The Human Skin

Macroscopic

The human skin as a whole conforms to the underlying parts of the body as a continuous organ. A baby, which at first may easily be held in the palms of two hands, grows in three dimensions, but the skin keeps pace with the change in size, always fitting the enlarging body perfectly without any bursting at the seams. The clothes, or adventitious skin, in which the child is encased by its parents are frequently too large, because of the hopeful expectation that he will grow to fit them eventually. Not so the marvelously pliable skin. Its smooth expanse is diversified by a few noteworthy elevations and depressions, as when it is stretched over the cartilaginous framework of the external ear, or descends into the ear passage itself. The innumerable tiny pits, appearing wherever there are emerging hairs or openings of the miniature volcano-like sweat glands, are microscopic depressions that do not entirely penetrate the skin or in any way interrupt its continuity.

Wrinkles and creases around the joints aid in accommodating the elastic integument to changing contours. In old age the skin frequently exhibits wrinkles, because it does not shrink as rapidly as the underlying muscles, in the process of diminishing repair attendant upon advancing years.

The skin is thinnest where it passes over the exposed part of the eyeball. It is so thin and translucent here, as well as in the double layer of the eyelids, that it is possible to perceive light through three layers of skin at once, a fact easily demonstrated by turning the closed eyes toward a brilliant light, when the difference between light and darkness is distinguishable through the double skin fold that is the eyelid, and the continuous conjunctiva which constitutes the front face of the cornea.

The thickest region of the skin is found on the soles of the feet. Corns, callouses, and other local thickenings wherever there is continued excessive friction or pressure, are evidences of increased thickness by use and are particularly pronounced on the soles of the habitually unshod.

According to Lamarck the thickened skin on the soles of a baby’s feet before they have been subjected to use is inherited from ancestors who acquired it while walking up and down the earth. This is by no means, however, the most plausible explanation. Mudpuppies (Necturus) likewise have the thickest skin on the soles of the feet. Since these primitive amphibians have their bodies always supported by the surrounding water, they do not use the soles of their feet, nor is it likely that any of their ancestors did so. Obviously there must be another reason for the differentiation in skin thickness on the soles of the feet.

According to Rauber the area of a typical human skin is about 1.6 meters square, or approximately five feet square. This fact is instructive when it is remembered that certain functions of the skin, as an organ of excretion or respiration, for example, depend upon its expanse.

The weight of the human skin with the subcutaneous fat removed, as determined in autopsies, is stated by Bischoff to be 3175 grams for a thirty-two year old female, and 4850 grams for a male thirty-three years old, or approximately 7 and 10.7 pounds.

The color of the skin depends upon two factors, namely, translucency, which permits the underlying capillaries to show through as in blushing, and the presence of pigments, of which there are several kinds, white, yellow, black, and red. Excepting in albinos, these pigments are all present in varying proportions in the different races of mankind. They are unequal in distribution, even in a single individual, being heavier on exposed parts of the skin, and around the axillae, nipples, and genitalia. The general color of the skin of so-called white people varies also with age, from pink babyhood to yellow senescence.

Characteristic blue-gray birth marks in the sacral region of newly-born mongoloid people, which fade out in the course of two or three years, are due to brown pigment granules located in the deeper translucent layers of the skin.

Nerve endings, except those of the most undifferentiated character, do not ordinarily extend into the intercellular spaces of the epidermis. Consequently stimuli which affect the body must reach the deeper-lying nerve endings of the corium through the protective barrier of the epidermis.

Microscopic

The cells of the epidermis are arranged in stratified layers, like the leaves of a book, with the most important and indispensable layer next to the corium. From it the other, more superficial, layers are derived, together with such accompanying modifications as hair, feathers, and nails. This remarkable life-giving restorative layer of germinative cells is called the Malpighian layer (Fig. 138), in honor of Marcello Malpighi (1628-1694) who first pointed out its significance, thereby erecting to his name a memorial far more enduring than an isolated mausoleum or a marble shaft.

Diagram of the skin, showing how the Malpighian layers gives rise to the superimposed layers of the epidermis

The cell progeny arising from the germinative Malpighian layer are gradually modified while they are being crowded toward the exposed surface of the skin. Their walls become thicker at the expense of the cytoplasm, while the breakdown of their nuclei is accompanied by a sequence of chemical changes in the cell substance. Finally, each cell flattens until eventually only a dead scalelike remnant remains, like the collapsed skin of a grape after the pulp has been squeezed out. The squamous husks of the outermost dead cells thus formed are constantly breaking free from the underlying layers, being shed with no interruption of function of the skin as a whole, while at the same time a continuous renewal from the Malpighian layer below is maintained. Dandruff is formed of the matted masses of the outermost dead epidermal cells.

