Resulting from of irritation or another cause, a cavity which ought naturally to contain a little fluid becomes distended, or the natural outlet from the cavity becomes blocked. Wens are caused by the blockage of the outlet from sebaceous glands in the skin, so that an accumulation of fatty matter takes place. RANULA is a clear swelling under the tongue, due to a collection of saliva in consequence of an obstruction to a salivary duct. Cysts in the breasts are, in many cases, the result of blockage in milk ducts, due to inflammation; these should be assessed to exclude cancer (see BREASTS, DISEASES OF). Cysts also form in the kidney as a result of obstruction to the free outflow of the urine.

Of these, the most important are the huge cysts that originate in the OVARIES. The cause is doubtful, but the cyst probably begins at a very early period of life, gradually enlarges, and buds off smaller cysts from its wall. The contents are usually a clear gelatinous fluid. Very often both ovaries are affected, and the cysts may slowly reach a great size – often, however, taking a lifetime to do so.

A similar condition sometimes occurs in the KIDNEYS, and the tumour may have reached a great size in an infant even before birth (congenital cystic kidney).

Dermoid cysts are small cavities, which also originate probably early in life, but do not reach any great size until fairly late in life. They appear about parts of the body where clefts occur in the embryo and close up before birth, such as the corner of the eyes, the side of the neck, and the middle line of the body. They contain hair, fatty matter, fragments of bone, scraps of skin, and even numerous teeth.

are produced in many organs, particularly in the liver, by a parasite which is the larval stage of a tapeworm found in dogs. They occur in people who keep dogs and which may contaminate their food. (See TAENIASIS.)