The Skin And Its Function
( Originally Published 1938 )
The significance of soap and water and a clean skin for a long life and a healthy one, can only be appreciated if the structure and the functions of the skin are understood. Strangely enough and it is to be regretted in spite of the importance of the skin our knowledge of dermatology is far from complete. Small wonder then, that the average person clings to strange superstitions about this organ and easily falls prey to quacks who spell bind him more likely her into throwing away money on magical liniments and salves guaranteed to cure at least twenty internal disorders over night, not to mention all the rare oils, herbs and creams which iron out wrinkles, fade out blemishes, remove two-out-of-three chins, dry out blackheads and bring back the beauty of Apollo and Venus before the first jar is emptied (which really would be very bad business). So, forgetting our pet skin tonic or lotion for the moment, let us see what the skin really looks like and how it functions.
First of all, even though your physiology text books emphasize that the liver is the largest solid organ of the human body, the skin really is the largest organ. Indeed, it is about six to eight times larger than the liver and that fact alone should convince the reader of its importance. Moreover, the structure of the skin is as complicated as it is vast. It is divided into three layers the epidermis, the corium or true skin and subcutaneous tissue. The outer layer or epidermis is composed of a horny layer of dead cells, a layer of translucent epithelial cells, a layer of coarsely granulated cells and a mucous layer which contains the pigment which distinguishes the blond from brunette and protects the underlying layers from the chemical action of the sun's rays. This layer also contains the openings of the sweat and fat glands.
The next layer the corium is divided into two parts, the papillary layer which is composed of fine connective tissue fibers and contains nerve endings and loops of blood vessels. With a high-powered magnifying glass you can see tiny, rounded depressions which are the openings of the sweat glands. A single drop of sweat can usually be discerned at each opening, especially after exercise. The lower or reticular layer has a coarser, loose structure than the papillary layer and contains some of the glandular structure of the skin (fat and sweat glands) as well as hair follicles, muscles and blood vessels.
There is no definite line of demarcation between the reticular layer and the subcutaneous tissue indeed, many authorities consider the latter a part of the corium. It varies in thickness and is composed of a network of interlacing bundles of connective tissue less clearly arranged than those in the corium and enclosing irregular spaces containing fat cells, so that it serves as a buffer or pad against external violence.
The nutrition of the skin is supplied by arterial blood, the vessels of which are controlled by the vasoconstrictor and vasodilator mechanism. In the corium there is a very abundant network of blood vessels, but in the epidermis there are no blood vessels, its nutrition being obtained from the fluid part of the blood which circulated between the layers of the lower part of the epidermis. The outermost layer of the epidermis, which consists of structureless, horny scales, has passed beyond the stage where it needs or can assimilate nutrition.
The vasoconstrictor and vasodilator nervous mechanism which controls the blood supply is very sensitive and is affected by many factors, but it is not under voluntary control. It may be affected by direct action on the nerve centers or on the peripheral mechanism of substances in the blood; by reflex stimulation after the application of heat or cold to the surface of the skin; and by various forms of cerebral or mental stimulation such as emotional excitement. Pallor, for instance, may be caused by shock the blood vessels of the skin contract and the blood supply of this area is diminished. Blushing is another example of the disturbance of the blood supply produced by emotional factors. In this instance the blood vessels are dilated and the blood rushes to the surface. Poisonous substances in the blood may produce similar disturbances. The most familiar example of this is the flushing of the face from alcohol or from improper eating. The control of the blood supply by the nerves exists throughout the skin, but it is more sensitive in the skin of the face, as most of us have learned through experience, often to our embarrassment. When you're trying to act nonchalant when the girl of your dreams puts a flower in your lapel, you feel like choking little brother who points his finger at you and shrills out that "he's blushing, he's blushing!"
The functions of the skin may be divided into five general classes: protective, sensory, respiratory, heat regulatory and secretory. As a protective covering, it guards against the harmful action of the sun's rays, the loss of body fluids, the entrance of toxic substances into the body, and against mechanical violence. The outer surface of the epidermis is highly resistant and very dense and insensitive. Therefore, although it is tissue thin, it serves perfectly to protect the living, highly sensitive layers lying beneath it, against all ordinary injury. It is remarkably resistant not only to physical contacts, but also to the most active chemical ones and is able to prevent the invasion of bacteria. The corium is an exceedingly tough tissue, and is therefore the layer that gives the skin its strength. The fatty layer which lies beneath the corium is a very loose network and therefore acts as the cushion or buffer against any physical violence. It also gives a smooth even contour to the body curves instead of knobs and angles. And as it is composed of a very loose network of fibers with spaces filled with roundish deposits of fat, it furnishes the skin with a comparatively large amount of freedom of movement over the underlying structures.
In connection with its protective function, the sensory function of the skin is very important. The skin is richly supplied with nerves, as we have said, and it is highly sensitive to heat, cold, pain, pressure and friction. This sensitiveness not only serves as a warning against harmful external forces, but it also controls the other functions, as well as the nutrition of the skin. As we shall see, this sensitiveness of the skin is of utmost importance in the effect that baths of varied temperatures have on the system.