It has been estimated that a person who has attained three score years and ten has, quite unawares and painlessly, gotten rid of over forty-five pounds of dead and discarded epidermal cells. Due to the eternal youth of the Malpighian layer, the skin in this way is cleaned over and over without wearing thin the way clothes do that are repeatedly scrubbed. The dead outermost layer is the corneum. The region between the outer corneum and the living Malpighian cells below is characterized in certain areas of the human body by the presence of two transitional layers, called the stratum granulosum and the stratum lucidum. The former is best seen in cross sections of skin taken from the soles of the feet or the palms of the hands. It is several cells in thickness next to the Malpighian layer, and is called “granulosum” because, upon the breakdown of the Malpighian nuclei, keratohyalin granules (Waldeyer) are formed, which give it an appearance of greater density.

The apparently homogenous stratum lucidum, which lies just outside the stratum granulosum and is derived from it, owes its semi-transparency and comparative resistance to all ordinary histological stains to the fact that the keratohyalin of the stratum granulosum becomes changed at this point into a different chemical compound, called eleidin (Ranvier). This layer is usually wanting except where the skin is particularly thick, but it reaches a conspicuous development in the nails, which it principally composes.

Skin pigment is usually located in the Malpighian layer of the epidermis, although in some vertebrates it is distributed among the deeper-lying cells of the corium.

The corium, or derma, is the distinctive part of the vertebrate skin, being unrepresented in invertebrate integuments. It is a network of connective tissues, consisting of cells and fibers produced by cells, felted together. It underlies the superficial epidermis and is many times thicker. When leather is made it is the corium that is tanned to produce it, the epidermis being discarded. The corium of the human skin as well as that of different animals can be made into excellent leather. As a matter of historical fact, during the French Revolution, shoes were made from the tanned skin of guillotined persons. It is related of one Johann Ziska, a fire-eating German patriot of olden days, that he stipulated in his will that his skin be tanned and made into a drumhead, the martial resonance of which should incite those who heard it to fight as valiantly as if his own voice were urging them on.

Among the many structures embedded in the corium are: capillaries and lymph vessels in abundance, nerve endings, sense organs, migrating pigment cells, deposits of glycogen and fat, smooth muscle cells, sweat glands, sebaceous glands, and hairs, the last three being downgrowths from the epidermis.

The deeper parts of the corium form the subcutaneous layer, characterized by the inclusion of masses of soft fat cells and by the looser weave of the felted reticulum, which allows greater freedom of motion to the underlying muscles. Its blood supply may include a large fraction of the total amount of blood in the entire body. Some of the fibers of the subcutaneous region interlace with the fibers composing the connective tissue sheaths that envelop the muscles, thus fastening the skin down, as it were, more firmly. This is demonstrated better in the palm than over the back of the hand where the skin is looser.

Diagram showing some of the details of friction skin

In regions of the body such as the finger tips that are much in contact with things, the outer part of the corium just under the epidermis is thrown up into rows of tiny projections, or papillae, that form ridges (Fig. 139). It is customary consequently to speak of a papillary layer of the corium, although stratification of the corium is not as pronounced as stratification of the epidermis. The roughened papillary layer helps possibly to hold the corium and epidermis together at points on the skin where friction or pressure is frequently applied, for the epidermis dovetails intimately into the minute hills and valleys formed by the dermal papillae.

There are two sorts of papillae in the papillary layer, namely, nutritive and sensory, the former containing a capillary knot, the latter occupied by a sensory nerve ending. It is possible to demonstrate these two kinds of papillae experimentally in the finger tips by patient manipulation with a very fine needle. When a nutritive papilla is punctured there is no particular pain, although a tiny drop of blood may appear, but when a sensory papilla is pricked no blood flows and pain is felt. Both kinds are so close together, and any needle point is relatively so large, that it requires nice discrimination to perform the experiment successfully.

Wherever papillae are present, three layers, which shade imperceptibly into each other, may be distinguished in the corium, namely, papillary, reticular, and subcutaneous.

Embryonic

As already indicated, the human skin as well as the vertebrate skin in general has a double embryonic origin. The epidermis, which is the primary component, arises from that part of the ectoderm remaining after the medullary tube, which forms the central nervous system, has migrated in from the outside by invagination. It consists at first of a single layer of ectodermal cells that soon gives rise to a temporary skin, the epitrichium (Fig. 140), a delicate outer layer of somewhat swollen cells which take certain stains distinctively, thus showing a specific chemical character. Corneal cells, derived from the Malpighian layer, soon appear under the epitrichium until, at the age of about three months in the human embryo, the epidermis of the fetus has acquired a thickness of three or four cells deep. About the fifth month of fetal life, when embryonic hairs begin to emerge from the skin, the gauzy epitrichium is shed from the entire body, excepting the palms and soles, into the amnionic fluid, and is never replaced in kind. The name epitrichium (epi, upon ; trichium, hair) signifies that this layer temporarily rests upon the tips of the budding hairs. A more inclusive term for this evanescent embryonic mantle is'periderm {peri, around; derm, skin), since it is present as a part of the embryonic skin of reptiles and birds also, where there are no hairs upon which it can rest.

Skin from the head of a human embryo of 2,5 months

The corium is derived from cells of the somatic mesoderm and the mesenchyme. It is secondarily wedded to the overlying epidermis, which it eventually exceeds many times in thickness.