The skin should be viewed as a complex organ with a number of distinct functions. In fact no other organ in the body has so many and varied duties to perform and none is more constantly exposed to so many injurious influences. To a certain extent the skin and its functions are essential to the life of the individual. If these functions be long suspended, or any considerable portion of the skin be destroyed, health is impossible and life doubtful; whereas one may enjoy a long period of comparative health after the loss of an extremity, an eye or a kidney. The intestinal tract, kidneys, lungs, and skin comprise the four emunctories of the body. To some extent partial suppression of the excretory action of any one of these four is equalized by extra activity on the part of the other three. A full appreciation of this fact may impress upon the student the importance of the excretory function of the skin which, strange as it may seem, is little appreciated. The physiological functions to be considered are those of protection, secretion, heat-regulation, respiration and sensation.
Protection may be styled the chief passive function of the skin and is easily explained when the nature of the epidermis, corium, subcutaneous tissue and their varied contents are noted. The epidermis, because of its thickness and impermeability and the insensibility of the corneous layer, serves to protect the body from the injurious effects of extreme temperature and from the absorption of many poisonous substances. The oily glandular secretion partially aids in this respect by preventing too great evaporation of water from within. The corium through its firmness, flexibility and elasticity, and the subcutaneous tissue with its loosely bound bundles of fibrous tissue and fat serve well to protect the muscles, nerves and blood-vessels from all external traumatic agencies. The weakest points in the line of defence offered by the skin are the glandular orifices. However, in the case of the sweat and in the pilosebaceous system the secretion which is decidedly pasty and fatty is somewhat defensive. The abundant hairy growth on the head serves as an additional protection for the brain.
The sebaceous and sweat-glands are the secretory organs of the skin. They furnish oil and moisture to make the skin soft and pliable, and assist in the regulation of the body temperature and in giving off waste material.
Sweat is secreted in the coil or gland proper and finds its way through a duct to the surface. Ordinarily it is rapidly evaporated from the surface in the form of vapor so that its presence is not perceived (insensible perspiration). If, on the other hand, evaporation is retarded or the secretion markedly increased, sweat accumulates on the surface in drops (sensible perspiration). In health sweating is more pronounced in some parts of the body than in others, such as the palms, soles, face, neck, axillae and genitocrural regions, due either to more numerous and larger glands or to a normal increase of heat in the parts specified. The quantity of water vapor normally given off by the skin varies from one to two pounds daily, and is nearly double that eliminated by the lungs. The nervous system is an undoubted factor in the production of sweat, because its secretion is dependent upon nerves, the centers of which are situated in the spinal cord as far up as the medulla oblongata, which probably contains the general center. These centers may be directly or reflexly stimulated, and act, through the local nerve-fibers, directly on the epithelia of the sweat-glands. Sweat is increased by external heat, changes in the blood, and by the ingestion of such drugs as camphor, pilocarpin and strychnine. Pilocarpin and some other alkaloids are believed to stimulate the secretion of sweat by acting directly on the peripheral nerves. The secretion of sweat is diminished by cooling the skin, by suspension of the blood supply and by such drugs as atropine, morphine, etc. A normal increase of sweat is attended with increased activity of the local circulation, but in abnormal conditions the perspiratory nerves may act independently of the vasomotor system and a free secretion of sweat occur when the skin is pale and cold (cold sweat).
The normal sweat is composed of about 99 per cent. of water and 1 per cent of organic and inorganic constituents. These chemical elements embrace urea, sodium chloride, volatile fatty acids such as acetic, formic, butyric, proprionic, caproic and caprylic in variable quantities and small amounts of sodium phosphate, earthy phosphates, neutral fats (palmitin and stearin) and cholesterin. The quantity of urea in the sweat is small and varies very little in health, but in a pathological condition of the kidneys, with suppression of the excretion of urea by these organs, it may be enormously increased. It has been intimated that the kidneys and skin normally hold compensating relations to each other; thus a reduction of sweat in cool weather is compensated for by an increase of urinary fluid, and vice versa in warm weather. Sweat is a clear, watery fluid with a salty taste, variable odor, and normally an acid reaction, but this acidity seems to be due to a mixture of sebaceous material. It is interesting to note that the sweat from the palms of the hands where there are no sebaceous glands is alkaline, and that profuse sweat caused by the administration of diaphoretics is neutral and alkaline in reaction. Investigations of Meissner and Unna show that the sweat-glands also secrete fat for the lubrication of the skin.
