Glands are cellular structures that produce either a secretion or an excretion. In addition to the integumentary glands considered in this chapter, there are other glands that open on the mucous membranes lining the internal passage-ways, and still others within the body which, in consequence of having lost their ducts, depend upon blood vessels for the disposal of their products.
All of the integumentary glands of vertebrates take their origin in the Malpighian layer of the epidermis. They may consist of single cells which have gone farther than their ectodermal neighbors in glandular specialization, or they may be composed of groups of similar cells that join in the common enterprise of producing some kind of substance which is, or is not, of use to the organism. In the former case it is a secretion, and in the latter, an excretion.
Glands such as the sebaceous hair glands are called holocrine because in the production of secretions individual cells are used up, extruded with their secretions, and replaced by new cells. Another type, like sweat glands for example, is merocrine in character, that is, the glands continue to elaborate secretions without fatal results to their structural units.
The simple one-celled glands of lower forms, such as the mucous glands in the hypodermis of an earthworm, have at first a surface exposure to the outside world. As glandular needs increase with the enlargement of the body, and the amount of available outside surface becomes inadequate, they push down into the underlying corium, thus adding enormously to the total secreting area without taking up any more room at the surface, just as large bays and inlets increase greatly the actual extent of the coast line between two points as a crow flies.
Many-celled epidermal glands which occur in land forms higher up in the scale are either tubular or alveolar (Fig. 151), and may be either simple, branched, or compound. The amount of space that a compound gland occupies at the surface is relatively small, being represented simply by a tiny pore for the escape of the secretion produced.
Invertebrate Skin Glands
Representatives from nearly every phylum of invertebrates exhibit integumentary glands of various sorts that serve a variety of purposes. The Cnidaria among coelenterates receive their name from the wide-spread occurrence of glandular stinging cells, or nematocysts, in the ectoderm, by means of which small prey is paralyzed, and the attacks of enemies probably are warded off.
Sedentary animals in some instances may gain anchorage by glandular activity. Thus, the cement glands of barnacles enable these curious crustacean cousins of the crabs to stand on their heads, securely fastened within their protective shells, in which position they can tranquilly kick food into their mouths in safety.
Many mollusks also, for example mussels, attach themselves to some solid foundation by the secretion of tough byssal threads from a byssal gland. Even microscopic rotifers, as they inch along, manipulate their tiny bodies by the aid of a sticky tail gland, while the lacquered cocoon in which an earthworm deposits its underground eggs is secreted by the glandular clitellum.
Many insects produce glandular secretions. The defensive odor of “stink bugs,” the protective millinery of woolly aphids, the poisoning or irritating power of myriapods, spiders, and brown-tail moths, as well as the thread and web spinning of caterpillars, are all due to the activity of hypodermal glands. Anyone who has picked up a fat-bodied blister beetle (Meloe) will remember the acrid yellow “elbow grease” that exudes glandular unfriendliness from its joints. Bee’s wax is another product of invertebrate integumentary glands.
The almost universal epidermal glands of fishes are superficial one-celled mucous glands, which are widespread both over the surface of scales, and wherever naked skin occurs. They are supplemented by two kinds of less common glandular cells, namely, granular gland cells, which are especially abundant throughout the epidermis of cyclostomes, and more deeply lying beaker cells that frequently extend from the Malpighian layer all the way to the surface. All three of these kinds of glands contribute to render fishes slippery and hard to grasp. Doubtless too by lubrication they may facilitate to a certain extent the passage of these submarines through the water, also effecting the constant removal of foreign substances that may adhere to their bodies.
The African lungfish, Protopterus, has skin glands that secrete a varnishlike cocoon in which the animal aestivates, buried in the mud, thus surviving the dry season.
Pterygopodial glands, associated with the pelvic “claspers” of male dogfish and other selachians, are multicellular mucous glands having to do with copulation.
Deep-sea fishes, that live in a world of darkness where no ray of sunlight can penetrate, are in many instances equipped with glandular integumentary organs of considerable complexity, which produce light. These luminescent organs (Fig. 152) are practically the only many-celled glands in the skin of fishes. They are usually accompanied in those species possessing them by enormously large eyes adapted for catching the faintest glimmer of luminescence, so that when Diogenes of the Deep Sea fares forth, his lantern may not pass by unnoticed. Other deep-sea fishes, without light-producing organs, are usually entirely blind or have only very degenerate eyes.
