The Causation of Tumours.—Innumerable suggestions as to the causation of tumours have been put forward from time to time. Here, reference can only be made to the more important.
First in point of time comes Virchow's hypothesis that tumours arise as the direct result of chronic irritation or injury. A cancer of the lip or tongue may follow the irritation of a clay pipe or a jagged tooth, cancer of the urinary bladder may result from the irritation of Bilharzia ova (see BILHARZIASIS), of the gall-bladder from irritation of a gall-stone, of the skin from prolonged ex posure to X-rays. But, on the other hand, there must be in numerable instances in which such causes of irritation have not been followed by a tumour. As a complete explanation Virchow's hypothesis is insufficient, but it is certain that chronic irritation has, at least, an accessory or pre disposing influence in tumour formation.
Another theory is Cohnheim's hypothesis of embryonic rem nants. According to Cohnheim more cells are, at times, produced in embryonic life than are re quired for the development of the body, and a remnant is left unap propriated. Owing to their em bryonic nature, these cells possess an exaggerated power of pro liferation, and if at a later period of life this should be roused into activity by some mechanical or other form of stimulus, their rate of growth will outstrip that of the adult cells and a tumour will develop. This is at best only a partial explanation which may be applicable to a small proportion of tumours.
The Parasitic hypothesis is still a matter of keen debate (see CANCER RESEARCH). In some degree cancer with its localized primary growth and widespread secondary deposits resembles cer tain infective diseases of microbial origin, such as pyaemia, where from a small primary site of infection the bacteria become dis seminated throughout the body. From this analogy it was argued that tumour formation was due to the activity of some parasite. But if the mode of dissemination of a cancer and of a micro organism be carefully examined this analogy is found to be false. When a micro-organism lodges in a gland or other part of the body, by its irritative action it stimulates the cells of that gland or tissue to increased activity, and any swelling that occurs is produced by the proliferation of those cells. But when a group
of cancer-cells is deposited in a gland the subsequent growth arises entirely from the multiplication of those cancer-cells, and the gland cells take no part whatever in its formation.
At the present time the general weight of evidence seems to favour the idea that tumour formation is ultimately due to some intrinsic cause, whereby the normal processes of growth are dis turbed, rather than to any extrinsic cause such as microbial in fection. Therefore it is from a careful study of the laws of growth, and from research directed along broad biological lines that the best results are to be looked for in the future.
Innocent Malignant Lipoma (fatty tumour).
Fibroma (fibrous tumour). Sarcoma.
Myoma (muscular tumour).
Osteoma (bony tumour).
Chondroma (cartilaginous tumour).
Odontoma (tumour in connection with teeth).
Myxoma (mucoid tumour).
Neuroma (tumour in connection with nerves).
Glioma (neuroglial tumour).
Angioma (tumour composed of blood or lymphatic vessels).
Innocent Malignant Papilloma. Carcinoma. Adenoma.
Innocent Malignant Some parotid and similar Some pleural, periboneal, and growths. similar growths.
I. Connective-tissue Tumours'.—Lipoina (fig. 5).—Of the connective-tissue group the fatty tumours are the most common. They often arise from the layer of fat beneath the skin on the back, though at times they are found on the limbs and elsewhere. They are soft, painless swellings, sometimes of great size ; though usually single, as many as a dozen may be present in the same individual. Lipomata are also found in the abdominal cavity, growing from the subperitoneal layer of fat.