Uses of the Integument
Among the various uses for which in different animals the structure of the integument is adapted, are the following: (1) protection; (2) reserve food storage; (3) heat regulation; (4) sensation; (5) excretion; (6) secretion ; (7) respiration; (8) locomotion; (9) sexual selection ; and (10) reproduction.
The skin is inevitably a protective organ. Four aspects of its protective function may be mentioned. First, it shields the animal body against mechanical injuries that may result from pressure, friction, or blows of various sorts. Like any other wrapper, the primary function of which is to protect the enclosed parts, the vertebrate integument is admirably adapted for this purpose, since typically it is closely woven in texture, resistant, and at the same time so pliable that it tends to “give” under mechanical stress rather than to rupture or break away. Although many of the individual cells that compose it are soft and delicate, for example in the skin stretched over the knuckles, they are crushed only by a hard blow against a solid object.
In addition to the enveloping skin itself, most animals are equipped with protective integumentary modifications, such as scales, bony plates, feathers, hair, or cushions of fat, which aid in minimizing the effects of blows or injurious contact of any kind.
Invertebrates, such as crustaceans and mollusks, are conspicuously fortified by exoskeletons against an unfriendly world, while turtles, armadillos, alligators, and porcupines are noteworthy instances among vertebrates of animals that go forth, like armored knights of old, well clad to resist the blows and harassments of their adversaries.
Second, the integument protects the body against foreig?i substances. Whenever skin infection from any outside source occurs, it is usually through some break, however slight, in the enveloping integument. Since the skin is practically germproof so long as it remains whole, the internal realm of the body is protected from invasion through it by foreign immigrants.
Cleaning the skin of whatever undesirable substances may stick to it is usually accelerated in the case of civilized man by the application of soap and water. However, in the absence of these aids the human skin automatically cleans itself by surface renewal. Among the unsoaped relatives of man the same result is accomplished in a variety of ways. The production of mucus over the skin of certain slippery fishes and amphibians, for example, makes a constantly renewable jacket of slime, which in sloughing off carries foreign accretions with it. In various other ways also the outermost dead layers of the skin, with such epidermal structures as hair and feathers, are, like soiled clothes, periodically cast off. When a snake “sheds its skin,” although only the outermost part of the epidermis is involved, it emerges bodily clean from the gauzy corneal envelope, which may have become besmirched.
Third, the skin protects the body tissues from excessive loss of moisture. This is a very important function, since in living tissues water plays a major role. Both terrestrial and aquatic organisms are equally dependent upon water. The protoplasm in every cell must maintain a certain degree of fluidity, otherwise it dies. The enclosed universe of the body tissues contains a considerable percentage of fluids. These tissues cannot successfully be subjected to unrestrained evaporation and still carry on the life processes. Moreover, water is the basis for all internal transport of materials, as well as being the great chemical solvent of the substances to be transported. It is also the indispensable agency by means of osmosis for transferring food substances and liquid wastes in and out of the closed body. These precious underground waters of the body are conserved very largely by means of the water-proof blanket of the integument. Its impervious character serves not only to retain the moisture within but also to keep out an unregulated amount of water in the case of submerged animals, such as fishes, whose more delicate underlying tissues would become water-soaked without such protection.
Fourth, the skin or integument acts as an organ of protection in all those animals exhibiting protective coloration, whereby some degree of invisibility, and consequent escape from enemies, is secured by close resemblance to the surroundings. Similarly, so-called warning colors, like the conspicuous black and white markings of the skunk, which serve as “hands-off” signals from its possessor, are integumental modifications, protective in function. Thus “it pays to advertise,” and it is the integument that provides the most available billboards.
It is obvious too that the poisonous skin glands of toads, the eluding slipperiness of certain water animals, and the embarrassing armor and spines of several well-known vertebrates, are all integumental protective devices against the attack of enemies.
Reserve Storage of Food
In the deeper subcutaneous layers of the skin reserve food in the form of fat is stored to be drawn upon, like a savings bank account, in times of need. The fat stored temporarily in the liver and in the muscles is for immediate daily use, whereas what is laid down in the subcutaneous part of the skin may be retained for weeks and months.
The manner of the irregular distribution of fat in cushions and pads forms the basis of those contours in “the outward form and feature” that have pleased the eye of the artist from time immemorial, while providing an anatomical reason for the familiar phrase, “beauty is only skin deep.”
In man stored subdermal fat may constitute as much as twenty per cent of the entire weight of the body. In whales and seals it forms an extensive blanket of considerable thickness, called blubber, that not only serves as food storage, but also acts as a non-conducting retainer of body heat.
The characteristic sexual differences in human body form and contour are largely dependent upon the distribution and manner of the dermal fat upholstery.
Regulation of Heat
Heat is being constantly generated by the oxidation of tissues within the animal body. Coming more abundantly from soft parts like muscles than from hard parts like skeletal organs, it is distributed and equalized by the flowing blood which permeates nearly every part of the body, so that in “warm-blooded” animals a practically constant temperature is maintained. From such an animal heat is lost in three ways: (1) with the expired warm breath; (2) with the expelled excreta; and (3) from the skin.
Every breath of warmed air carries away a certain amount of body heat, for cold air that is drawn into the lungs is warmed at the expense of the body before it is expired.
