In order to observe the organization of the nails advantageously, it is necessary to take those that are very distinct, as those of the great toe, the thumb, &c. We then see evidently that a single lamina occupies the whole of their convex surface. Behind, this lamina exists alone; hence the extreme thickness of the nails at this place. But as we examine towards the front, we see new laminæ successively added to it, on the concave surface; so that the nail becomes successively thicker. These laminæ can be easily raised up layer by layer. The most anterior are the shortest. They often exhibit upon the concave surface of the nail an infinite number of very evident small striæ, all longitudinal and parallel, and which make us attribute to it a fibrous texture. At other times this arrangement is less evident.
What is the nature of the laminæ which form the nails? I believe that they are almost precisely the same as the epidermis. What proves it is, 1st, that the most superficial is evidently continued with it by its edges; there is no intermediate agent between them. 2d. I have already observed that the nails are detached, and then regenerated exactly like the epidermis. They have two modes of increase; one in length, when we cut their extremities; the other in thickness, when we detach only a lamina, which is soon formed again. When the whole of the nail comes off, all the portion of the dermis which covers the back of the last phalanx, contributes at the same time to form it anew by its external surface. 3d. There is the same obscurity in the vitality of the nails as in that of the epidermis. No trace of animal sensibility is discoverable in them. The excruciating pain that is experienced when they are pulled out arises solely from the sensibility of the subjacent pulpy texture; it is from the same cause as in pulling out the hair. There is no organic sensibility, no internal circulation and consequently no heat inherent in the texture of the nails; thus the horns of animals are nearly of the same degree of temperature as the atmosphere, whilst some external productions with evident vital forces, though raised up like horns, have a temperature equal to that of the body. Such are the combs of the cock of our country, and those which are more striking of the cock-turkey. Compare these excrescences with those on the feet of these animals, which are horny, and the difference of temperature is evident. 4th. The nails give out when burning a disagreeable odour, analogous to that of the epidermis under the same circumstances; they exhibit then the same phenomena. Their combustion is supported, like that of the epidermis, by an oil of which they contain a great quantity. 5th. If maceration and stewing do not produce upon the nails that want of consistence, that kind of brittleness, if I may so express myself, which they produce upon the epidermis, it appears to be owing only to their greater solidity. 6th. The action of the nitric, sulphuric acids, &c. has exhibited to me nearly the same phenomena as upon the epidermis.
Every thing then appears to establish the most exact analogy in the composition, organization and properties of the nails and the epidermis. There is no doubt a difference of principles between them, since the appearance is not the same, and since, though many epidermoid layers may be in juxta position as on the soles of the feet and the palms of the hands, they do not exhibit the form and texture of the nails; so that we cannot consider these as mere layers of epidermis applied to each other. Chemists must ascertain what these differences are, which are certainly very slight. Thus nature often employs indifferently the two organs for the same uses; it is thus that on the sole of the foot of man and many analogous species, there is a thick epidermis; whilst on the feet of animals with hoofs, we see a horny substance of the nature of the human nail.
An evident proof of the slight degree of internal motion which is going on in the epidermis and the nails, of the kind of inertia in which they are in relation to the constant motion of composition and decomposition, which constitutes nutrition, and of the insensibility which they exhibit to various excitants, is the ease with which they are penetrated by different colouring substances, and the very long time they retain them. We know this effect with regard to the nails of dyers. Many savage people who paint the face, various parts of the body, and often even the whole of the external surface of the body, preserve for a long time, without a new coat, the colour which they have artificially given themselves. I have removed the epidermis of a portion of skin of the arm of a dead body, which was coloured blue during life; this colour was not only on the surface of the membrane, but penetrated the whole of it, like a piece of cloth that had been soaked in it. Yet the pores were as evident as before, and the sweat could pass through them; I presume this secretion goes on as usual in savages who paint the skin. Thus the cloth which is immersed in a dye, has not its pores closed by it. I may make use of this comparison, as the epidermis and the nails are really species of inorganic bodies. Lay any organ bare and paint it in this way; the colour, together with the contact of the air will irritate and inflame it, and the suppuration arising from this inflammation will soon throw out the colouring particles, which nutrition would have done, if inflammation had not. There is a means however which can perpetuate the colour, even upon organs, which, very sensible like the skin, are constantly subject to the double nutritive motion; it is that of using the colours with a red hot iron. It is in this way I am convinced that the letters or coloured figures which most soldiers mark upon themselves, with a red hot pin, have their seat not only in the epidermis, but also in the chorion itself.
Development.
The nails have in the fœtus a very considerable consistence, whilst the skin is still pulpy; but their tenuity is then extreme. But they thicken and acquire greater consistence as the fœtus increases in size. They have not at birth a length proportionable to what they are afterwards to have. They do not extend beyond the ends of the fingers which are often much the longest; so that it is not till after birth that they are bent over and exceed the fingers in length, for both of these would be useless in the womb of the mother, as there is nothing there for the fœtus to seize upon. Their transparency allows us evidently to see, at the moment of birth, first the black colour of the blood which before circulated in the arteries, and then the vermilion colour which respiration suddenly imparts to it. As we advance in age, the nails grow in the same proportions as the epidermis, but they have nothing peculiar in their growth. In old people they become extremely thick.
These organs experience during life those diseases only which are analogous to those of the epidermis. These are excrescences, augmentations of size, &c. and other productions, the texture of which is precisely the same as that of the nail, and in which there is neither more sensibility, nor more circulation, nor more heat, nor more life; a remarkable character which distinguishes them from those tumours which arise upon the other organs with very active vitality, as upon the skin, the muscles, &c. tumours the texture of which is very different from that of the organs which have produced them, and which most usually have properties entirely different. But the epidermoid excrescences are in every respect analogous to the epidermis.
PILOUS SYSTEM.
The adjective by which I characterize this system, is derived from the latin substantive which signifies the organs of which it is composed. Hair is found less generally upon man than upon most other animals. It forms upon them a kind of covering external to the skin, which, lessening in part the contact of external bodies, makes the cutaneous animal sensibility perform a less important part, and establishes less numerous relations between these bodies and them. External life is then, in this respect, more limited in them than in man, in whom a delicate epidermis and a few hairs thinly scattered over it, separate the organ of feeling from surrounding objects, the least impression of which is felt, and which, owing to this, keep the animal sensibility in permanent activity; thus man is designed to live more without than within himself. The pleasures of reproduction and digestion constitute exclusively the happiness of animals. That of man is in part the result of them; but an order of pleasures wholly different, purely intellectual and in relation only with external sensations, enlarges immensely by its presence, and contracts by its absence, the field of this happiness.