Sebum, the product of the sebaceous glands, is in its normal state a fluid or semifluid fat which in health lubricates the hair and epidermis. There are many variations in the amount and consistency of this secretion within the limits of perfect health. Not only do individuals, but different parts of the same individual, show marked differences in this respect. The chemical constituents of sebum are water, palmitin, olein, palmetic and oleic acids, saponified fats, cholesterin, a casein-like, albuminoid and inorganic salts. The function of the sebum is mainly preservative. It partially protects the surface of the skin from external infection, from the softening effect of long-continued moisture, and, at the mucous outlets, from the contact of irritating excretions. It may prevent too rapid evaporation from the cutaneous surface and consequent undue loss of heat and probably contributes to the nutrition of the hair and preserves its external surface. Unlike the secretion of sweat, the nerves are not supposed to be intimately concerned in the production of sebum. However, the exact nature of the sebaceous gland secretion is not understood and some authorities believe that this secretion, as in the case of sweat, is under the control of the nervous system and subject to variations. Bowen suggests that sebum is increased by dilatation of the blood-vessels and increased cutaneous temperature. In any case, the secretion is doubtless a continuous one, although subject somewhat to fluctuations.
The skin plays a most vital part in the regulation of the amount of heat given out by the body. In this respect the epidermis, especially the horny layer, is an important factor because it is a bad conductor of heat. By this essential quality it prevents a too great caloric loss from the superficial blood-vessels and by its firm substance exerts a variable degree of pressure on the underlying capillaries, preventing overfilling and resultant heat-loss. The vasomotor nerves assist in this function through their regulation of the cutaneous blood supply by reflex impressions from without and from excessive heat-producing processes from within. A variation in bodily temperature of ten degrees. either above or below normal, is incompatible with health and life; yet the wide atmospheric variations are endured without harm and with little or no change in systemic heat. It is estimated that from 70 to 80 per cent. of the total loss of bodily heat occurs by evaporation, radiation and conduction from the skin. When the surface is cooled, the amount of blood sent to the skin is greatly diminished by shrinking of the blood-vessels through the contraction of their muscular coats, controlled by the vasomotor nerves. At the same time increased tension of the cutaneous muscles reduces its superfices, and thereby diminishes the discharge of secretions. In this manner the loss of heat is brought to a minimum required to preserve a normal internal temperature. Excess of external heat or of heat physiologically produced within the body tends to relax the tension of the blood-vessels and the skin. A larger flow of blood toward the surface follows, the superficies of the skin are increased, and loss of heat takes place by augmented evaporation and radiation from its surface. Heat-loss is aided by dry air and retarded by moist air.
The respiratory function of the skin is similar to that of the lungs in that it embraces the inhalations or absorption of oxygen and the exhalation of water and carbonic acid. Its activity in this respect is insignificant as compared with that of the lungs, for it is estimated that it does one-fiftieth of the respiratory work of the latter. Scherling says that ten grams of carbonic acid are given off in twenty-four hours, and Kaposi asserts that the carbonic acid exhalation is 1/220 and the oxygen inhalation 1/180 of the lung total for the same length of time. The amount of nitrogen and ammonia exhaled is doubtful but water is given off in large quantities. The method of exit for these various substances is probably through the sweat-glands, but it is possible that carbonic acid and water may escape between the cervices of the epidermis.
