Examined chemically, the cuticle is found to be inso luble in water and alcohol, but completely soluble in pure potash and soda, and Irom the circumstance of its re ceiving a deep orange colour on the application of am monia, alter it has been tinged yellow by nitric acid, it is concluded that this membrane is merely a modificatiun of coagulated albumen.
The cuticle of the foetus scarcely differs from that of 5 I. '2 the adult, except in its greater absolute thinness, though, as we have before remarked, even at this early period of the membrane is thickest on the palms of the hands and soles of the feet. As old age advances, the cuticle becomes dry, harsh, and more scaly, than at any former period.
The nails are evidently appendages of the cuticle, as they may be separated by the same means, and appear to be composed of the same substance, though of greater density. They are generally distinguished into three parts, one next the joint of the linger or toe, which we may tall the proxinuil part of the nail, which is firmly attached to the cuticle, and forms about t of the whole length ; a second quite detached lrom the neighbour ing parts, and varying in length and figure, according to circumstances, which we may call the distal part ; and a tuird [RAN\ cell these, constituting by far the great er part of the nail, and covering toe whole sensible ex tremities 01 tic fingers and toes on their anconal and ro tular surlae, s. '11,, proximal part of the nail, is fixed between to o plates on tile cuticle, that on its convex sur face hat Mg a portion of the skin betwixt it and the nail. The middle portion is fixed by its concave surface to the skin of the linger or toe, by a thin fold of the cuticle, which appears to be reflected over the part, and to have in.,inuated itself between the skin and the nail, to the latter of which it serves as a lining.
We have said that the length and form of the dis tal part of the nail varies according to circumstances. If permitted to grow, it gradually extends beyond the extremity of the finger or toe, and if not accidentally broken, it turns over the point, so as to cover it com pletely. Hence it appeal s that the practice of cutting the nails is not merely a point of cleanliness, but is of advantage in preserving the delicacy of touch.
The nails are made up of several layers, the number of which is greatest at the proximal extremity of the nail, and gradually diminishes towards the distal part, where the nail is thinnest. Though the free margin of
the nails is well known to be quite insensible, consider able pain is excited in cutting or tearing the middle por tion. This arises from the close attachment of that por tion to the skin beneath, and not, as some have supposed, from any greater sensibility of the nail itself in that part ; for we cannot believe that this portion of the nail posses ses any innate sensibility, more than the other parts, since no vascular or nervous connection can be traced be tween them. See Bichat's G en. torn. iv. p. 788.
The hair though not derived from the cuticle, may be considered as an appendage to that membrane, as it passes through pores in the cuticle, and receives from it membranous productions. Bichat, at the conclusion of his „inatomie Generale, has given us an elaborate and prolix account of the human hair, which constitutes what he calls systeme pqeux. The first article or this part of his work contains little more than well-known facts respecting the parts of the body on which the hair grows, with remarks on the disposition, form, varieties and colour of the hair of the head, the eyebrows, the eye lashes, and the beard. The three succeeding sections are of more importance, and treat of the organization, the properties, and developcment of the hair. Of these the following is a brief abstract.
The hair appears to originate within the cellular sub stance that lies below the skin, where its roots form 2 sort of bulb, and irons its origin to its emersion beyond the cuticle, each particular hair is invelopcd in a small membranous, transparent canal, of a cylindrical form, quite distinct from the hair itself, but of which the ori gin is unknown. Surrounded by this membranous tube, the hairs pass through the oblique pores of the skin, the rete inufUsuin, and the pores of the cuticle. Preceding writers have supposed that the hairs du not pass through the cuticle, but that they push it outwards, each hair de riving from the cuticle an external covering. This strange supposition is controverted by Bichat, chiefly on the grounds that each hair is of the same diameter within its callal below the cuticle, as when it has emer ged beyond it, and that when this canal is opened at its extremity next the skin, the hair may easily be drawn out, without producing any rupture in the cuticle.