Certain fishes, as well as amphibians and reptiles, also have integumentary cells of a glandular nature, called chromatophores, by means of which the color of the body may be modified to conform to the color of the environment in which the animal temporarily finds itself. It has been demonstrated by Parker that the operation of these color changes depends not so much on direct stimulation through the nervous system as upon certain “neurohumors” or hormones produced by ductless glands within the body.
With the exception of the so-called Leydig’s glands found in the larvae of some anurans, one-celled epidermal glands, so characteristic of the fish skin, do not appear in amphibians, being replaced by many-celled alveolar glands which also provide mucus.
One of the functions of the skin glands of both fishes and amphibians, that does not recur as an integumental activity in higher vertebrates, is the production of irritating or poisonous substances as a means of defense against enemies. In fishes such poison glands are usually at the base of puncturing spines or sharp fin rays, but in amphibians they are more generally distributed over the body. Toads, for example, are usually left alone on account of the noxious secretions from their skin glands. The glandular rings that alternate with the tiny embedded scales of the blind caecilians are equipped not only with many-celled mucous glands, characteristic of amphibians, but also with peculiar giant poison glands.
Another function of epidermal glands is shown by tree frogs (Fig. 153) and certain salamanders which have glandular feet that enable them to stick to vertical surfaces, and by some male frogs that are unique in having glandular thumbs swollen during the breeding season, making it possible for them to saddle on to the slippery backs of the females until the extrusion of the sperm and eggs is accomplished (Fig. 154).
Epidermal glands are much reduced in reptiles and birds, and whenever they do appear are quite localized. For instance, from neck to tail down the long back of an alligator there is a crowded row of degenerate glands between the first and second row of scales on either side of the midline, the use of which has not been determined. On the underside of the lower jaw also there is a pair of evertible glandular structures that during the mating season give forth a strong musky odor which probably has something to do with the sexual psychology of these animals.
Similar odoriferous glands occur in other reptiles. They are a most notable possession, for example, of the “stink-pot” turtle, whose scientific name, Aromochelys odorata, is almost as descriptive as its common name. Odor glands are located particularly about the cloacal opening of copperheads and certain other snakes.
The so-called femoral “glands” of male lizards, extending in a row along the inside of each hind leg from knee to cloaca like a row of tiny portholes (Fig. 155), produce a dry gummy secretion which hardens into short spines, or “teeth” (Fig. 156), that are useful as a holdfast gripping device during copulation.
The uropygial, or preening glands, are best developed in water birds, and are reported as being odoriferous during sexual activity, which suggests that their ancestral function was sexual allurement, although their chief use now has come to be that of supplying pomatum for use in preening. They are paired structures usually with a single outlet from which the bird squeezes out the greasy secretion with its beak when dressing the feathers. In ducks and pelicans there are several ducts, instead of a single opening, that allow the oily secretion to escape.
Aside from this curious uropygial gland at the base of the tail, the only other integumental glands found in birds are oil-glands in the external ear passages of certain gallinaceous birds, like the European capercaillie (Tetrao urogallus), and the American turkey.
Integumental glands reach their greatest variety and differentiation in the mammalian skin. Never unicellular, they are either tubular or alveolar in character.
Sweat glands are the most common and generally distributed of mammalian tubular glands. Dr. Oliver Wendell Holmes, in his delightful lectures to Harvard medical students, likened sweat glands to “fairies’ intestines.” Each one is an elongated tube, the walls of which are composed of cells (Fig. 157). The deeper glandular portion is usually coiled up to occupy a minimum of space, while the outermost part, that serves as a duct and opens at the surface with a funnel-shaped pore, often spirals like a corkscrew as if it found difficulty in penetrating the compacted outer corneal layer of the skin. Although originating in the epidermis like all other integumentary glands of vertebrates, sweat glands by a process of growth push deep down into the corium where their terminal coiled parts come into intimate contact with the capillaries, making possible the extraction of sweat from the blood.
In a healthy man the fluid sweat, visible and invisible, amounts to a daily loss of from one to five pints, and in extreme instances to as much as two per cent of the entire body weight. It has been estimated that there are two and one half million sweat tubules in an average human skin each with a separate pore just at the limit of visibility to the naked eye. Stated more graphically, there are about four hundred printed words on this page, and in the entire book approximately one eighth as many words as sweat glands in the author’s skin, all of which it might be added have been exercised in laboriously arranging the letters as they stand.
Sweat glands in the human skin are not equally distributed, being more numerous on the palms and soles than elsewhere, and attaining a notably greater size under the arm pits.