Absorption by the skin of such substances as tar, iodine, arsenic and a few others is undoubted, even though not applied in ointment or oily form. Many volatile materials are easily absorbed to a varying degree. The naturally lubricated, firm, horny layer of the epidermis, with its continuity interrupted only by the glandular openings, offers the chief obstruction to absorption by the skin. If the epidermis is removed and a soluble substance applied directly to the corium, absorption takes place quite rapidly. Rubbing and the removal of the oil from the surface with alcohol, chloroform or ether (before bringing a substance to be absorbed in contact with the surface) will facilitate absorption. In this way many medicated applications are introduced into the system and exert their effects almost as effectually as when given internally. It is probable that all substances which are absorbed by the skin pass through the epidermis (sometimes aided by friction), by way of the gland-ducts and hair- follicles, whose walls are only lined with a single layer of epithelia and thus present a comparatively slight barrier to absorption. Mercury in ointment is absorbed in this way in the form of vapor or after being dissolved by the acid secretions of the skin. Watery vapor is readily absorbed by the skin from the surrounding air, and water in contact with the surface may enter the epidermis in considerable quantity by soakage, without actual absorption. Micro-organisms may be absorbed by the skin; thus furuncles have been produced by rubbing into the sound skin cultures of staphylococci. According to Wasmuth, the different bacteria do not enter the skin by the sweat or sebaceous passages, but by way of the opening between the hair-shaft and the sheath.
This important function of the skin provides a means of protection and discrimination. Formerly it was thought that different sensations were caused by varying degrees and kinds of irritant applied to a single nerve ending, but now it is well known that there is a distinct disassociation of sensation. General sensation is provided for every part of the skin; the thinnest portions of the skin being most sensitive, and the thickest portions the least sensitive. Ordinarily contact becomes painful, if applied directly to the corium. The acuteness of tactile sensation depends on the distribution of the sensitive papillae of the corium. Where these are abundant, as in the skin of the end of the third finger, sense of touch exists in a high degree. Webber found by experiment, that at these parts two distinct sensations of touch could be felt, only one twenty-fourth of an inch apart. The middle of the thigh and forearm appear to be the least endowed with the sense of touch, the distance at which two points of contact can be distinguished in these regions being more than two inches apart.
The tactile sense not only makes known the size, shape and other properties of bodies, but it also differentiates the varieties of pain and temperature. Goldscheider believes there are two kinds of sensitive nerves of touch. The office of the tactile corpuscle, in the light of later investigations, appears to be to give greater mechanical protection to the nerve terminations. That the quality of touch can be educated to a surprising degree is demonstrated in the blind who, by their delicacy and expertness of touch, seem almost to supply a substitute for the loss of vision. The distribution of temperature sensation is very like that of common sensation and varies in different parts of the skin, but is not modified by the relative thickness of the skin to the same extent as general sensation.
Temperature perception was once thought to be a variety of general sensation. The experiments of Blix and Goldscheider not only tend to disprove this, but seem to show that there is a separate nerve mechanism for cold, for heat and for pressure. Experimenting independently of each other, these investigators found that the same irritant produced on some parts of the skin a sense of cold; on other parts heat; and on others, only the ordinary sense of pressure. It is well known that in some diseases attended with paralysis of ordinary sensation, sensitiveness to heat and cold may remain intact. The degree of temperature felt depends, in great measure, on the extent of surface exposed. For instance, one finger can be comfortably borne in hot water which would become painfully hot to the whole hand. The tip of the tongue, the fingers and face are most sensitive to temperature changes, one-half to one degree variation being readily appreciated by these parts.
The non-striated muscles of the arteries are well supplied with vasomotor nerves, and although these elements are not concerned in sensory function, they must be briefly mentioned. They control the constriction and dilatation of cutaneous vessels causing increased or diminished blood supply and hence affecting the temperature of a part. Many morbid conditions are directly affected by these actions which originate from the depression or stimulation of the nerve centers in the medulla or from a similar action exerted upon the peripheral nerve terminations. So little is known about the action of the so- called trophic nerves that, although there will be affections of the skin which will hereafter be referred to as trophoneuroses, it is impossible to give any detailed description of their working. For the most part the nutrition of the peripheral parts is influenced or controlled by the spinal cord.