Page:The American Cyclopædia (1879) Volume XV.djvu/87

 SKIN 79 ie hands and fingers, feet and toes, where they are arranged in double rows in parallel curved lines ; the average length of the papillae, including the height of the ridge upon which md Papillae of the Skin from the surface of the Hand, showing 1 double, triple, and quadruple divisions. Base of a compound papilla. 6, b, b. Its upper extremi- ties, c, c, c. Points of other papillae, the base of which is not visible. they are placed, is about T ^y of an inch, and the diameter at the base -^^ ; they are abun- dantly supplied with blood, which explains their erectile turgescence under stimuli ; they adhere more or less firmly to the cuticle. The f^aceous or oil glands of the skin are formed the same plan as the sudoriparous, and can Vertical Section of the Skin, magnified. a. Epidermis, b. Inferior layer of epidermis, or rete Malpi- ghianum. c. Papilla? of the skin. d. Corium, or dermis. ,/. Lobules of adipose tissue, g. Perspiratory glands. h. Ducts of the perspiratory glands. *. Their external orifices. Jc. Hair follicle. I. Hairs projecting from the skin. m. 1 1 air papilla, n. Hair bulb. o. Shaft of hair in the hair follicle, p. Openings of the sebaceous glands. often be distinguished from them only by the nature of their oily secreted product ; "they are distributed over the whole surface of the body, being least abundant where the perspi- ratory glanduhio are most numerous, and vic'e 74G VOL. xv. 6 versa; they are absent on the palms and soles, but abundant on the face and scalp ; they vary considerably in size, but the tubes are general- ly wider and straighter than those of the sweat glands ; the structure is sometimes complicated. In the parts of the skin covered with hair, there is usually a pair of sebaceous ducts opening into the follicle of each hair. The object of their secretion is doubtless to prevent drying and cracking of the hair by the sun and air ; this secretion is most abundant in tropical nations, and in some dark races has a characteristic odor, as in the case of the negro ; its protective action in the tropics is often assisted by vege- table oils applied externally. The Meibomian glands on the edges of the eyelids are a double row of sebaceous follicles set along a straight duct ; they secrete an oily matter for the lu- brication of these parts, which in diseased con- ditions frequently sticks them together. An- other modification of sebaceous glands is to be found in the external ear passage, where is secreted the cerumen or waxy matter ; they consist here of long, highly contorted tubes, well supplied with blood vessels. The color of the skin depends on pigment cells mixed with the inferior epidermic ones, in what is called the rete mucosum, or mucous layer, and considered by Flourens and other authors as a distinct membrane ; all the hues of the races of man depend on the relative abundance of these cells and the tint of the contained pig- ment. The folds of the skin are for the most part produced by the contractions of the su- perficial muscles. The skin is pierced at the eyes, ears, nostrils, mouth, rectum, and genito- urinary opening ; it is continuous internally with the mucous membrane, consisting of the same elements modified according to the va- riety of functions to be performed ; it is very vascular, and freely supplied with nerves and lymphatics. The skin is the seat of the sense of touch in man, though in most animals hairs, scales, bony or horny plates and envelopes, and shells, render it nearly insensible to external influences, this sense in them being confined to particular portions or projecting organs ; even in man the sensibility varies much in different parts, being most acute at the ends of the fingers and on the lips, and dullest on the back and limbs. Aeration of the blood takes place to a certain extent through the skin, and in some naked-skinned fishes and batrachians this is a very important part of the respiratory process. It has been shown by experiment that in a frog, after the removal of the lungs, one fourth of a cubic inch of car- bonic acid is exhaled from the skin in eight hours ; in the human subject the amount of this gas given off by the skin varies from -fa to fa of that exhaled from the lungs during the same time ; where the lungs perform their of- fice imperfectly, the temperature of the skin is often elevated ; in all febrile diseases the skin should be kept moist. The absorbent powers of the skin are noticed under ABSORPTION.