_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.
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