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.