Sebaceous Glands

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 hair, 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.