perspiration is a direct product of a vital process, and not a mere exudation of watery particles through the pores of the skin.

Taking even the lowest estimate of Lavoisier, we find the skin endowed with the important charge of removing from the system nearly two pounds of waste matter every twenty-four hours; and when we consider that the quantity not only is great, but is sent forth in so divided a state as to be invisible to the eye, and that the whole of it is given out by the very minute ramifications of the blood vessels of the skin, we perceive at once why these are so extremely numerous and fine that a pin's point cannot touch any spot without piercing them; and we see an ample reason why checked perspiration should prove so de

trimental to health, because for every twenty-four hours that such a state continues, we must either have two pounds of useless and hurtful matter accumulating in the body, or have some of the other organs of excretion over-tasked to an equal amount, which obviously cannot happen without disturbing their regularity and wellbeing. People know the fact, and wonder that it should be so, that cold applied to the skin, or continued exposure in a cold day, often produces bowel complaint, a severe cold on the chest, or inflammation of some internal organ. But were they taught, as they ought to be, the structure and uses of their own bodies, they would rather wonder that it did not always produce one of these effects. It is remarked, for example, that such organs as belong to the same class of functions sympathise closely with each other. Thus the skin, the bowels, the lungs, the liver, and the kidneys, have all the common object of throwing waste and injurious matter out of the system, each in a way peculiar to its own structure. Now, if we suppose the exhalation from the skin to be stopped by long exposure to cold, the large quantity of waste which it was charged to excrete, being in itself hurtful to the system, will be thrown upon the other excreting organs, in addition to their regular amount of work; and if any of them, from constitutional or accidental causes, be already weaker than the rest, as often happens, it is quite natural to expect that its health will suffer from the extra demand made upon it. In this way the bowels become irritated in one individual, and give rise to bowel complaint; while in another it is the lungs which suffer, giving rise to catarrh or common cold, or perhaps even to inflammation. If, on the other hand, all these organs are in a state of vigorous health, a temporary increase of function takes place in them, and relieves the system, without leading to any local disorder, and the skin itself speedily resumes its activity, and restores the balance between them.

One of the most obvious illustrations of this reciprocity of action is afforded by any convivial company seated in a warm room in a cold evening. The heat of the room, the food and wine, and the excitement of the moment, stimulate the action of the skin, cause an efflux of blood to its surface, and increase in a high degree the flow of the insensible perspiration, which thus, while the heat continues, carries off an undue share of the fluids of the body, and leaves the kidneys almost at rest. But the moment the company goes into the cold external air, a sudden reversal of operations takes place; the cold chills the surface, stops the perspiration, and drives the blood inwards upon the internal organs, which presently become excited,—and, under this excitation, the kidneys, for example, will in a few minutes secrete as much of their peculiar fluid, as they did in as many of the pre

ceding hours. The reverse of this, again, is common in diseases obstructing the secretion from the kidneys, for the perspiration from the skin is then altered in quantity and quality, and acquires much of the peculiar smell of the urinary fluid.

When the lungs are the weak parts, and their lining membrane is habitually relaxed, accompanied with an unusual extent of mucous secretion from its surface, cold applied to the skin throws the mass of the blood previously circulating there inwards upon the lungs, and increases that secretion to a high degree; but this secretion, were it to accumulate, would soon fill up the air-cells of the lungs, and cause suffocation. To obviate this danger, the Creator has so constituted the lungs, that any foreign body coming in contact with them excites the convulsive effort called coughing, by which a violent and rapid expiration takes place, with a force sufficient to hurry the foreign body along with it, just as we see boys discharging peas with much force through short tubes by a sudden effort of blowing. A check given to perspiration, thus naturally leads very often to increased expectoration and cough, or, in other words, to common cold; because the matter which ought to have gone off by the skin, must find some outlet, and its presence disorders those organs most which are naturally weakest.

The lungs excrete, as we shall afterwards see, a large proportion of waste materials from the system; and the kidneys, the liver, and the bowels, have in so far a similar office. In consequence of this alliance with the functions of the skin, these parts are more intimately connected with each other in healthy and diseased action than with other organs. But it is a general law, that wherever an organ is unusually delicate, it will be more easily affected by any cause of disease than those which are sound. So that, if the nervous system, for example, be weaker than other parts, a chill will be more likely to disturb its health than that of the lungs, which are supposed in this instance to be constitutionally stronger; or, if the muscular and fibrous organizations be unusually susceptible of disturbance, either from previous illness or natural predisposition, they will be the first to suffer, and rheumatism will ensue; and so on. And hence the utility to the physician of an intimate acquaintance with the previous habits and constitutions of his patients, and the advantage of adapting the remedies to the nature of the cause, when it can be discovered, as well as to the disease itself. A bowel complaint, for instance, may arise from overeating as well as from a check to perspiration; but although the thing to be cured is the same, the means of cure ought obviously to be different. In the one instance, an emetic or laxative to carry off the offending cause, and in the other a diaphoretic to open the skin, will be the most rational and efficacious remedies; and

hence, too, become apparent the glaring ignorance and effrontery of the quack, who affirms that his one remedy will cure every form of disease. Were the public not equally ignorant with himself, their credulity would cease to afford his presumption the rich field in which it now revels.