As we explained before, the source of heat and energy in the body is the constant oxidation of tissue. While oxidation is taking place all the time, the rate of oxidation varies with the output of energy. During sleep, for example, the burning of tissue is much slower than during exercise. It is obvious that a delicate heat regulating mechanise is required to keep the body at even temperature. This mechanism is chiefly provided for by the skin. The surface of the body, it will be recalled, has just beneath it a very abundant network of blood vessels. The constant flow of blood through the vessels causes continuous radiation of heat from the surface. When the surface is cold and the body needs to retain heat, minute muscles in the skin contract and cause "goose flesh". By this mechanism the blood vessels are emptied, perspiration is checked and the amount of heat given off is much less. On the other hand, in the presence of excessive heat, or when heat is produced during muscular exercise, the cutaneous blood vessels dilate, the skin becomes congested with blood and at the same time the perspiration becomes "sensible". This increased supply of blood rapidly gives off heat by radiation and the body is further cooled by the evaporation of sweat.
The skin is also important as an organ of elimination and excretes the waste products of the body just as the lungs and kidneys do. The waste products are given off through the pores of the skin in the form of sweat. It is estimated that about one liter of water is given off during twenty-four hours, and during exercise, two or three as much.
The secretion of sweat is constant unless stimulated to an increased degree through exercise, emotion, drugs or illness. It evaporates as rapidly as it is formed and is known as "insensible perspiration". When it is given off in abnormal amounts it is called "sensible perspiration".
Sweat is about 99 percent water and through its secretion the chief function of perspiration is performed the elimination of water. This regulates the temperature of the body and, as we pointed out above, renders the horny layer of the epidermis soft and pliable by percolating through the spaces between the cells and thereby keeping them moist.
The sebaceous or fat glands are also constantly secreting a semi-fluid substance called sebum. This also tends to keep the skin soft and pliable as well as to protect it from external injury. As sebum tends to harden in the ducts and form blackheads which may cause infection, the necessity of keeping the pores clean and open is imperative. This is particularly true in adolescence when the glands are secreting more than at any other period in life.
Before leaving the subject of the functions of the skin, a few words particularly for the female sex's benefit should be said on the subject of the skin as an absorptive agent. The skin is primarily an organ of elimination and of protection against foreign substances. In no sense is it an active organ of absorption, although some substances, such as certain vitamins and fats, may be absorbed and utilized in some measure when properly applied. There is practically no absorption of fats when merely placed on the skin. Fats and oils, however, may be absorbed by inunction or friction. And if the natural fat of the skin is removed by either or if the blood vessels of the skin are dilated by the application of heat, absorption takes place more readily.
This fact is of utmost importance when considering the effect of the various preparations used in the care of the skin soaps, lotions, medicaments, as well as external applications for internal cures, such as counterirritants, for two reasons. First, abrasive substances may break down the physical structure of the skin and therefore lower its resisting powers. Likewise, strong chemicals, such as caustics, may break down the chemical composition of the skin and lower its resistance.
The horny layer of the skin is markedly acid and as such has the power to inhibit the growth of or to destroy many germs. On the other hand, the lower layers are alkaline. It is in this alkaline medium that many germs multiply. Hence the necessity of keeping the acid layer clean, intact and free from excess perspiration which lowers the resistance of the outer layer and acts as food for bacteria and other micro-organisms.
The horny substance of the outer layer of the skin not only protects the skin from mechanical irritation, but it can stand prolonged macerations in 50 percent solutions of mineral acids. On the other hand, it is soluble in cold, weak solutions of potash and soda, quick lime, alkaline earths and sulphides. These substances can dissolve the horny material that holds the hair roots in place and act as depilatories.
We could go on describing the prophylactic powers that the skin possesses in addition to its protective powers against mechanical and chemical injury, if time and space permitted. But we shall have to close the subject with a warning that it should be kept in mind that the resistance of the skin is only relative, nor is it the same all over. It is slower and slighter on the external surfaces than on the flexor surfaces, the differences not being governed by the thickness of the skin. It is such inequalities as these that make the skin less resistant to external noxae. And, with particular reference to the effect of soap on the skin, it should be kept in mind that alkalies are the chemical agents against which the outer layers of the skin have the least power of defense.
Before entering the all important subject of how to keep the skin in good condition, let us quickly sum up what we have found, for truly our findings have been many and complicated. The skin, we learned, is one of the most important organs of the body, from a social and from a physiological point of view. It is composed of three layers of a highly complicated physical and chemical structure. Each layer has its own particular functions to perform. The outer layer is composed of a horny substance which serves as a protective covering against mechanical and chemical injury, not only to the skin itself, but to organism as a whole. This layer also has self disinfecting powers which are particularly efficacious when the skin is clean and especially when it is free from fatty substances. Moreover perspiration is known to be an excellent culture medium for bacteria and fungi. It is obvious, therefore, that the skin must be kept clean and intact if it is to perform its functions of protection, elimination, heat regulation, respiration and sensation. And we shall soon see that the best way to keep the skin clean and intact is through the soap and water bath provided the soap is mild, nonirritating and yet thoroughly detergent.