7th. Cold carried to a great degree acts also upon the cutaneous organ, and produces different effects, according to its intensity. The first of these effects is very analogous to the first effect of a slight degree of caloric. It consists of a kind of local inflammation. The tip of the nose, the ears and the fingers, the cheeks, &c. become red from a slight degree of cold. I have not accurately observed the other effects between this and the last, which consists in a sudden privation of life. But there is this difference between the gangrene that then takes place, and that produced by a high degree of caloric, that the blackness is sudden in this last, whereas it takes place only as a consequence in the other. Observe in fact that there is in gangrene two things which physicians do not sufficiently distinguish, 1st, the mortification of the part; 2d, its putrefaction. The mortification is always antecedent; it is produced by a thousand different causes; sometimes by the ligature of an artery, as in aneurism; sometimes by that of a nerve; often by violent inflammation; sometimes by a contusion, attrition, a bruise, &c. Whenever a part is dead in the midst of those which live, whatever may be the cause of its death, it becomes putrid precisely like a dead body, every part of which life has left. Putrefaction takes place then even sooner, because on the one hand the natural heat of the body, and on the other the moisture of the surrounding parts, favour it remarkably. This putrefaction varies according to the state in which the part was at the instant of death. If much blood infiltrated it, as when inflammation destroys life, it quickly becomes putrid, blackens immediately and allows a fetid sanies to escape; this putrefaction is called moist. If there is but little blood in the part at the instant of death, its putrefaction is less sudden; it first putrifies, then blackens, and allows but little sanies to escape; this is the dry gangrene. Thus in a dead body, if one part is much loaded with blood, as the head of those that have died of apoplexy, its putrefaction is much more rapid and moist than that of the parts in which this fluid is less abundant. In the gangrene which succeeds mortification produced by cold, there is often dryness of the part, because there was but little blood in it at death. How little many physicians know of the progress of nature in the employment of antiseptics, which they apply in the living economy, as upon flesh without life. Antiseptics are applied for one of two purposes, either to prevent the death of the part, or its putrefaction. 1st. If it is with the first intention, antiseptics should be varied. By untying the artery of a limb of an animal that has been tied, you will perform an antiseptic operation. Bleeding and emollient applications which lessen the violence of inflammation in a phlegmon, are antiseptics. A tonic as wine and all stimulants which excite the vital forces in a part in which they are languid after a bruise, are antiseptics. This word is then extremely improper when it is applied to medicines designed to prevent the mortification of the parts. Antiseptics are employed to prevent a dead part in the midst of living ones becoming putrid; some effect is obtained; thus by sprinkling cinchona, muriate of soda, or any neutral salt, by moistening a limb, a portion of skin, the extremity of the nose, &c. which is dead from any cause, the putrefaction will be arrested, as in a dead body upon which the same means are employed. But what will be the result of it? a little less fetor in the surrounding parts and less danger of their receiving the influence of the emanations of the dead part; but it is always necessary that this should come off; antiseptics will never bring it to life. Hence it is evident that these means should be considered in two points of view entirely different. The first prevent mortification, and vary remarkably though they are designed to effect the same object; thus our means of curing retention of urine are very variable, oftentimes opposite, according to the cause which tends to produce this retention. The others prevent putrefaction, without restoring the parts to life; now these are uniformly the same, whatever may have been the cause of the local death.
Action of the Air.
The air acts incessantly upon the cutaneous organ. In the ordinary state, it constantly removes from its surface the sweat that is exhaled from it. Fourcroy, who has paid particular attention to the solution of the transpired fluid by the surrounding air, appears to me to have allowed too much influence to this solution upon transpiration. In fact there are two very distinct things in this function; 1st, the action of the exhalants which throw out the fluid; 2d, the action of the air which dissolves and evaporates it. Now the first of these is wholly independent of the other. Whether the fluid is dissolved or not, more is still furnished by the exhalants. If the solution does not take place, the fluid accumulates upon the skin, which remains moist; but this moisture does not obstruct the exhalant pores and prevent new moisture from being added to it. A comparison will render this very evident. In the natural state, the serous fluids are constantly exhaled and absorbed; the absorbents perform for them the functions of the air which dissolves the sweat; now, though these vessels cease to act, as in dropsies, the exhalants continue their action; there arises only a serous collection, which, though applied to the orifices of the exhalants, does not prevent them from pouring out more serum. The bladder in vain contains urine which presses upon the opening of the ureters, these ducts do not pour less into it. Though the mucous juices become stagnant on their respective surfaces, new juices are however poured upon these surfaces. So though the skin remains moist from the want of solution of the transpiration, more transpiration is nevertheless exhaled. Solution is a physical phenomenon wholly foreign to the vital phenomenon of exhalation. We transpire in a bath as well as in the air; only the fluid which arises from it is mixed with the water, instead of being reduced to vapour.
