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.
SECT. IX.
Of the External Senses.
Feeling is the universal sensation of the nerves, on which all other senses depend, as the subtile spring of life itself. What it is, and whence it proceeds, has already in the foregoing been explained; namely, a sensation of the nerves, by which we perceive an idea of hard or soft, wet or dry, hot or cold, or in general terms, pleasure or pain. The peculiar seat thereof seems to be placed at the ends of our fingers; whether this is the absolute seat ordained by nature, or rendered so by custom, is not my business here to enquire; perhaps, both conjoin to make it there most perfect, as best suited for that purpose.
Taste is the next sensation of the nerves, regarding an immediate approach and action of substances themselves, in order to distinguish betwixt the nature of foods, by which we are enabled to form an idea of salt, sweet, sour, bitter, sharp, or mild; which according to their mixture and proportion, render taste agreeable or disagreeable. Its seat is principally in the tongue; but that it extends to the very stomach itself, is evident, from the immediate consent of parts; namely, that what is disagreeable to the palate, will ever be nauseous to the stomach: Of which I shall say more hereafter.
Smelling is that sensation given to the animal creation, as a friendly messenger of taste, in order to enable us the better to chuse our food. It is actuated by means of the olfactory nerves spread in the pituitary membrane of the nose, and has an immediate communication with the brain; which nervous plexus is actuated by odorous particles, exhaling from substances, by which the olfactory nerves are stimulated agreeably, or disagreeably, as the first intelligence to the palate, or to the whole consent of the nervous system.
Hearing is that sensation of the nerves which is occasioned by the vibration of the air, and forms the sound, which enters the cavity of the ear, and tremulates and puts in action, the organs of hearing. The ear is perhaps as curious a piece of mechanism to our conception, as any part whatever we shall meet with in anatomy.
To the organs of hearing we must reckon the external and internal auditory, divided by a thin membrane, which is a thin spreading of nerves, called the drum; within this, or in the internal cavity, we observe, 1st. Four little bones called, from their forms, the hammer, malleus; the anvil, ancus; the stirrup, stapes; and roundling, orbicularis. 2dly. Their peculiar muscles. And, 3dly. Two considerable cavities, the labyrinth and aqueduct. How hearing is actually brought about, Anatomists have not as yet agreed; and which would be too tedious here to enquire into. That the little hammer is perpetually moving on the tympanum, according to the different sounds, may be one part; which, however, is greatly assisted by the other organs.