This is a very common affection, especially between the ages of fifteen and thirty years. The seat of the disease is the sebaceous follicles or oil-glands of the skin. The eruption consists in pimples scattered over the face, neck, back, and chest. The inflammation of each follicle may run its course in three or four days, or may continue for a week or ten days. When the inflamed part becomes indurated, or hardened, the inflammation may continue for several weeks. Several varieties of the disease are observed; that just described is the most common. Another form consists in obstruction of the outlets of the sebaceous glands, producing what are sometimes termed flesh-worms, or grubs. This form of acne is indicated by little black specks, seen upon different parts of the face, but chiefly upon the skin of the nose. Each speck makes the obstructed outlet; and if pressure is made on either side, something having the appearance of a small grub may be pressed out. Upon careful examination, this so-called grub proves to be a mass of hardened sebaceous matter, or sebum, which has assumed its grub-like form by being pressed through the small mouth of the follicle. The black speck, giving to this little cylinder of fat the appearance of a head, is simply a small accumulation of dirt. The technical term for one of these little masses is comedo. When examined under a microscope, these are often found to contain a whole family of parasites, male, female, and their numerous progeny.

In Plate VIII may be seen an excellent representation of these parasites, which rejoice in the title of demodex folliculorum. It is not probable that this parasite gives rise to the disease, but rather that the distended follicle furnishes an agreeable home for this insect, which is closely related to the acarus scabieir, or itch mite. In another form of acne, in which t he nose and the adjoining portions of the cheek are chiefly involved, in addition to the pimples described there is intense congestion and redness of the parts, due to enlargement of the blood vessels which are sometimes so much distended as to be distinctly visible. This form of the disease is termed acne rosacea. In still another form of the affection the inflammation is chiefly confined to the roots of the hairs. This form is sometimes known as barbers itch. The chief causes of acne are erroneous dietetic habits. People suffering with acne can bring on an attack at any time by the use of rich pastry, fried food, and large amounts of sugar or sweet food, etc. Doughnuts, griddle cakes, cheese, hot bread, preserves, candies, and similar dietetic abominations, are very active causes of different forms of this affection. Acne rosacea is very frequently the result of using alcoholic liquors in some form, on which account it is sometimes termed, when seen in persons addicted to drinking, the "rum-blossom." Acne is sometimes the result of debilitating habits, particularly secret vice in young persons, though it should be by no means supposed that every young person affected with this disease is addicted to secret vice.