These belong to the outer skin, and commonly open into the follicles of the hairs, but also appear on the free surface of the lips and prepuce, etc., where no hairs exist.
The secretion cannot be collected in great quantity in a normal condition, but, as far as can be made out, it is composed of neutral fat, soap, and an albuminous body allied to casein, and organic salts and water, about 60 per cent.
The secretion contains the remains of numerous epithelial cells which are thrown off from the inner surface of the glands, while they are undergoing a peculiar kind of fatty change. These cells gradually get quite broken down during their sojourn in the gland alveoli, and the secretion is finally pressed out by the band of smooth muscle which usually embraces the gland and squeezes it against the hair fallicle.
Fig. 166. Section of Skin showing the roots of three hairs and two large sebaceous glands (d).
This secretion, the use of which is to lubricate the surface with a fatty material, is cast Off with the desquamated epithelium and the hairs. The Meibomian glands of the eyelids are analogous structures, and are specially elaborated for the lubrication of the ciliary margin. The glands about the prepuce and clitoris are also analogous to the sebaceous glands; in some animals (Castor) they secrete a peculiarly odoriferous material.