SECT. VIII.
Of the Skin.
The whole human automaton is covered with a skin, the mechanism of which is not less curious than what we have already described.
It is a texture curiously interwoven with an infinite number of tendons, arteries, veins, nerves, and lymphatic vessels. The tendons form the net-work of the whole, and give it an elasticity, from whence it requires a muscular motion; and the rest add each their parts, as I shall hereafter describe.
The skin, universally over the whole body, is divided into three parts; but in most parts it is found to have four.
The external, is the cuticula, or scarf skin; this is a horny and insensible substance, formed by nature as a shelter for the next part of the skin, which is exceedingly sensible; on examination it appears to be a horny substance, and as it were, little scales continued from the immediate skin, which on any external injury, such as repeated friction, fire, scalding, or blistering, separates from the main skin. The next is the real skin, called cutis; This is strongly interwoven with tendinous and nervous fibres, interspersed with numberless ramifications of arteries, veins, and lymphatic vessels; whence it is not only exquisitely sensible, but elastic, and on the least injury, subject to bleed. On the external surface is a very thin plexus, called the reticulare; which has the tinge of the natural hue of the skin, and distinguishes the complexion of the negro, the tawny, and white, from each other.
The structure of the cutis is peculiarly curious, and may justly be considered as a continued secretory and excretory gland; or more properly, like the bark of a tree; for whilst there is continual transpiration from the body, it, at the same time, is capable to absorb the subtile particles of whatever surrounds it, and exclude the grosser. Over the whole surface are innumerable little pores, which are nothing but the little openings of the various lymphatics, and of the blood vessels corresponding with their peculiar glandules, called the milliary glans; between those on the hairy parts, the hair is planted as it were, with their distinct roots, corresponding with the hue of the subject they belong to. These pores widen or contract according to the internal or external warmth of the body; which when they contract, cause a kind of roughness, or continuation of little warts; and on the contrary, when dilated, smooth the skin. Next to this follows the fat skin, or membrana adiposa; which is a continuation of cells, wherein the fat is contained, and has an immediate communication with the neighbouring blood and lymphatic vessels. This part is not universal, for some parts of the face, the genitals, &c. are without it. The last of all is the membranosa, a thin parchment-like texture, which adheres spontaneously to whatever part it covers, either periosteum, the tendons, ligaments, or muscles.
These are the four principal divisions of the skin; but that which is the principal, is the second mentioned, namely, the cutis; on which I shall have occasion to offer something hereafter, that may prove of the greatest advantage to the sea-faring people, for whose benefit I have particularly intended these Lectures.