Racial differences in the abundance of sweat glands have been observed, as shown in the following counts per square centimeter on the finger tips: American, 558; Filipino, 653; Negrito, 709; Hindu, 738. Negroes can endure the tropics better than the whites because they are more generously supplied with sweat glands. There is so much individual variation in whites with respect to the ability and ease to perspire, that army authorities of England and the Netherlands take cognizance of this fact and do not detail for service in their tropical colonies those men who are unable to sweat freely.
In mammals that are abundantly clothed with hair, the sweat glands become crowded out or localized in restricted areas. Thus, in cats, rats, and mice these glands are confined to the soles of the feet; in bats, to the sides of the head; in rabbits, to an area around the lips; in deer, to the region at the base of the tail; in shrews, to a line down either side of the body; in ruminants, to the muzzle and the skin between the toes; while in the hippopotamus sweat glands occur only on the ears, which are the parts of the body of these semi-aquatic monsters most exposed to air. Sweat glands are wanting in Echidna, some insectivores and the water-dwelling sirenia and cetacea.
The male of the giant kangaroo is named Macropus rufus because its sweat is reddish in color, and the African antelope, Cephalophus pygmaeus, is said to produce albuminous sweat that forms a bluish lather. It will be remembered that in the “horse and buggy days,” when a harness chafed an overheated horse, white lather appeared because of albumin present in the sweat.
The ciliary (cilium, eyelash) glands of Moll, that are the center of trouble whenever a sty is formed, are modified sweat glands.
While tubular glands are confined to mammals, alveolar glands of various kinds occur not only in the mammalian skin but also in the skin of other land vertebrates as has already been noted.
The most universally distributed of the mammalian alveolar glands are sebaceous glands which produce an oily secretion (sebum), usually in connection with hairs (Fig. 158), although they are also found independent of hairs at the edge of the lips and about the genitalia, where the skin passes over into the mucous membrane. On the tip of the nose, particularly the bulbous noses of the indulgent, middle-aged type, the openings of the free sebaceous glands may be seen as tiny pits, marking the locality of ancestral hairs that have been lost in the evolutionary shuffle.
Sebaceous hair-follicle glands number frequently two or three to each hair, opening into the pocket from which the hair shaft projects, rather than directly upon the surface. The size of sebaceous glands is not in relative proportion to the size of the hairs with which they are associated. They frequently become enlarged in the absence of hairs, which suggests that their primary function is not so much concerned with oiling the dry hair, as is commonly assumed, as with providing the surface of the skin with a filmy coating of oil.
The two-toed sloth, Choloepus; the Cape mole of South Africa, Chrysochloris; the scaly anteater, or “pangolin,” Manis; and the water-inhabiting sirenians and cetaceans, already cited for their lack of sweat glands, are equally deficient in sebaceous glands, although the first two are abundantly hairy animals.
Other Alveolar Integumentary Glands
Along the edge of each eyelid there is a line of modified sebaceous glands, called tarsal or Meibomian glands, which produce an oily film across the exposed part of the eyeball between the edges of the eyelids and the eyeball itself, a film that retreats and advances with every wink. This oil seal ordinarily retains a film of tears which constantly moistens the surface of the eyeball. In the case of weeping the oily dam is broken by the flood pressing from behind, and tears trickle down the cheeks (Fig. 159).
Another kind of integumentary alveolar glands is associated with sexual activity in various mammals. These structures should not be confused in any way with the so-called primary “sex glands” which produce eggs and sperm, since they are derivatives of the epidermis having usually only a lubricating function in connection with the genital organs. Examples of such glands are the preputial and vulval glands in the male and female respectively, and scent glands which act as an allurement to the opposite sex. These latter glands are usually located near the anus, as in the musk deer, beaver, civet cat, dog, fox, and skunk. The scent glands between the toes of goats, whatever their effect on humankind, may have a meaning for the goats themselves.
In the external ear passages of most vertebrates are found the ceruminous or wax glands, which in form show affinities with the tubular type but in function resemble sebaceous glands, since they produce a gummy or waxy secretion more like oil than sweat. They serve to arrest dust particles, and to discourage adventurous crawling insects that might otherwise be tempted to invade the sacred precincts of the ear, a function not so apparently needful in the case of man as of a dog sleeping in the sunshine with a halo of busy insects buzzing around its head.
