The skin plays an important role in the reg ulation of bodily temperature. The action of excessive heat from within or without, upon the vasomotor nerves distributed to the blood-ves sels of the skin, causes the latter to dilate. An increased amount of blood is thus brought to the skin, where it is cooled by conduction and radiation and also by evaporation. The latter effect results from the increase of moisture on the skin, following an acceleration of sweat gland_secretion, due to augmented blood-supply.
On the other hand, cold, acting upon the vaso motors, leads to contraction of the vessels; a diminished amount of blood enters the skin; evaporation is reduced by the checking of sweat secretion; and bodily heat is conserved.
The secretory and excretory functions of the skin are carried on by the sebaceous and sweat glands. The product of the former, a fatty, unctuous material, cannot be strictly re garded as a secretion. Its purpose rather is to anoint the hair and skin, thus keeping the lat ter soft and preventing heat loss by evapora tion, and maceration of the epidermis by moisture. The sweat, however, is a true excre tion and as organs for the removal of waste products the sweat-glands are most important. Various estimations have been made of the amount of sweat produced in 24 hours; the average is probably about one and one-half pounds. This can be greatly increased by mus cular activity, by high external temperature and by the use of certain drugs; it is obvious, there fore, that considerable material can be removed from the body through the skin, a fact of great importance in the treatment of certain diseases. The sweat itself is about 99 per cent water; the remaining 1 per cent represents a number of complex fatty bodies and mineral salts. It may be alkaline or acid in reaction; acidity is in part due to admixture of fatty acids from the seba ceous secretion. It has been claimed that the sweat-glands produce no fat, but it is generally accepted now that this is not true, since the secretion in the palm, a part devoid of sebaceous glands, unquestionably contains fatty products.
The skin possesses a respiratory function, but it is comparatively insignificant. Carbonic acid gas is given off and oxygen absorbed, the relative amounts of each as compared with those from the lungs, being 1-220 of the former, and 1-135 of the latter. The interchange probably takes place through the sweat-pores. The skin also exhibits a certain degree of absorptive power. This is but slight in the undamaged skin. If the horny layer be removed, or if the substance be forced into the glandular ducts, absorption may take place readily, especially of certain substances.
Diseases of the Skin.—Abnormal or dis eased conditions of the skin, as of other or gans, are recognized by manifestations called symptoms. These are all referable to one of two categories: subjective symptoms and ob jective symptoms. By the former is meant those which are appreciated only by the patient him self ; by the latter, those which may he de tected by the examining physician. Subjective symptoms in purely cutaneous diseases are, in many instances, wanting; when present they are manifested in some form of abnormal sensa tion, as burning, prickling, tingling, smarting, creeping or itching; in diminished sensibility or anaesthesia; in increased sensibility or hyper msthesia ; or in pain, which latter may be burn ing, shooting, boring or aching. Of all subjec tive symptoms, itching or pruritus is most com mon; it is constantly present in many condi tions, though often in varying degree.
Objective symptoms are vastly more impor tant. To the person afflicted they often seem the visible manifestations of disfiguring, repul sive disease, and their appearante is therefore dreaded; to the dermatologist • they stand for facts and principles 'which, when aggregated.
constitute the major portion of his special knowledge, and which he must know and under stand most thoroughly if he shall hope to be proficient. Any alteration • from the normal in the texture and appearance of the skin is termed a lesion. Lesions are variable in appearance, character and manner of evolution, which fact is the basis of their subdivision into two gen eral classes: elementary or primary lesions; and consecutive or secondary lesions. These terms are self-explanatory; the former are lesions which at their inception present a certain type; the latter are those which are produced by pro gressive alteration from a previously existing type. The classification is not entirely accurate, for under certain conditions a lesion of a primary type may appear as secondary. Briefly described, the objective cutaneous char acters which are considered primary, eight in number, are: (1) Macules (spots or stains), circumscribed alterations in the color of the skin, without elevation or depression of the sur face. They are variable in size, shape, color, dura tion and as to cause of the dvschromia. A familiar example is the "freckle." (2) Papules (pimples), small, usually superficial, pinhead to pea-sized, circumscribed, solid elevations of the skin. These, too, differ in size, shape, color, duration and cause. The epimples" or papules of acne in young persons, illustrate this type of lesions. (3) Tubercles (nodules), solid, fairly circumscribed, pea-sized, deep-seated elevations, usually persistent in character. These are in reality large papules, or more properly, inter mediate lesions between the papule and the tumor, to be next described. In severe acne tubercles are sometimes found. (4) Tumors, soft or firm, more or less circumscribed eleva tions of the skin, varying in size and shape, and seated deeply, in the corium and subcutaneous tissues. As the papule merges into the tubercle, so the tubercle passes over into the tumor; thus at one extremity of the series stands the pin head-sized papule, at the other, the egg-sized or larger tumor. (5) Wheals, circumscribed, red dish-white, oedematous elevations of variable size and shape, usually evanescent in character. This form of lesion occurs typically in the con dition known as urticaria (q.v.) or nettle-rash. It is angioneurotic in origin and is always ac companied with subjective symptoms, especially itching. (6) Vesicles (little blisters), cir cumscribed elevations of the horny layer of the epidermis, pin-point to pea-sized, with limpid. lactescent or sanguinolent contents. This form of lesion may at times be secondary, since it can develop on a previously existing papule, as il lustrated in smallpox. (7) Pustules, pin-point to filbert-sized, circumscribed cutaneous ab scesses, having an epidermal roof-wall. The skin surrounding the base usually shows an in flammatory areola. They are developed very frequently from vesicles, less often from papules. It is, in fact, somewhat questionable to class the pustule as a primary lesion. They are seen commonly in acne. (8) Blebs (large blis ters), irregularly shaped elevations of the epi dermis, varying in size from a pea to a goose egg, containing clear or opaque fluid.• This lesion is well illustrated by the blister following a severe burn.