The peculiarly placed fibres have been supposed, by their contraction, to assist in the flow of sweat along the tubule. In certain cutaneous glands of the frog, of a relatively simple nature, there is evidence that the secretion is ejected from the comparatively large lumen by the contraction of plain muscular fibres in the wall of the gland, or by a contraction of the wall itself, which is contractile without being distinctly differentiated into muscular tissue. And this rather supports the above view, but the matter is at present by no means clear.
The coil of a sweat gland is well supplied with blood- vessels in the form of capillary networks, and nerves have been traced to the tubes; but the exact manner in which these end is not as yet known.
Though present in all regions of the skin (of man), the sweat glands are unequally distributed, being more abundant in some regions, such as the palms of the hand, than in others. In the axilla are glands of very large size, and in these the ducts possess distinctly muscular coats.
The sebaceous glands are appendages of the hairs, and are seated in the corium; their ducts open into the hair follicle at the neck in the case of the larger hair follicles; but in the case of the smaller, or downy hairs, the relative position of the glands and hair follicle is altered, so that the minute hair follicle leads into the duct of the sebaceous gland, which opens directly on the surface. These glands are absent from the palm of the hand, the sole of the foot, and the dorsum of the third phalanges of the fingers and toes, and there are few about the penis. The largest are found about the nose, scrotum, anus, and labia.
Just where the corneous layer abruptly leaves off in the upper part of the hair follicle, a sebaceous gland opens into the cavity of the follicle, on each side of the hair. Each gland consists of a short rather wide duct which divides into a cluster of somewhat flask-shaped alveoli. The basement membrane both in the alveoli and in the duct, is lined with a layer of rather small cubical cells continuous with the layer of perpendicularly disposed cells which form the innermost layer of the outer root sheath, as of the Malpighian layer of the skin generally. This layer of cells leaves a wide lumen both in the alveoli and in the duct; this lumen, however, is occupied not as in other glands with fluid, but with cells. Both alveoli and duct, in fact, are filled with rounded or polygonal cells which may be regarded as modified cells of the Malpighian layer. The whole gland, indeed, is a solid diverticulum of the Malpighian layer.
In the alveoli the cells next to the layer of cells immediately lining the basement membrane, though larger than these, resemble them in so far that each consists of ordinary cell substance surrounding a nucleus of ordinary character. The more central cells are different; their cell substance is undergoing change; numerous granules or droplets, some of them obviously of a fatty nature, make their appearance in them, and the nuclei are becoming shrunk and altered. The cells are manufacturing fatty and other bodies and depositing the products in their own substance, which, however, is not being removed, but is dying. These changes are still more obvious in the cells lying within the duct; the cells as indicated by the breaking up of the nuclei are dead, and the whole of the cell substance has been transformed into the material constituting the secretion of the gland called sebum, which is discharged on to the surface of the skin through the mouth of the hair follicle.
In these sebaceous glands, secretion, if we may continue to use the word, takes place after a fashion different from that which we have hitherto studied. In an ordinary gland the cells lining the walls of the alveoli manufacture material which they discharge from themselves into the lumen to form the secretion, their own substance being at the same time renewed, so that the same cell may continue to manufacture and discharge the secretion for a very prolonged period without being itself destroyed. In a sebaceous gland the work of the cells immediately lining the wall of an alveolus appears limited to the task of increasing by multiplication. Of the new cells thus formed, while some remain to continue the lining and to carry on the work of their predecessors, the rest thrust toward the centre of the alveolus are bodily transformed into the material of the secretion, and during the transformation are pushed out through the duct by the generation of new cells behind them. The secretion of sebum, in fact, is a modification of the particular kind of secretion taking place all over the skin, and spoken of as shedding of the skin. It is chiefly the chemical transformation which is different in the two cases. In the skin generally the protoplasmic cell substance of the Malpighian cells is transformed into keratin; in the sebaceous glands it is transformed into the fatty and other constituents of the sebum.
The so-called “ceruminous glands” of the external meatus of the ear are essentially sweat glands. They are wrongly named, since the fatty material spoken of as “wax” of the ear is secreted not by them but by the sebaceous glands belonging to the hairs of the meatus, or by the general epidermic lining. The ceruminous glands appear at most to supply the pigment which colors the “wax”.
The Meibomian glands of the eyelids, on the other hand, are essentially the sebaceous glands of the eyelashes, the glands of Mohl being in turn sweat-glands.
A hair is a development, in the form of cylinder, of a cap of corneous epidermis surmounting a papilla of the dermis sunk to the bottom of a tubular pit, or involution of the skin, called a hair-follicle. In the upper part of the hair-follicle the walls consist of ordinary skin with all its parts, dermis, Malpighian layer, and corneous layer, the latter as usual of considerable thickness. At some little distance from the mouth of the follicle the corneous layer suddenly ceases, and in the follicle below this the epidermis is represented by the Malpighian layer, now called the outer root-sheath, and two layers of peculiar cells, forming the inner root-sheath, of which the outer is called Henle’s and the inner Huxley’s layer; these may, perhaps, be considered as corresponding to the stratum granulosum lucidum respectively.
The dermis of the wall of the follicle is at the same time developed into an outer layer with bundles of connective tissue disposed chiefly longitudinally, and an inner layer of peculiar nature, the arrangement of which is transverse, and which at least simulates, if it really be not, a muscular transverse coat. Between this dermis of the follicle and the outer root-sheath or Malpighian layer is a very conspicuous definite hyaline basement membrane, so thick that it presents a very easily recognized double contour.
At the bottom of the follicle the dermis of the wall of the follicle is continuous with the substance of the (dermic) papilla, while the outer root-sheath or Malpighian layer, which here becomes extremely thin and reduced to one or two layers, is reflected over the papilla, and there expands again into a mass of cells, which like the cells of the Malpighian layer in the rest of the skin multiply, and by their multiplication give rise to the corneous body of the hair. It is said that in those hairs which possess a medulla the vertically disposed lowermost cells of the Malpighian layer are at the actual summit of the papilla continued upward in the axis of the hair, as the medulla.
The layer of Henle, following the Malpighian layer or outer root-sheath on which it rests, is similarly reflected and forms over the hair a single layer of flat transparent imbricated scales known as the cuticle of the hair; Huxley’s layer, similarly reflected, forms a similar layer of similar scales, but this is considered as belonging to the root-sheath, and is called the cuticle of the root-sheath.
Muscles of the skin: There appear to be two kinds of muscles found in the skin-the voluntary, or striated, and the involuntary. The former are to be detected in the face, beard, and nose, “ascending sometimes obliquely, sometimes vertically, between the hairs and the sebaceous follicles to terminate in the corium” (Biesiadecki).They come from below. The organic or non- striated muscles are more abundant. They occur forming a kind of network in the scrotum. Over the general surface of the skin bands of fibres are detected in connection with the hair follicle, and are called errectores pili. These muscles exist as single fasciculi. 045 to.22 of a millimetre, sometimes on one, sometimes on both sides of the hair follicle, in immediate relation to the sebaceous glands, which they enclose more or less. They run from the corium above to the part of the hair follicle just below the glands, and there end in the inner sheath of the hair follicle. Some authors affirm that bundles go down to the subcutaneous tissue and send off vertical and horizontal branches. Neuman, who is of this opinion, states that bands run above and under the sweat glands, more especially in the axilla. He describes also independent bundles of muscle in the corium quite unconnected with the hair follicles.