Of paramount importance in the life of mammals are the milk glands which characterize this order of vertebrates. The mammary glands, although resembling the necrobiotic sebaceous glands in structure, are intermediate in method of secretion between sweat and sebaceous glands. Their derivation in all probability should be traced not to sweat or sebaceous glands, but to some common ancestral type less differentiated than either. Their activity is periodic instead of continuous and, for the most part, finds expression only in the female.
The mammary apparatus includes not only the mammary glands themselves, but also the elevated nipples, that furnish an outlet for the glands, and breasts, or mammae, which are integumentary swellings produced by the localized presence of the enlarged mammary glands in the skin (Fig. 160).
The normal number of nipples varies from two in the horse, bat, whale, elephant, and man, to twenty-five in the opossum, Didelphys henseli (Fig. 161). Carnivores usually have six or eight; rodents, two to ten; pigs, eight to ten; and ruminants, four. In those species where several young are born in a litter there is a corresponding provision in the number of nipples.
The number of ducts per nipple that drain the glands is also subject to considerable variation. In mice, ruminants, and insectivores, there is only one; in the pig, two or three; and in carnivores, three to six. In man there is a cluster of about twenty separate ducts opening into each nipple.
Milk, which is secreted by these glands, is the natural food of young mammals. It is composed of water derived from the blood stream, butter-fat, milk-sugar, albumin, and certain salts in varying proportions. Albumin in milk favors rapid growth of the young. The milk of a reindeer, which lives in a habitat where it is desirable for the young to attain enough maturity to care for themselves as soon as possible, has a large albumin content. The guinea pig, whose milk contains approximately ten per cent of albumin, doubles its weight after birth in six days, while the human infant feeding upon milk with less than two per cent of albumin, requires 180 to 200 days in which to double its weight. Other factors, in addition to the kind of milk, enter into this difference in rapidity of growth, but the fact is apparent that different kinds of milk are adapted in nature to different requirements.
Mammary glands may develop in various places on the mammalian skin. Instances are recorded in medical literature of the abnormal occurrence in human beings of mammae under the arm pits, on the shoulders, and even upon the hips. Their normal distribution in different species of mammals, however, holds a definite relation to the accessibility of the nipples to the suckling young. Thus in carnivores and swine, which attend to their nurslings while lying flat on the side, the nipples are arranged in two rows along the ventral side of the body. Those quadrupeds which habitually stand while nursing their young usually have the nipples in a protected situation between the legs, either anterior as in elephants, or posterior as in cattle and horses, while the nurslings brace themselves on stiltlike legs as they drain the maternal udders. Arboreal animals that hold their “babes in arms” have conveniently located pectoral nipples. Mankind, with a probable arboreal ancestry, also has pectoral nipples. The grotesque sea-cows, which enfold their single offspring between their anterior flippers and “stand” with the head elevated out of the water, likewise have pectoral nipples. This circumstance has no doubt contributed to the mermaid myths among sailors who have chanced to glimpse at a distance the intimate family life of these rare strange creatures.
Unlike the young of the sea-cow, the baby whale is a marine “trailer,” for the maternal nipples from which it secures milk while navigating the high seas are situated far posterior on either side of the sexual orifice entirely out of the mother’s sight, in pockets which fit over the snout of the baby whale in such a way as to minimize the chance of the milk becoming too much diluted with salt water.
The opposite extreme to the position of nipples in the cetaceans is found in the topsy-turvy bats and flying lemurs (Fig. 162), whose offspring literally “cling for dear life” to the breasts of their aerial mothers, the accessible nipples of which are axillary in location, or under the arm pits.
The development of the mammary apparatus is initiated by the formation of an epidermal ridge down either side of the belly from axilla to groin, called the milk-line stage (Fig. 163). It appears in man near the beginning of the second fetal month when the embryo is still less than half an inch in length. The milk-line stage is succeeded by the milk-hill stage (Fig. 163), which results when the epidermal ridge of the milk-line becomes absorbed except for a beadlike row of remnants, each one of which marks the possible location of a future mammary gland. These tiny milk-hills are compact masses of cells that later sink down into the underlying tissue, leaving no visible trace of the developing mammary apparatus. The double row of depressed “hills,” thus embedded in the corium, becomes the milk-feld stage. As the leveled hills of the milk-feld stage sink deeper and become valleys, there is formed where the hills formerly were a double row of pits along the lateral walls of the belly, converging posteriorly from the anterior region. This represents the milk-pocket stage (Fig. 164c). It is the cells that line the sides and bottom of these milk pockets which directly give rise to the mammary glands.
