As to the action of the bath upon the cutaneous texture, we know but little of it during life. They say in medicine that it softens, relaxes and unbends this texture; this is vague language to which no precise meaning is attached, and which is no doubt borrowed from the relaxation which the skin of dead bodies undergoes, or even tanned leather, when exposed to water. A bath acts upon the vital forces of the skin, raises or diminishes them, as I shall say; but it leaves its texture in the same state; it is only that of the epidermis which it alters, as we shall see.
Macerated in water of a moderate degree of temperature, in that of cellars for example which does not vary, the human skin softens, swells but little, becomes evidently whiter, and remains for a long time without experiencing any other alteration than that of a putrefaction infinitely less than that of the muscular, glandular, mucous textures, &c. subjected to the same experiment. This putrefaction, which removes the epidermis, appears to be much greater on the side nearest to this membrane; at the end of two or three months the skin loses but little of its consistence. It does not become pulpy as the tendons and muscles in this length of time when macerated; it does not become a fetid pulp till the end of three or four months. I have preserved some of it for eight months, which has still its primitive form, but which feels liquid under the fingers when pressed a little. In the half putrid state, the skin still preserves the faculty of crisping from the action of caloric; it moves about when placed on burning coals, or when plunged into boiling water. When once reduced to a really putrid state it loses this property.
Exposed to ebullition, the dermoid texture when well separated from the cellular, furnishes less scum than the muscular, the glandular and the mucous; it resembles in this respect the tendons, no doubt because being almost wholly gelatinous, it contains but little albumen. In the horny hardening that takes place a little before ebullition commences, it twists and then always becomes convex on the side of the epidermis, and concave on the opposite side; and for this reason; the fibres of the chorion in contracting by the horny hardening, are pressed against each other; all the spaces which exist between them are effaced; now, as these spaces are very large in the second direction, the dermoid texture necessarily becomes more contracted there, whilst in the first, the spaces hardly existing at all, every thing being almost solid, the fibres have less space to contract, they remain longer, and the surface continues larger. In the natural state the cavity of these spaces, being filled with cellular texture, increases the extent of the internal surface; this space then disappearing, this surface becomes contracted.
The moment this kind of twisting takes place upon the skin, it is covered, as I have said, with an infinite number of vesicles filled with serum, and which are formed by the epidermis. As this membrane is very thick on the soles of the feet, and the palms of the hands, it cannot contribute in those places to their formation, and we see nothing there similar to them. Yet by removing it from feet that have been boiled, I have observed that it contained between its layers many small vesicles, which were scarcely visible. I have not analyzed the water of these vesicles, but presume it is analogous to that of blisters. Besides a greater or less quantity of it is poured out, and the vesicles are consequently larger or smaller, according to the state of the external capillary system at the instant of death.
By the horny hardening, the skin becomes hard, elastic, very resisting, thicker, but not so broad. It soon loses its semi-transparency and yellowish colour, like the boiled fibrous organs. Then the hardness it had acquired at the instant of the horny hardening is gradually lost; it softens, gives out much gelatine in the water in which it is boiled, does not lessen in size, but even increases in thickness. Every kind of fibre, vacant space and organization is then gone; it is a membranous mass, homogeneous in appearance, semi-transparent and gelatinous. In this soft state, it does not lose the elasticity it had acquired in the horny hardening, like the mucous, serous, cellular textures, &c. &c. The great quantity of gelatine it contains still preserves this property in it. The least motion that is communicated to it produces a general trembling, a sort of vibration of all its parts, exactly analogous to that of the various kinds of animal jellies, half coagulated, which vacillate in the vessel from the least jar.
Finally, the ebullition still continuing, the gelatine is almost all dissolved, and there remains only a residuum like membrane and which disappears with great difficulty; it requires even a very long time for common boiling water to reduce the skin to this residuum. Such are the phenomena of the ebullition of the human skin as I have carefully observed them. Chemists have paid great attention to the dermoid texture of many other animals; they have formed different ideas of its nature; they have admitted that there are two substances in it; one fibrous and the other gelatinous. I refer to their works upon this point, particularly to the labours of Seguin, and the work of Fourcroy; for in general I do not relate what is detailed by others, it would be only a useless repetition.
Action of the Acids, the Alkalies and other Substances.
The sulphuric, nitric and muriatic acids act upon the skin, when in contact with it, as upon all the other animal substances. I have remarked however that their action is much slower, especially on the side of the epidermis, though this membrane may have been previously taken off. The first of these acids reduces it easily to a blackish pulp; the others bring it to a pulpy state with more difficulty, even when they are very little weakened; the oxy-muriatic acid produces hardly any effect upon it.
Some authors have said that the lapis infernalis produces the same phenomena on the dead as on the living body. I wrapped up in a piece of skin, as in a rag, many fragments of this substance, so that they were in contact with the epidermis; at the end of a day they were reduced to a kind of pap of a yellowish red, by the moisture which they had absorbed. The dermoid texture, crisped and contracted, had not been penetrated; it did not appear even to have been injured on the exterior. In general the action of the alkalies appears to be wholly different during life, and it varies even according to the different degrees of vitality. We know that flaccid and fungous flesh burns much less easily than that which is red and vigorous. It is the same with the acids. Never during life do they produce any thing analogous to that pulp of different colours according to the acids that are employed, which is always after death the result of their action.
We know that an alkaline solution, put in contact with the skin, produces a kind of unctuous and slippery feel, which is no doubt owing to the combination of the alkali with the oily deposit of the skin, from which arises a sort of soap.