It is in consequence of this sympathy and reciprocity of action between the skin and the internal organs, that burns and even scalds of no very great extent prove fatal by inducing internal, generally intestinal, inflammation. By disordering or disorganizing a large nervous and exhaling surface, an extensive burn causes not only a violent nervous commotion, but a continued partial suspension of an important excretion; and when death ensues at some distance of time, it is almost always in consequence of inflammation being excited in the bowels or sympathizing organ. So intimate, indeed, is this connexion, that some surgeons of great experience, such as Baron Dupuytren of the Hotel Dieu, while they point to internal inflammation as in such cases the general cause of death, doubt whether recovery ever takes place, when more than one-eighth of the surface of the body is severely burnt; and whether this estimate be correct or not, the facts from which it is drawn clearly demonstrate the importance of the relation subsisting betwixt the skin and the other excreting organs.

It

The Skin a regulator of Animal Heat.-One very important use of the cutaneous perspiration, we have seen to be the removal from the system of a great quantity of waste materials, which would prove noxious if they remained. The Creator has, in his omniscience and foresight, and with that regard to simplicity of means which betokens a profoundness of thought inconceivable to us, superadded to this another purpose scarcely less important, and which is in some degree implied in the former; I mean the proper regulation of the bodily heat. is well known that, in the polar regions, and in the torrid zone under every variety of circumstances, the human body retains nearly the same temperature, however different that of the air may be by which it is surrounded. This is a property peculiar to life, and, in consequence of it, even vegetables have a power of modifying their own temperature, though in a much more limited degree. Without this power of adaptation, it is obvious that man must have been chained for life to the climate which gave him birth, and even then have suffered constantly from the change of seasons; whereas, by possessing it, he can enjoy life in a temperature sufficiently cold to freeze mercury, and, for a time, sustain unharmed a heat more than sufficient to boil water, or even to bake meat. Witness the wintering of Captain Parry and his companions in the Polar Regions; and the experiments of Blagden, Sir Joseph Bankes, and others, who remained for

many minutes in a room heated to 260°, or 50° above the temperature of boiling water. The chief agents in this wonderful adaptation of man to his external situation, are undoubtedly the skin and the lungs, and in both the power is intimately connected with the condition of their respective exhalations; but it is of the skin alone, as an agent in reducing animal heat, that we have at present to speak.

When the body is exposed to a warm air and powerful sun, or, as in many trades, to the intense heat of a furnace, or when it is engaged in severe exercise, an increased production of heat takes place, which would soon kill the man were it not as rapidly carried off, and an equilibrium thus kept up. Attention to the order of events affords the requisite knowledge of the means employed for this purpose. At first the body is actually felt to be warmer, the skin becomes dry and hot, and the unpleasant sensation of heat is soon at its maximum. By-and-by a slight moisture is perceived on the skin, followed by an immediate increase of comfort. In a short time afterwards this moisture passes into free and copious perspiration, moistening and bedewing the skin in the shape of sweat; and if the heat or exertion is still kept up, the sweat becomes profuse, and drops from the body or wets the clothes which envelope it. A decrease of animal heat unavoidably accompanies this, because, independently of any vital action contributing to this effect, as is most probable, the mere physical evaporation of so much fluid is itself sufficient to take up and carry off a large quantity of caloric. On this latter account, the evaporation of water from a rough porous surface is constantly resorted to in the East and West Indies, and other warm countries, as an efficacious means of reducing the temperature of the air in rooms, and of wine and other drinks, much below that of the surrounding atmosphere. Franklin was, we believe, the first to point out the analogous use of the perspiration in reducing the heat of the body; and that he has not erred in ascribing this effect to it, must be evident to every one who chooses to make a very simple experiment: For, if we cover the bulb of a thermometer with a piece of wet rag, and swing it rapidly round in a dry air, we shall presently see the mercury fall, and indicate a reduction of temperature proportioned to the extent of surface exposed, and the rapidity with which the evaporation is accomplished. Keeping this in view, and setting aside (because it may be disputed) all reduction of temperature arising from heat becoming latent in the passage of the fluid perspiration into the state of vapour, we shall still find evaporation a perfectly adequate provision for the gradual and safe dispersion of the superfluous animal heat. The amount of fluid evaporated from the skin during profuse sweat, cannot be less than double of what is carried off by the lowest insensible