An excessive and erratic growth of epithelium is the essential and distinguishing feature of these tumours. The innocent forms are the papilloma and the adenoma; the malignant, the carcinoma or cancer.
Papilloma.—A papilloma is a tumour which projects from a cutaneous or mucous surface, and consists of a central axis of vascular fibrous tissue with a covering of epithelium resembling that of the surface from which the tumour grows. In the papillomas of the skin—commonly known as warts—the covering consists of epidermis; in those growing from mucous surfaces it consists of the epithelium covering the mucous membrane. When the surface epithelium projects as filiform processes, the tumour is called a villous papilloma, the best-known example of which is met with in the urinary bladder. Papillomatous growths are also met with in the larynx, in the ducts of the breast, and in the interior of certain cystic tumours of the breast and of the ovary. Although papillomas are primarily innocent, they may become the starting-point of cancer, especially in persons past middle life and if the papilloma has been subjected to irritation and has ulcerated. The clinical features and treatment of the various forms of papilloma are considered with the individual tissues and organs.
Adenoma.—An adenoma is a tumour constructed on the type of, and growing in connection with, a secreting gland. In the substance of such glands as the mamma, parotid, thyreoid, and prostate, adenomas are met with as encapsulated tumours. When they originate from the glands of the skin or of a mucous membrane, they tend to project from the surface, and form pedunculated tumours or polypi.
Adenomas may be single or multiple, and they vary greatly in size. The tumour is seldom composed entirely of gland tissue; it usually contains a considerable proportion of fibrous tissue, and is then called a fibro-adenoma. When it contains myxomatous tissue it is called a myxo-adenoma, and when the gland spaces of the tumour become distended with accumulated secretion, a cystic adenoma, the best examples of which are met with in the mamma and ovary. A characteristic feature of the cystic variety is the tendency the tumour tissue exhibits to project into the interior of the cysts, constituting what are known as intracystic growths. They are essentially innocent, but intracystic growths, especially in the mamma of women over fifty, should be regarded with suspicion and therefore should be removed on radical lines. Transition forms between adenoma and carcinoma are also met with in the rectum and large intestine, and these should be treated on the same lines as cancer.