All the fingers have at their extremity, on the outer side or that of extension, hard, transparent and elastic layers, of the nature of the horns of many animals, and which are called nails.
I. Forms, Extent, Relations, &c.
The nails of man differ from those of most other animals, in their breadth and want of thickness. The first makes them better adapted to support the extremity of the fingers, which is broader than in most animals for the perfection of touch; the second renders them less fit to serve for defence or as a means of aggression.
Most people cut their nails even with their fingers, so that the length of these bodies which is seen is not what is natural to them. When allowed to grow, they lengthen and turn over on the side of flexion, and cover entirely the lower end of the fingers. This growth has a certain limit which the nail cannot pass, and which it attains when it exhibits at its extremity a cutting and sharp edge. As long as this edge has the appearance of having a part cut off, the nail continues to grow.
We usually think that the habit of cutting our nails is a thing of mere decorum. But if we reflect a little upon society and the numerous arts to which it gives rise, upon the perfection, delicacy, precision and rapidity of the motions which the fingers are often forced to execute, upon the necessity of approximating them, crossing them in a thousand different ways, &c. we shall soon see that this habit is almost inevitably the result of the social state, and what appears to us the effect of fashion is that of necessity. The sense of touch in man in a natural state is coarse and obscure; it is only necessary that he should seize objects destined for his nourishment, his defence, his aggressions, &c. that he should climb especially and attach himself to trees to keep himself upon them; now his nails are for this purpose of great use. What he loses in this respect in society, he seems to gain by the precision and extent which are added to his touch, and by the faculty which the fingers acquire of distinguishing the most delicate tangible qualities. In the first state, his hands are of great assistance to him in locomotion. In the second, they contribute hardly at all to this use, and they gain in the partial motions of their fingers what they lose in their motions as a whole, which become of less urgent necessity.
The nail has three distinct parts in the natural state; one posterior, concealed on both sides by the integuments; another middle, free only on one side, and the third anterior, without adhesion at either side.
The posterior portion of the nail is nearly a sixth part of its extent. Its convex surface adheres very intimately to the epidermis, which goes in the following manner to fix it. After having covered over the portion of the finger corresponding with flexion, it is reflected upon the concave edge where the skin terminates and where the nail begins to become external; it commonly forms all around this edge a kind of small string that is very distinct and has a small groove in the top of it, and which is evidently composed entirely of epidermis, since we can cut the whole of it without giving any pain, and which is afterwards easily reproduced. After having formed this string, which is in the form of a parabola, the epidermis is again reflected, passes between the skin and the nail, and is glued, if we may so say, to the concave surface of the latter, without being intermixed with it; for we can remove it with ease by scraping with a scalpel. So that the dermis which covers the superior portion is really between two layers of epidermis. After having thus fixed the nail, and having arrived at its posterior edge, the epidermis is continued and identified as it were with this edge, whose evident delicacy and softness approximate it in nature to this membranous layer. Hence it follows that without the adhesion of the epidermis to the nail, there would be between them, a kind of cul-de-sac. Some authors have thought that the extensor tendon is extended as far; but it is easy to see that it does not go beyond the tubercle which terminates behind the phalanx. The nail does not reach this tubercle, there is a space of three lines between them. The concave surface of the posterior portion of the nail corresponds with the same substance as the middle portion.
This middle portion is bare on its convex surface, which is smooth, whitish behind where this colour forms a kind of half moon, reddish in the greater part of its extent, a colour which is foreign to it and which it derives from the subjacent texture. Upon the sides, the skin covers this surface a little, and terminates afterwards by continuing the concave and free edge of which we have spoken. The epidermis forms also in this place a small string which is continued on each side with that pointed out above; then it unites to the nail and adheres to its lateral edges with which it is identified. The concave surface of this middle portion is fixed in front by the epidermis, which, after having covered the extremity of the fingers, and having arrived at the place where the nail ceases to be free, is separated from the dermis, and adheres to the whole length of the nail in a curved line; then by mixing with it, it seems to form its internal lamina. The dermis on the contrary is continued on the convexity of the last phalanx, has there a remarkable consistence, a reddish appearance, and a texture like pulp and wholly different from what is observed elsewhere; more vessels run through it; there is no distinct space in it, and no elongation goes from it to the surface of the nail of which the epidermis forms a part. We do not see on this surface, as on those of the other parts of the epidermis, those threads, which are the remains of the broken exhalants and of which we have spoken; thus the sweat never passes through the nail. There is neither any oily oozing upon its surface; whence it follows that water is not formed into little drops on the exterior of its horny laminæ. Hence the nail is evidently insulated from all the other organs except the epidermis, with which it is continued on its concave face and especially on its posterior and lateral edges. Thus observe that when collections of pus or other affections have broken this continuity behind or on the sides, the whole of the nail, though unaffected in the middle falls off.
The free or anterior portion of the nail is of a length which it is difficult to determine. I have never seen it allowed to take its natural growth. I have only observed that if it is permitted to grow to a considerable size, we see evidently that it has a greater thickness than the posterior and middle portion. In general, the thickness, resistance and hardness of the nail increase in a gradual manner from the posterior to the anterior part; we shall now examine to what this is owing.