A Treatise on Anatomy, Physiology, and Hygiene Part 41

617. The skin is a membrane which envelops the muscles and other parts of the system. In youth, and in females particularly, it is smooth, soft, and elastic. In middle age, and in males, it is firm and rough to the touch. In old age, in persons who are emaciated, and about the flexions of the joints, it is thrown into folds. The interior of the body, like the exterior, is covered by a skin, which, from the constantly moistened state of its surface, is called the mucous membrane. At the various orifices of the body, the exterior skin is continuous with the internal.

ANATOMY OF THE SKIN.

618. The SKIN, to the naked eye, appears composed of one membrane. But examination has shown that it consists of two layers of membrane, namely, the _Cuti-cle_, (scarf-skin,) and the _Cutis Vera_, (true skin.) These layers are widely different from each other in structure, and perform very different offices in the animal economy.

619. The CUTICLE (sometimes called the _ep-i-dermis_) is the external layer of the skin. This membrane is thin and semi-transparent, and resembles a thin shaving of soft, clear horn, and bears the same relation to other parts of the skin that the rough bark of a tree does to the liber, or living bark. The cuticle has no perceptible nerves or blood-vessels; consequently, if it is cut or abraded, no pain will be felt, and no fluid will ooze from it.

617. What is the skin? Mention its different appearances in its different conditions in the human frame. Is the interior of the body, as well as the exterior, covered by a skin? What is the interior membrane called? Why has it received this name? 618-636. _Give the anatomy of the skin._ 618. What is said of the skin? What is said relative to these layers of membrane? 619. Describe the cuticle. What name is sometimes applied to the cuticle?

_Experiment._ Pa.s.s a pin through the portion of the cuticle that skirts the nails, or remove a thin shaving from the palm of the hand, and no painful sensation will be experienced unless the pin or knife penetrates deeper than the cuticle.

620. This membrane varies in thickness on different parts of the body,--from the thin, delicate skin upon the internal flexions of the joints, to the thickened covering of the soles of the feet. The greater thickness of the cuticle of the palms of the hands and soles of the feet, is manifestly the intentional work of the Creator; for it is perceptible in infants, even at birth, before exercise can have had any influence.

621. The CUTIS VERA (sometimes called the _cori-on_) is composed of minute fibres, which are collected into small bundles or strands.

These are interwoven with each other so as to const.i.tute a firm, strong, and flexible web. In the superficial part of the true skin, the web is so close as to have the appearance of felt-cloth; but more deeply, the pores become progressively larger, and, upon the lower surface, have a diameter of about a line, or one twelfth of an inch.

This gives the under surface the appearance of a coa.r.s.e web. The strands of the under surface of the true skin are connected with the fibrous web, in which the sub-cutaneous fat of the body is deposited; while the upper surface gives support to the sensitive, or papillary layer, which is bedded upon it.

Give the experiment. 620. What is said of the thickness of the cuticle in different parts of the body? 621. Describe the cutis vera. By what name is it sometimes called? What is the appearance of the upper surface of the cutis vera? Of the under surface?

_Observation._ When the skins of animals are immersed in a strong solution of oak or hemlock bark, a chemical union takes place between the gelatin, of which the true skin is mostly composed, and the tannin of the bark. By this process leather is formed, and its peculiar markings are owing to the papillary layer.

[Ill.u.s.tration: Fig. 111. An ideal representation of the papill. 1, 1, The cutis vera. 2, 2, The papillary layer. 3, 3, The arteries of the papill. 4, 4, The veins of the papill. 5, 5, The nerves of the papill.]

622. The sensitive layer of the skin is thin, soft, uneven, pinkish in hue, and composed of blood-vessels, which confer its various tints of red; and of nerves, which give it the faculty of sensation. The unevenness of this layer is produced by small, elongated, conical prominences, called _Pa-pill_.

623. Each PAPILLA is composed of a minute artery, vein, and nerve.

Some of the prominences are arranged in concentric ovals, as may be seen on the ends of the fingers; others are more or less parallel, and pursue a serpentine course; some suddenly diverge, and again reunite, as may be seen in the palm of the hand. Papill are found in every part of the skin. Consequently, their number is very great.

How is leather formed? 622. What is the appearance of the sensitive layer? What causes the unevenness of this layer? Explain fig. 111.

623. Describe the papill.

624. The cutis vera contains not only _Arteries_, _Veins_, and _Nerves_ but _Lymphatics_, _Oil-Glands_ and _Tubes_, and _Perspiratory Glands_ and _Tubes_.

[Ill.u.s.tration: Fig. 112. The arteries and veins of a section of the skin.

A, A, Arterial branches. B, B, Capillary, or hair-like vessels, in which the large branches terminate. C, The venous trunk, collecting the blood from the capillaries.]

