A cancer is a malignant tumour which originates in epithelium.  The cancer cells are derived by proliferation from already existing epithelium, and they invade the sub-epithelial connective tissue in the form of simple or branching columns.  These columns are enclosed in spaces—­termed alveoli—­which are probably dilated lymph spaces, and which communicate freely with the lymph vessels.  The cells composing the columns and filling the alveoli vary with the character of the epithelium in which the cancer originates.  The malignancy of cancer depends on the tendency which the epithelium has of invading the tissues in its neighbourhood, and on the capacity of the cells, when transported elsewhere by the lymph or blood-stream, of giving rise to secondary growths.

Cancer may arise on any surface covered by epithelium or in any of the secreting glands of the body, but it is much more common in some situations than in others.  It is frequently met with, for example, in the skin, in the stomach and large intestine, in the breast, the uterus, and the external genitals; less frequently in the gall-bladder, larynx, thyreoid, prostate, and urinary bladder.

Tissues appear to be most liable to cancer when, having attained maturity, they enter upon the phase of decadence or involution, and this phase is reached by different tissues at different periods.  It is not so much, therefore, the age of the person in whom it occurs, as the age of the tissue in which it arises, that determines the maximum incidence of cancer.  Cancer of the stomach appears and attains a maximum frequency earlier than cancer of the skin; cancer of the uterus and mamma is more frequent towards the decline of reproductive activity than in the later years of life; rectal cancer is not infrequently met with during the second and third decades.  There is evidence that the irritation caused by alcohol and tobacco plays a part in the causation of cancer, in the fact that a large proportion of those who become the subjects of cancer of the mouth are excessive drinkers and smokers.

A cancer may appear as a papillary growth on a mucous or a skin surface, as a nodule in the substance of an organ, or as a diffuse thickening of a tubular organ such as the stomach or intestine.  The absence of definition in cancerous tumours explains the difficulty of completely removing them by surgical measures, and has led to the practice of complete extirpation of cancerous organs wherever this is possible.  The boundaries of the affected organ, moreover, are frequently transgressed by the disease, and the epithelial infiltration implicates the surrounding parts.  In cancer of the breast, for example, the disease often extends to the adjacent skin, fat, and muscle; in cancer of the lip or tongue, to the mandible; in cancer of the uterus or intestine, to the investing peritoneum.