(FIGURE 2.284. The human skin in vertical section (from Ecker), highly magnified, a horny layer of the epidermis, b mucous layer of the epidermis, c papillae of the corium, d blood-vessels of same, ef ducts of the sweat-glands (g), h fat-glands in the corium, i nerve, passing into a tactile corpuscle above.)
The observations of the comparative anatomist are in complete accord with this view. He tells us that large numbers of the lower animals have no nervous system, though they exercise the functions of sensation and will like the higher animals. In the unicellular Protozoa, which do not form germinal layers, there is, of course, neither nervous system nor skin. But in the second division of the animal kingdom also, the Metazoa, there is at first no nervous system. Its functions are represented by the simple cell-layer of the ectoderm, which the lower Metazoa have inherited from the Gastraea (Figure 1.30 e). We find this in the lowest Zoophytes—the Gastraeads, Physemaria, and Sponges (Figures 2.233 to 2.238). The lowest Cnidaria (the hydroid polyps) also are little superior to the Gastraeads in structure. Their vegetative functions are accomplished by the simple visceral layer, and their animal functions by the simple cutaneous layer. In these cases the simple cell-layer of the ectoderm is at once skin, locomotive apparatus, and nervous system.
(FIGURE 2.285. Epidermic cells of a human embryo of two months. (From
Kolliker.))
When we come to the higher Metazoa, in which the sensory functions and their organs are more advanced, we find a division of labour among the ectodermic cells. Groups of sensitive nerve cells separate from the ordinary epidermic cells; they retire into the more protected tissue of the mesodermic under-skin, and form special neural ganglia there. Even in the Platodes, especially the Turbellaria, we find an independent nervous system, which has separated from the outer skin. This is the "upper pharyngeal ganglion," or acroganglion, situated above the gullet (Figure 2.241 g). From this rudimentary structure has been developed the elaborate central nervous system of the higher animals. In some of the higher worms, such as the earth-worm, the first rudiment of the central nervous system (Figure 1.74 n) is a local thickening of the skin-sense layer (hs), which afterwards separates altogether from the horny plate. In the earliest Platodes (Cryptocoela) and Vermalia (Gastrotricha) the acroganglion remains in the epidermis. But the medullary tube of the Vertebrates originates in the same way. Our embryology has taught us that this first structure of the central nervous system also develops originally from the outer germinal layer.
Let us now examine more closely the evolution of the human skin, with its various appendages, the hairs and glands. This external covering has, physiologically, a double and important part to play. It is, in the first place, the common integument that covers the whole surface of the body, and forms a protective envelope for the other organs. As such it also effects a certain exchange of matter between the body and the surrounding atmosphere (exhalation, perspiration). In the second place, it is the earliest and original sense organ, the common organ of feeling that experiences the sensation of the temperature of the environment and the pressure or resistance of bodies that come into contact.
The human skin (like that of all the higher animals) is composed of two layers, the outer and the inner or underlying skin. The outer skin or epidermis, consists of simple ectodermic cells, and contains no blood-vessels (Figure 2.284 a, b). It develops from the outer germinal layer, or skin-sense layer. The underlying skin (corium or hypodermis) consists chiefly of connective tissue, contains numerous blood-vessels and nerves, and has a totally different origin. It comes from the outermost parietal stratum of the middle germinal layer, or the skin-fibre layer. The corium is much thicker than the epidermis. In its deeper strata (the subcutis) there are clusters of fat-cells (Figure 2.284 h). Its uppermost stratum (the cutis proper, or the papillary stratum) forms, over almost the whole surface of the body, a number of conical microscopic papillae (something like warts), which push into the overlying epidermis (c). These tactile or sensory particles contain the finest sensory organs of the skin, the touch corpuscles. Others contain merely end-loops of the blood-vessels that nourish the skin (c, d). The various parts of the corium arise by division of labour from the originally homogeneous cells of the cutis-plate, the outermost lamina of the mesodermic skin-fibre layer (Figure 1.145 hpr, and Figures 1.161 and 1.162 cp).
