The tumour tissue may undergo necrosis, and when the overlying skin or mucous membrane gives way an ulcer is formed. The margins of a _cancerous ulcer_ (Fig. 57) are made up of tumour tissue which has not broken down. Usually they are irregular, nodularly thickened or indurated; sometimes they are raised and crater-like. The floor of the ulcer is smooth and glazed, or occupied by necrosed tissue, and the discharge is watery and blood-stained, and as a result of putrefactive changes may become offensive. Hæmorrhage is rarely a prominent feature, but discharge of blood may constitute a symptom of considerable diagnostic importance in cancer of internal organs such as the rectum, the bladder, or the uterus. [Illustration: FIG. 57.--Carcinoma of Breast with Cancerous Ulcer.] _The Contagiousness of Cancer._--A limited number of cases are on record in which a cancer appears to have been transferred by contact, as from the lower to the upper lip, from one labium majus to the other, from the tongue to the cheek, and from one vocal cord to the other; these being all examples of cancer involving surfaces which are constantly or frequently in contact. The transference of cancer from one human being to another, whether by accident, as in the case of a surgeon wounding his finger while operating for cancer, or by the deliberate introduction of a portion of cancerous tumour into the tissues, has never been known to occur. It is by no means infrequent, however, that when recurrence takes place after an operation for the removal of cancer, the recurrent nodules make their appearance in the main scar or in the scars of stitches in its neighbourhood. In the lower animals the grafting of cancer only succeeds in animals of the same species; for example, a cancer taken from a mouse will not grow in the tissues of a rat, but only in a mouse of the same variety as that from which the graft was taken. While cancer cannot be regarded as either contagious or infectious, it is important to bear in mind the possibility of infection of a wound with cancer when operating for the disease. A cancer should not be cut into unless this is essential for purposes of diagnosis, and the wound made for exploration should be tightly closed by stitches before the curative operation is proceeded with; the instruments used for the exploration must not be used again until they have been boiled. The greatest care should be taken that a cancer which has softened or broken down is not opened into during the operation. Investigations regarding the cause of cancer have been prosecuted with great energy during recent years, but as yet without positive result. It is recognised that there are a number of conditions which favour the development of cancer, such as prolonged irritation, and a considerable number of cases have been recorded in which cancer of the skin of the hands has followed prolonged and repeated exposure to the Röntgen rays.