SKIN. 216 SKIN. great extensibility, with elasticity, is required, the yellow (elastic) elemeut predominates; and where strength and resistance are specially re- quired, as in the sole of the foot, the corium is chiefly composed of a dense interweaving of the white (inelastic) element. The thickness and strength of this layer ditl'er greatly in different parts, according to the amount of resistance re- quired against pressure. The skin is thicker on the hinder surface of the body than in front, and on the outer than on the inner sides of the limbs. It is unusually thin over the flexures of the joints. It is particularly delicate in the eyelids, and proportionately so in some other situations where great mobility is demanded. In regions which are most subject to external pressure, as the soles of the feet, it is firmly united by very dense laminae to the subcutaneous fascia; and the intervals between these are provided with pellets of fat, forming a cushion, as an additional means of protection to the delicate organs it in- closes and covers. It is on the external surface of the cutis that the tactile papillw, or true or- gans of touch are developed. The corium is divided into the 'reticular' and 'papillary' portions, the latter being the reddish-gray external superficial layer which contains the upper portion of the hair follicles and cu- taneous glands, and whose most important elements are these tactile papillae. They are most abimdant and largest in the palm of the hand and the sole of the foot, while in the back and in the outer sides of the limbs they are almost en- tirely absent. They occur as small, semi-trans- parent, flexible elevations, which are usually con- ical or club-shaped in form ; but in certain parts, as the palm of lihe hand, present numerous points (in which case they are termed compound papil- lae). In one square line of the palm of the hand, it has been calculated that there are 81 com- pound and from 150 to 200 smaller papilla^, ar- ranged in tolerably regular rows. The glands occurring in the skin are the sudor- iparous or su^eut glands, the sehacenus or fat fllands, and the ceruminous filaiuls. The su'^at glands exist in almost every part of the human skin. They lie in small pits in the deepest parts of the true skin, and sometimes eiitirely below the skin. Their orifices can be seen in the mid- dle of the cross grooves that intersect the ridges of the papilh-E on the hands and feet, their ar- rangement being here necessarily regular, while in other parts they are irregularly scattered. Their size and number in different regions of the skin correspond with the amount of perspiration yielded by each part ; thus they are nowhere so much developed as in the axilla or armpit. In that part of the region which in the adult is more or less covered with hair, they form a layer of a reddish color, about an eighth of an inch thick. They are soft, and more or less flattened by their pressure on one another, being imbedded in delicate connective tissue, and covered and permeated with a network of capillaries. On isolating one of these glands, and highly magni- fying it, it is found to consist of a solitary tube, intricately raveled, one end of which is closed and hidden within the glandular mass, while the other emerges from the gland. The wall of the tube consists of an outer or hnsement mrmhrane, with which the Wood-vessels are in contact, and aa epithelium, lining the interior, the former disappearing when the tube reaches the surface of the papillfe. The duct, on leaving the gland, follows a meandering and rather spiral direction through the reticular portion of the cutis to the interval between the papillae, when it becomes straight; and it again assumes a spiral course in perforating the cuticle. The sebaceous ijlunds are small whitish glands, wliieh exist in almost every part of the skin, ex- cept the palms and soles, and are especially al)undant in the scalp, face (the nose being par- ticvilarly rich in them ) , and about the anus. They are usually connected with the hairs, and consist of a duct terminating in a blind pouch- like or ])ear-shaped extremity. The basement memhranc of these glands is lined by an epithe- lium, in the particles of which arc included gran- ules of fatty or sebaceous matter, which, having become detached, constitutes the secretion. These glands are the seat of the parasite known as Acarus foUieulorum (q.v.). The ceruminous glands are brown simple glands, in external appearance like the sudoripa- rous glands, occurring in the cartilaginous por- tion of the external meatus of the ear. They yield an adhesive bitter secretion, w'hich protects the membrane of the tympanum from the access of dust, insects, etc. Regarded as a protective covering, the skin possesses the combined advantages of toughness, resistance, flexibility, and elasticity: the connec- tive framework being the part which mainly con- fers these properties, although the epidermis co- operates with it. The subcutaneous layer of fat, and the modifications of epidermis in various forms, as hairs, wool, feathers, scales, etc., serve for the preservation of warmth, and occasionally ( when they occur as claws, talons, etc. ) as means of oft'ense or defense. Besides preserving the warmth of the body, the skin has also the power of reducing body temperature by the evaporation of sweat. The skin is the seat of a twofold ex- cretion, viz. of that formed by the sudoriparous glands and that formed by the sebaceous glands. The fluid secreted by the sudoriparous glands is usually formed so gradually that the watery por- tions of it escape by evaporation as soon as it reaches the surface; but in certain conditions, as during strong exercise, or when the e.xternal heat is excessive, or in certain diseases, or when the evaporation is prevented by the application of a texture impermeable to air, as for example oiled silk, or mackintosh, or india-rubber cloth, the se- cretion, instead of evaporating, collects on the skin in the form of drops of fluid. When it is stated that the sweat contains urea, lactates, ex- tractive matters, etc., and that the amount of wa- tery vapor exhaled from the skin is on an average two pounds daily, the importance of the sudo- riparous glands as organs of excretion will be at once manifest. The secretion of the sebaceous glands is a semi-fluid oily mass, which often solidi- fies into a white viscid tallow-like matter on the surface or in the glandular ducts, from which it can be removed by pressure, in a form resembling that of a small whitish womi or maggot. The skin is, moreover, an organ of absorption. ^Mercurial preparations, when rubbed into the skin, have the same action as when given inter- nally. Potassio-tartrate of antimony, when rubbed into the skin in the form of ointment or solution, may excite vomiting, or an eruption ex-