The moisture of the skin is owing to two causes wholly foreign to each other; 1st, to the increase of the fluid furnished by the cutaneous exhalants; now the action of these exhalants may be increased from three causes. First, every thing which accelerates the motion of the heart, as running, the paroxysm of acute fevers, &c. drives to the skin, as it is commonly expressed. In the second place, every thing which tends to relax and expand the cutaneous organ by a direct action exerted upon it by the surrounding bodies, increases also the action of these exhalants, as in the great heat of summer, as in a bath and after coming out, as in a heated room, &c. In the third place, the action of the skin is in many cases, sympathetically increased. Here may be classed the sweats of phthisis of which the lungs are the source; those of fear, which depend upon a sudden affection of an epigastric organ; those of many acute diseases, &c. Now in all these cases, however active the solution by the air may be, the skin will be constantly moist, because there is thrown out upon it more fluid than the air can dissolve. Thus in catarrhs of the lungs, in which more mucous juices are thrown into the bronchia than the air can remove, it is absolutely necessary that there should be cough and expectoration to carry off the remainder.
2d. There are cases in which the moisture of the skin arises from the solution not being sufficient. This is what takes place in the moisture of the bed in which the air is not changed, in damp weather, &c. There is not then more fluid exhaled; but the ordinary fluid becomes evident, because it is not dissolved. It is under this point of view that we must consider the action of the air upon the cutaneous organ which transpires. It carries off nothing in this organ; it has no real action upon it; it takes only what its vessels throw off. Solution is merely accessory, it is always subsequent to exhalation, and has no relation with it. In the same day, in which the temperature has remained the same, the skin is often dry, moist, humid and even wet with sweat. If the air acts upon transpiration, it is by contracting or relaxing the exhalants, and not by dissolving what they throw out. If the skin formed a sac without an opening, like the serous surfaces, transpiration would go on though it was removed from the contact of the air, the same as if in contact with it. Why in fact should not that take place there, which does upon these surfaces?
If we consider the action of the air upon the skin of the dead body, we see that it produces two different effects, according to the state of the body. If the air penetrates the skin on all sides, it dries it, and it then acquires a sort of transparency, like the fibrous organs, unless a large quantity of blood had been accumulated in it at the moment of death, in which case it becomes black or of a deep brown. Thus dried, 1st, it is firm and resisting, but can be bent in various directions without breaking, as is the case with many textures thus dried, as the cartilaginous, the muscular, &c. &c. 2d. It is much less easily altered than most of the other textures in a dried state. 3d. It absorbs moisture less easily than them, though however when immersed for a long time in water, it finally resumes nearly its original colour and loses its transparency. 4th. It does not exhale a very disagreeable odour, like many of the other textures. Hence why the skins of animals, merely dried, are used in many of the arts; why some barbarous people make use of them for clothing, &c. The aponeuroses, and the mucous, serous and fibrous membranes could not be thus employed. It is to this also that must be attributed the little alteration that takes place in the exterior of mummies, which would never last for ages, if clothed with a fleshy or serous covering.
When the skin is left upon the dead body, or exposed to a moist air, it becomes putrid instead of drying. Then it takes at first a dull colour, then a green and finally a black one. It exhales a very great fetor, swells and thickens, because the gases which are disengaged there fill the cellular texture in its little spaces. A mucous covering is spread upon its external surface, which is deprived of its epidermis. Nothing similar to this covering is seen on the internal surface. Finally, when all the fluids it contains are evaporated, there remains a black residuum, very different from that which is left after combustion.
Action of Water.
This action during life, is relative either to the substances that are deposited on the surface of the skin, or to the cutaneous texture itself.
The sweat deposits incessantly upon the epidermis many substances, the principal of which are taken away by the air, but many being slightly soluble in it, as the salts for example, remain on its surface, and adhere to it when not removed by friction. Mixed with the unctuous fluid which oozes out upon this surface, and with the different foreign particles that the air deposits there as everywhere else, these substances form upon the skin a deposit which cannot, like the transpiration, be carried off by solution. Now water removes all this deposit; hence why the use of baths is truly natural. All quadrupeds bathe themselves. All birds frequently plunge into the water; I do not speak of those for whom this fluid is as it were the element. It is a law imposed upon all species of animals whose skin throws out a considerable quantity of fluid. All the human races hitherto observed frequently plunge into brooks, rivers, or lakes, along which they take up their abode. The countries that are well watered are those which animals prefer. They avoid those where this fluid is wanting, or in which it is only sufficient for their drink. We oppose nature in every thing in society. In our own, numerous classes hardly ever use a bath; thus you must seek especially in these classes for cutaneous diseases. We have seen that the mucous juices, remaining too long upon their surfaces, irritate and stimulate them and cause there various affections. Is it astonishing that the residuum of the cutaneous exhalation which the air does not remove, should occasion various alterations upon the skin? In summer, baths are more necessary, because as many excretions are taking place by the skin, more substances are deposited there. In winter, in which every thing passes off by the urine, the cutaneous surface becomes less dirty, and has less need of being cleansed. After severe diseases, in which there has been copious cutaneous evacuations, one or two baths terminate the treatment advantageously. Let us consider water then as acting as accessory to the air upon the skin, as removing from its surface substances which the first cannot dissolve, substances, which varying remarkably like those that compose the urine, have presented the transpiratory fluids to chemists, sometimes as alkaline, sometimes as acid, oftentimes as containing salts, sometimes charged with odoriferous substances, &c. Water is the general vehicle; when it is evaporated, it leaves the substances that are not volatilized like it. It is on this account that dry frictions are also advantageous; they clean the exterior of the body.