The excreta, both urine and feces, are kept at body temperature until expulsion, when a loss of heat occurs. Probably nine-tenths of the heat loss of a warm-blooded animal like a mammal, however, is through the skin. Regulation of bodily heat consequently is very largely an integumental function.
The skin effects the regulation of the loss of heat in two ways, one physiological and the other physical. Physiological regulation is brought about by the expansion, or relaxation, and the contraction of the skin and the walls of the capillaries contained therein. When exposed to cold air the skin tends to contract, sometimes to the point of forming “goose flesh,” with the result that the capillaries carrying the warm blood are reduced to a smaller size and buried somewhat from the surface. The amount and degree of cold to which the circulating blood is exposed is thus diminished, and there is a lessened loss of heat. In warm air the skin relaxes, affording the capillaries, now more exposed and with walls more expanded, an opportunity to permit greater loss of heat from the blood.
Physical regulation is accomplished by the evaporation of sweat which is constantly being excreted from the mammalian skin, even though it does not always appear in visible drops. Heat is universally required for the physical process of changing a liquid into a gas. During the evaporation of sweat the necessary heat is abstracted from the body through the skin and is thus eliminated.
Loss of heat from the body is further controlled by the fact that parts of the integument, like the blubber of a whale, serve as a non-conducting blanket to hold in the generated heat, making life endurable even in icy waters. “Cold-blooded” animals do not specialize in dermal fat.
A film of oil produced by the sebaceous glands in the mammalian integument serves the same heat-retaining function. When Gertrude Ederle swam the English Channel, she was generously greased all over in order to meet the unusual thermal conditions of that famous adventure.
In birds the dissipation of bodily heat is regulated through a covering of adjustable feathers by means of which a blanket of warmed air is retained next to the skin. The thickness of this blanket of warm air can be adjusted with meticulous nicety and almost instantaneously by fluffing the feathers. Thus, an English sparrow on a hot summer’s day is streamlined with its feathers hugged tight down close to the body, but in cold weather it assumes adequate underwear by fluffing out its feathers and so surrounding itself with a more generous layer of body-insulating air.
The same result is accomplished more awkwardly in the case of civilized man by means of clothing, which in reality is nothing more than extra layers of non-conducting artificial integument added to that which nature has provided. Skin and clothes in themselves are not warm. They are simply devices for retaining heat generated within. Even the finest sealskin cloak thrown over the marble shoulders of the famous statue of Venus de Medici would not warm up that attractive work of art, whatever may have been the probable temperature of the original Venus.
The most universal of all the senses, the great confirmatory sense of touch, has its receptors located in the skin. The allied senses of pressure, temperature, and pain are also referable for the most part to integumentary nerve endings. Even the chemical senses of taste and smell, which occupy the neighboring mucous membrances of the nose and mouth cavity of higher forms, are still found on the outside of the body in the skin of the lower aquatic vertebrates.
Among mammals the sweat glands supplement the kidneys in removing waste products from the blood. The mammalian skin has been referred to quite appropriately as an “unrolled kidney,” since each sweat gland in the skin, with its accompanying capillaries, is a complete kidney apparatus in miniature. Whenever the activity of the sweat glands is accelerated by exercise, heat, or diuretic and diaphoretic drugs, such as aspirin, caffein, or pilocarpin, there is less work for the kidneys to do.
The constant shedding of corneal material from the surface of the vertebrate skin also may be regarded as a kind of excretion from the integument.
The most notable example of the integument functioning as an organ of secretion is found in mammals, whose mammary glands, which are a very specialized form of the integument, develop as an indispenable part of the reproductive apparatus of these vertebrates.
There are also present in the mammalian skin, associated with hairs, a great number of sebaceous glands which secrete an oily substance that tends to spread over the skin, rendering it supple and more or less resistant to soaking by water and to loss of heat. In fishes and amphibians the mucous glands, already mentioned, are also important organs of secretion.
Other instances of the integument functioning as an organ of secretion could be cited, particularly among invertebrates, as, for example, the “crust” of the crustaceans, which is a product of the hypodermis, or invertebrate skin.
The moist skin of the amphibians accomplishes to a remarkable degree the exchange of gases which constitutes the process of respiration. Cutaneous arteries, for example, supplying the skin of a frog, are larger than the pulmonary arteries that go to the lungs. Even in man the skin supplements the work of the lungs.
The gills of water-dwelling animals may be regarded morphologically as extensions of the skin, as are also the tracheae, or breathing tubes of insects.
The cilia and flagella by means of which microscopic aquatic forms move about are derivatives of the outer envelope of these skinless animals, while in the diversified group of the arthropods, which include much over half of all known kinds of animals, locomotion is accomplished by lever-like appendages, the actuating muscles of which are attached to the inside of the integumentary exoskeleton. The wings of insects are entirely integumentary.
Among vertebrates the fins of fishes and the wings and tail feathers of birds that are essential to locomotion, are also integumentary in origin. The skin takes a conspicuous part too in the wings of bats, and in the flying mechanism of all gliding animals, such as flying squirrels, flying lemurs, and the “flying dragon” (Draco) of India, as well as in the wings of the extinct pterodactyls, which had a web of skin stretched between the fourth finger and the sides of the body. Web-footed animals like ducks and frogs depend upon the skin between the toes to enable them to paddle in the water.