Part 41 (1/2)

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.]