In forms that do not have two complete rows of nipples, some of the pockets fail to develop. In man, for example, it is the fourth pair of embryonic milk pockets at the anterior end that become the permanent mammae.
The final differentiation of the mammary apparatus takes place when the milk-pocket stage is succeeded by the nipple stage. According to the two ways of their formation true and false nipples are distinguished. Among marsupials, rodents, and primates the floor of the milk pocket, into which the ducts of the mammary glands open, elevates, carrying the elongated ducts of the milk glands with it, thus causing them to open at the tips of the true nipples. In the case of false nipples, which characterize pigs, carnivores, horses, and ruminants, the floor of the milk pocket with its ducts remains unelevated, while the margins of the pocket pull up all around to form a hollow nipple. There is thus formed a secondary tube or elongation upward of the milk pocket itself, called the milk canal, into which the mammary glands pour their secretion, to be pumped to the tip of the false nipple.
The mammary apparatus develops equally in both sexes up to the time of puberty, when it degenerates in the male and becomes potentially functional in the female. The male may produce milk, as in the primitive monotreme Echidna, and also in exceptional instances among higher mammals, even in man. Such abnormal behavior is termed gynecomastism.
In man as well as other mammals, extra nipples (hyperthelism) (Fig. 165) not infrequently occur, as also do extra breasts (hypermastism) (Fig. 166). Such persistent embryonic relics, particularly in the case of hyperthelism, occur quite as often in males as in females. Usually these supernumerary parts are arranged along the vanished embryonic milk-line.
The mammary apparatus of monotremes presents many exceptions to that of other mammals. Instead of being alveolar in form the mammary glands are branched-tubular, producing a sort of nutritious sweat instead of the usual milk (Fig. 167). No nipples are present, tufts of hair serving in their stead. The young monotreme does not have muscular lips and is further handicapped by a horny beak. In consequence it is quite unable to suckle, so it licks the nutritious sweat from the makeshift tufts of hair on the mother’s breast, with its protrusible tongue. The skin on the belly of Echidna forms a temporary pouch, or incubatorium, that surrounds the mammary area while the young are being cared for. Into this pouch is deposited the single leathery-shelled egg, which soon hatches into a very premature helpless embryo, there to undergo the preliminary perils of early development which other mammals accomplish in greater safety within the protective uterus of the mother.
Even an incubatorium is lacking in Ornithorhynchus, which broods its egg in a hole in the ground that serves as a nest. The ventral mammary area is depressed, as in the milk-pocket stage of development, and from the depression tufts of hair project, which serve as nipples. It is probable that gynecomastism occurs in both Echidna and Ornithorhynchus with both parents sharing in the feeding of the young.
Most marsupials regularly possess a permanent pouch for carrying the immature young, although between incubations it may decrease somewhat in size. In the Didelphyidae to which the opossum belongs, the marsupium, or pouch, is mostly wanting or represented by two insignificant folds of skin. True nipples are present within the marsupium and typical milk glands supply real milk. The nipples project, however, only during lactation. At other times, like a disappearing gun, they retract within a surrounding pit in the skin. The young marsupial retains its hold on the nipple within the enveloping edge of the marsupium by means of powerful muscles around its mouth. At first the young animal is so helpless that it can only stay passively attached to the nipple, and it is necessary for the mother to pump the milk down its waiting throat by means of the contraction of breast muscles. Later, as it becomes able to use its own muscles and nerves, it feeds itself as ordinary mammalian youngsters do, by its own efforts.
Young marsupials, even after they attain considerable size and have gained some degree of independence, are glad to retreat into the maternal marsupium on the approach of danger.
In placental mammals the marsupial pouch disappears, since the fetus is cradled in greater safety within the uterus until at birth a stage of development has been reached that makes bodily protection on the part of the mother less imperative. There are, however, certain dim vanishing reminders of a marsupial pocket around the nipple even in the human embryo, for at about the beginning of the second fetal month, when the future mammary apparatus is being set up, there develop around the milk-hills transitory epithelial thickenings that possibly represent the last remnants of an ancestral marsupium (Fig. 168).
The long period of obligatory milk-feeding among the higher mammals not only allows ample time for more extended development of the young but is also a necessary preliminary to the invaluable process of learning through prolonged association with the parent. This opportunity is denied to all those unmothered kinds of animals that are born equipped with instincts which make it unnecessary for them to learn how to live. The dominant mammals are fortunate because they must work out their salvation by learning how, and have been endowed with the capacity to do it.