[20] This is altogether inaccurate; a nail in growing is not nourished, any more than the mucus is nourished in the nasal fossæ, or the urine in the bladder. The nails, the hair on the various parts of the body and the hair of the head, all in a word epidermoid productions, are the result of real secretions which do not differ from the secretions of which we have just spoken, only in this, that the product instead of remaining fluid like the urine, or viscid like the mucus, hardens as it comes out of the secretory organ, like the thread of the silk worm, or that of the spider. A certain number of these organs is commonly arranged in such a manner, that the matter secreted by each of them is found in a fluid state in contact with that of the neighbouring organs, with which it is agglomerated in hardening. Arranged in concentric circles around a small cone, they produce a hollow cylinder; extended in parallel lines upon a broad surface, they produce a flattened lamina. Such is the manner in which the nails and the hair are formed. We see from this that the epidermoid productions grow, but are not nourished. The hair exhibits, it is true, an internal cavity, filled with a coloured fluid, which appears to be necessary for its preservation; but we can easily conceive how an oily fluid may help to preserve it, by giving it suppleness and thus preventing it from breaking. This fluid is poured into the canal in which it is found, and it is not the hair which draws it in, any more at least, than a capillary tube draws in the fluid into which its extremity is plunged. [21] The idea of endowing each texture with a peculiar kind of sensibility in relation with its uses is one which pleases the imagination. The ligaments are designed to oppose the separation of the bones; they should remain insensible to every kind of stimulus that does not tend to disunite these parts, and pain consequently, should not be produced but from distension or twisting. Unfortunately this supposition is not well founded, the facts on which it rests were not accurately observed. It is very true that in twisting these ligaments, the animal almost always cries out, but it is because we at the same time stretch some neighbouring parts endowed with sensibility. When this is prevented and the experiment is made with proper precaution, we can twist, distend or tear the ligament, without appearing to give the animal any pain. [22] So, as long as the fluid is retained in the artery, which is easily done by means of ligatures, no pain is manifested; but when the irritating substance is carried by the vessels to the heart or to any other sensible part, we can easily conceive that the animal must experience pain, for the irritant always produces its effect, whether it be carried directly to the part or arrive there by means of the circulation.