In the same way, all the parts and appendages of the epidermis develop by differentiation from the homogeneous cells of this horny plate (Figure 2.285). At an early stage the simple cellular layer of this horny plate divides into two. The inner and softer stratum (Figure 2.284 b) is known as the mucous stratum, the outer and harder (a) as the horny (corneous) stratum. This horny layer is being constantly used up and rubbed away at the surface; new layers of cells grow up in their place out of the underlying mucous stratum. At first the epidermis is a simple covering of the surface of the body. Afterwards various appendages develop from it, some internally, others externally. The internal appendages are the cutaneous glands—sweat, fat, etc. The external appendages are the hairs and nails.
The cutaneous glands are originally merely solid cone-shaped growths of the epidermis, which sink into the underlying corium (Figure 2.286 1). Afterwards a canal (2, 3) is formed inside them, either by the softening and dissolution of the central cells or by the secretion of fluid internally. Some of the glands, such as the sudoriferous, do not ramify (Figure 2.284 efg). These glands, which secrete the perspiration, are very long, and have a spiral coil at the end, but they never ramify; so also the wax-glands of the ears. Most of the other cutaneous glands give out buds and ramify; thus, for instance, the lachrymal glands of the upper eye-lid that secrete tears (Figure 2.286), and the sebaceous glands which secrete the fat in the skin and generally open into the hair-follicles. Sudoriferous and sebaceous glands are found only in mammals. But we find lachrymal glands in all the three classes of Amniotes—reptiles, birds, and mammals. They are wanting in the lower aquatic vertebrates.
(FIGURE 2.286. Rudimentary lachrymal glands from a human embryo of four months. (From Kolliker.) 1 earliest structure, in the shape of a simple solid cone, 2 and 3 more advanced structures, ramifying and hollowing out. a solid buds, e cellular coat of the hollow buds, f structure of the fibrous envelope, which afterwards forms the corium about the glands.)
The mammary glands (Figures 2.287 and 2.288) are very remarkable; they are found in all mammals, and in these alone. They secrete the milk for the feeding of the new-born mammal. In spite of their unusual size these structures are nothing more than large sebaceous glands in the skin. The milk is formed by the liquefaction of the fatty milk-cells inside the branching mammary-gland tubes (Figure 2.287 c), in the same way as the skin-grease or hair-fat, by the solution of fatty cells inside the sebaceous glands. The outlets of the mammary glands enlarge and form sac-like mammary ducts (b); these narrow again (a), and open in the teats or nipples of the breast by sixteen to twenty-four fine apertures. The first structure of this large and elaborate gland is a very simple cone in the epidermis, which penetrates into the corium and ramifies. In the new-born infant it consists of twelve to eighteen radiating lobes (Figure 2.288). These gradually ramify, their ducts become hollow and larger, and rich masses of fat accumulate between the lobes. Thus is formed the prominent female breast (mamma), on the top of which rises the teat or nipple (mammilla). The latter is only developed later on, when the mammary gland is fully-formed; and this ontogenetic phenomenon is extremely interesting, because the earlier mammals (the stem-forms of the whole class) have no teats. In them the milk comes out through a flat portion of the ventral skin that is pierced like a sieve, as we still find in the lowest living mammals, the oviparous Monotremes of Australia. The young animal licks the milk from the mother instead of sucking it. In many of the lower mammals we find a number of milk-glands at different parts of the ventral surface. In the human female there is usually only one pair of glands, at the breast; and it is the same with the apes, bats, elephants, and several other mammals. Sometimes, however, we find two successive pairs of glands (or even more) in the human female. Some women have four or five pairs of breasts, like pigs and hedgehogs (Figure 1.103). This polymastism points back to an older stem-form. We often find these accessory breasts in the male also (Figure 1.103 D). Sometimes, moreover, the normal mammary glands are fully developed and can suckle in the male; but as a rule they are merely rudimentary organs without functions in the male. We have already (Chapter 1.11) dealt with this remarkable and interesting instance of atavism.