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. The Skin, the external investment of the body, is a most complex and important structure, serving (1) as a protective covering ; (2) an organ for tactile sensibility ; (3) an organ for the elimination of excrementitious matters.
The Amount of Skin investing the body of a man of average size is about twenty feet, and varies in thickness, in different situations, from the f to the dy of an inch.
The skin consists of two principal layers, viz., a deeper portion, the Corium, and a superficial portion, the Epidermis.
The Corium, or Cutis Vera, may be subdivided into a reticulated and a papillary layer. The former is composed of white fibrous tissue, nonstriated muscular fibers, and elastic tissue, interwoven in every direction, forming an areolar network, in the meshes of which are deposited masses of fat, and a structureless amorphous matter; the latter is formed mainly of club-shaped elevations or projections of the amorphous matter, constituting the papillæ ; they are most abundant, and well developed, upon the palms of the hands and the soles of the feet; they average the ido of an inch in length, and may be simple or compound; they are well supplied with nerves, blood vessels, and lymphatics.
The Epidermis or Scarf Skin is an extra-vascular structure, a product of the true skin, and composed of several layers of cells. It may be divided into two layers, the rete mucosum or the Malpighian layer, and the horny or corneous.
The former closely applies itself to the papillary layer of the true skin, and is composed of large, nucleated cells, the lowest layer of which, the “prickle cells,” contain pigment granules, which give to the skin its varying tints in different individuals and in different races of men ; the more super
; ficial cells are large, colorless, and semi-transparent. The latter, the corneous layer, is composed of flattened cells, which, from their exposure to the atmosphere, are hard and horny in texture ; it varies in thickness from Ys of an inch on the palms of the hands and feet, to the goo of an inch in the external auditory canal.
APPENDAGES OF THE SKIN. Hairs are found in almost all portions of the body, and can be divided into (1) long, soft hairs, on the head ; (2) short, stiff hairs, along the edges of the eyelids and nostrils; (3) soft, downy hairs, on the general cutaneous
surface. They consist of a root and a shaft, which is oval in shape, and about the of an inch in diameter; it consists of fibrous tissue, covered externally by a layer of imbricated cells, and internally by cells containing granular and pigment material.
The Root of the hair is embedded in the hair follicle, formed by a tubular depression of the skin, extending nearly through to the subcutaneous tissue ; its walls are formed by the layers of the corium, covered by epidermic cells. At the bottom of the follicle is a papillary projection of amorphous matter, corresponding to a papilla of the true skin, containing blood-vessels and nerves, upon which the hair root rests. The investments of the hair roots are formed of epithelial cells, constituting the internal and external root sheaths.
The hair protects the head from the heat of the sun and cold, retains the heat of the body, prevents the entrance of foreign matter into the lungs, nose, ears, etc.
The color is due to the pigment matter, which, in old age, becomes more or less whitened.
The Sebaceous Glands, imbedded in the true skin, are simple and compound racemose glands, opening, by a common excretory duct, upon the surface of the epidermis or into the hair follicle. They are found in all portions of the body, most abundantly in the face, and are formed by a delicate, structureless membrane, lined by flattened polyhedral cells. The sebaceous glands secrete a peculiar oily matter, the sebum, by which the skin is lubricated and the hairs softened; it is quite abundant in the region of the nose and forehead, which often present a greasy, glistening appearance; it consists of water, mineral salts, fatty globules, and epithelial cells.
The vernix caseosa which frequently covers the surface of the fetus at birth consists of the residue of the sebaceous matters, containing epithelial cells and fatty matters ; it seems to keep the skin soft and supple, and guards it from the effects of the long-continued action of water.
The Sudoriparous Glands excrete the sweat; they consist of a mass or coil of a tubular gland duct, situated in the derma and in the subcutaneous tissue ; average the 15 of an inch in diameter, and are surrounded by a rich plexus of capillary blood vessels. From this coil the duct passes in a straight direction up through the skin to the epidermis, where it makes a few spiral turns and opens obliquely upon the surface. The sweat glands consist of a delicate homogeneous membrane lined by epithelial cells, whose function is to extract from the blood the elements existing in the perspiration.
The glands are very abundant all over the cutaneous surface, as many as 3528 to the square inch, according to Erasmus Wilson.
The Perspiration is an excrementitious fluid, clear, colorless, almost odorless, slightly acid in reaction, with a specific gravity of 1.003 or 1.004.
The total quantity of perspiration excreted daily has been estimated at about two pounds, though the amount varies with the nature of the food and drink, exercise, external temperature, season, etc.
The elimination of the sweat is not intermittent, but continuous; but it takes place so gradually that as fast as it is formed it passes off by evaporation as insensible perspiration. Under exposure to great heat and exercise the evaporation is not sufficiently rapid, and it appears as sensible perspiration.
COMPOSITION OF SWEAT.
0.043 Fatty matters,
0.014 Alkaline lactates,
0.317 Alkaline sudorates,
1.562 Inorganic salts,
1000.00 Urea is a constant ingredient.
Carbonic acid is also exhaled from the skin, the amount being about zoo of that from the lungs.
Perspiration regulates the temperature, and removes waste matters from the blood; it is so important, that if elimination be prevented death occurs in a short time.
Influence of the Nervous System.—The secretion of sweat is regulated by the nervous system. Here, as in the secreting glands, the fluid is formed from material in the lymph spaces surrounding the gland. Two sets of nerves are concerned, viz.: vasomotor, regulating the blood supply; and secretory, stimulating the activities of the gland cells. Generally the two conditions, increased blood flow and increased glandular action, coexist. At times profuse clammy perspiration occurs, with diminished blood flow.
The dominating sweat center is located in the medulla, though subordinate centers are present in the cord. The secretory fibers reach the perspiratory glands of the head and face through the cervical sympathetic ; of the arms, through the thoracic sympathetic, ulnar, and radial nerves; of the leg, through the abdominal sympathetic and sciatic nerves.
The sweat center is excited to action by mental emotions, increased temperature of blood circulating in the medulla and cord, increased venosity of blood, and many drugs, rise of external temperature, exercise, etc.