[2] For the histology of tumours the reader is referred to a text-book of pathology.

A tumour or neoplasm is a localised swelling composed of newly formed tissue which fulfils no physiological function. Tumours increase in size quite independently of the growth of the body, and there is no natural termination to their growth. They are to be distinguished from such over-growths as are of the nature of simple hypertrophy or local giantism, and also from inflammatory swellings, which usually develop under the influence of a definite cause, have a natural termination, and tend to disappear when the cause ceases to act.

The etiology of tumours is imperfectly understood. Various factors, acting either singly or in combination, may be concerned in their development. Certain tumours, for example, are the result of some congenital malformation of the particular tissue from which they take origin. This would appear to be the case in many tumours of blood vessels (angioma), of cartilage (chondroma), of bone (osteoma), and of secreting gland tissue (adenoma). The theory that tumours originate from fœtal residues or “rests,” is associated with the name of Cohnheim. These rests are supposed to be undifferentiated embryonic cells which remain embedded amongst fully formed tissue elements, and lie dormant until they are excited into active growth and give rise to a tumour. This mode of origin is illustrated by the development of dermoids from sequestrated portions of epidermis.

Among the local factors concerned in the development of tumours, reference must be made to the influence of irritation. This is probably an important agent in the causation of many of the tumours met with in the skin and in mucous membranes—for example, cancer of the skin, of the lip, and of the tongue. The part played by injury is doubtful. It not infrequently happens that the development of a tumour is preceded by an injury of the part in which it grows, but it does not necessarily follow that the injury and the tumour are related as cause and effect. It is possible that an injury may stimulate into active growth undifferentiated tissue elements or “rests,” and so determine the growth of a tumour, or that it may alter the characters of a tumour which already exists, causing it to grow more rapidly.

The popular belief that there is some constitutional peculiarity concerned in the causation of tumours is largely based on the fact that certain forms of new growth—for example, cancer—are known to occur with undue frequency in certain families. The same influence is more striking in the case of certain innocent tumours—particularly multiple osteomas and lipomas—which are hereditary in the same sense as supernumerary or webbed fingers, and appear in members of the same family through several generations.