a greyish-yellow pultaceous and semi-fluid material having a peculiar stale odour.  It is probable that the decomposition of the contents is the result of the presence of bacteria, and that from the surgical point of view they should be regarded as infective.  A sebaceous cyst may remain indefinitely without change, or may slowly increase in size, the skin over it becoming stretched and closely adherent to the cyst wall as a result of friction and pressure.  The contents may ooze from the orifice of the duct and dry on the skin surface, leading to the formation of a sebaceous horn (Fig. 100).  As a result of injury the cyst may undergo sudden enlargement from haemorrhage into its interior.

Recurrent attacks of inflammation frequently occur, especially in wens of the face and scalp.  Suppuration may ensue and be followed by cure of the cyst, or an offensive fungating ulcer forms which may be mistaken for epithelioma.  True cancerous transformation is rare.

Wens are to be diagnosed from dermoids, from fatty tumours, and from cold abscesses.  Dermoids usually appear before adult life, and as they nearly always lie beneath the fascia, the skin is movable over them.  A fatty tumour is movable, and is often lobulated.  The confusion with a cold abscess is most likely to occur in wens of the neck or back, and it may be impossible without the use of an exploring needle to differentiate between them.

[Illustration:  FIG. 99.—­Multiple Sebaceous Cysts or Wens; the larger ones are of many years’ duration.]

Treatment.—­The removal of wens is to be recommended while they are small and freely movable, as they are then easily shelled out after incising the overlying skin; sometimes splitting the cyst makes its removal easier.  Local anaesthesia is to be preferred.  It is important that none of the cyst wall be left behind.  In large and adherent wens an ellipse of skin is removed along with the cyst.  When inflamed, it may be impossible to dissect out the cyst, and the wall should be destroyed with carbolic acid, the resulting wound being treated by the open method.

#Moles.#—­The term mole is applied to a pigmented, and usually hairy, patch of skin, present at or appearing shortly after birth.  The colour varies from brown to black, according to the amount of melanin pigment present.  The lesion consists in an overgrowth of epidermis which often presents an alveolar arrangement.  Moles vary greatly in size:  some are mere dots, others are as large as the palm of the hand, and occasionally a mole covers half the face.  In addition to being unsightly, they bleed freely when abraded, are liable to ulcerate from friction and pressure, and occasionally become the starting-point of melanotic cancer.  Rodent cancer sometimes originates in the slightly pigmented moles met with on the face.  Overgrowths in relation to the cutaneous nerves, especially the plexiform neuroma, occasionally originate in pigmented moles.  Soldau believes that the pigmentation and overgrowth of the epidermis in moles are associated with, and probably result from, a fibromatosis of the cutaneous nerves.