625. The ARTERIES AND VEINS of the skin are very numerous. The larger branches of the arteries pa.s.s through the open meshes of the true skin, and are subdivided into a myriad of minute capillary vessels, which form a beautiful net-work on the upper surface of the true skin.

This vascular net sends a branch to each of the papill, which opens into and terminates in a minute vein. The capillary veins are as numerous as the arteries which they accompany. They unite and form larger trunks, as small springs from the hill side coalesce to form rivulets.

624. What vessels are found in the cutis vera? Explain fig. 112. 625.

What is said of the cutaneous arteries? Of the cutaneous veins?

626. The NERVES that are spread over every part of the sensitive layer of the true skin, proceed from the spinal cord. As a proof of the great number of nervous filaments in the skin, no part of this tissue can be punctured with a fine needle without transfixing a nerve, and inducing pain. In some parts of the system, however, the nerves are more abundant than in others; where the sense of feeling is most acute, we find the greatest number of nerves, and those of the largest size. Those parts that are most exposed to injury are most sensitive.

_Examples._ 1st. The conjunctiva, or skin of the eye, is pained by the presence of a particle of dust, because it would render vision imperfect.

2d. The lungs, also, would be injured by the smallest particle of matter; they are therefore protected by the exquisite sensitiveness of the lining membrane of the trachea, so that a particle of food or dust is ejected by a convulsive cough before it reaches the lungs.

627. The nerves are more numerous in the upper than lower extremities; in greater numbers upon the palm than the back of the hand. They are, likewise, more abundant and larger at the extremities of the fingers, and in the lips, than in any other part of the skin.

_Observation._ The proboscis of the elephant, the extremities of the tails of certain species of monkeys, and the tentacula of some kinds of fish, receive a more abundant supply of sensitive nerves than other parts of their systems.

626. Where do the nerves of the skin proceed from? Are they numerous in this membrane? How is it proved? What is said of those parts most exposed to injury? Give example 1st. Example 2d. 627. Mention the difference in the distribution of the nerves in various parts of the body. Is this difference found in the lower order of animals?

628. In the small papill, the nerve forms a single loop, while in papill of larger size, and endowed with a power of more exalted sensation, the nerve is bent several times upon itself previous to completing the loop. These little loops spring from a net-work of nerves, imbedded in the upper porous layer of the true skin, at the base of the papill. This net-work of nerves receives its influence through nerves which take their winding course through the fat distended openings of the deeper layers of the true skin.

[Ill.u.s.tration: Fig. 113. 1, 1, The cuticle. 2, 2, The colored layer of the cuticle. 3, 3, The papillary layer, exhibiting the nerves as they form loops. 4, 4, The net-work of nerves. 5, 5, The true skin. 6, 6, 6, Three nerves that divide to form the net-work (4, 4.) 7, 7, 7, The furrows between the papill. 8, 8, 8, Three papill magnified fifty diameters.]

629. The LYMPHATICS are found in great numbers in the true skin, and they are so minute that they cannot be seen with the naked eye; but when these hair-like vessels are injected with quicksilver, (a work of great difficulty,) the surface injected resembles a sheet of silver.

In this way their existence can be imperfectly demonstrated. They are a part of the vascular net-work situated upon the upper surface of the true skin. Each papilla is supplied with a lymphatic filament, the mouth of which opens beneath, and lies in contact with the under surface of the cuticle. This net-work of vessels communicates through the open meshes of the true skin with larger lymphatic trunks, that open into the venous system.

628. How are the nerves of the small papill arranged? How in the large papill? What does fig. 113 represent? 629. What is said of the cutaneous lymphatics? How is their existence proved?

[Ill.u.s.tration: Fig. 114. A plexus of lymphatic vessels in the skin, considerably magnified from an injected preparation.]

630. The OIL-GLANDS are small bodies imbedded in the true skin. They connect with the surface of the skin by small tubes, which traverse the cuticle. In some parts, these glands are wanting; in others, where their office is most needful, they are abundant, as on the face and nose, the head, the ears, &c. In some parts, these tubes are spiral; in others, straight. These glands offer every shade of complexity, from the simple, straight tube, to a tube divided into numberless ramifications, and const.i.tuting a little rounded tree-like ma.s.s, about the size of a millet seed.

Of what are they a part? 630. Describe the oil-glands. With what do they connect? Do they exist in every part of the body? Of what form are their tubes?

631. In a few situations, these small glands are worthy of particular notice, as in the eyelids, where they possess great elegance of distribution and form, and open by minute pores along the lids; in the ear-pa.s.sages, where they produce that amber-colored substance, known as the _ce-rumen_, (wax of the ears,) and in the scalp, where they resemble small cl.u.s.ters of grapes, and open in pairs into the sheath of the hair, supplying it with a pomatum of Nature's own preparing.