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TUMOR, in surgery, any morbid parasitic growth, generally, though not always, attended by swelling. Tumors are primarily divided into two classes, the first innocent, including non-malignant, solid, benign, or sarcomatous, and the second malignant growths. Tumors of the first type occur in comparatively few tissues, and do not alter the adjacent parts unless the tumor produces pressure and partial inflammation; they have no tendency to ulcerate or slough, and, if extirpated by a surgical operation, they do not grow again. They vary considerably in structure, being fatty, cellular, fibrous, fibroid or tendonous, encysted, vascular, cartilaginous, osseous, or fibro-cartilaginous. Fatty and cartilaginous tumors often reach a size so large that they weigh many pounds. They should be excised while they are small. A tumor of the second type, on the contrary, may arise in almost any part of the body, though some parts are more liable than others to attacks. They tend to propagate their morbid action to the adjacent parts, or, by means of the blood, even to spots remote from their formative seat; they ulcerate or slough, and, when extirpated by surgical operation, grow again, either at the original or some other place. The cancer and tubercle are leading types of malignant tumors. A third type of tumor, the semi-malignant, is intermediate between the first two, and includes some forms of sarcomic and of melanotic tumor, the painful subcutaneous tumor or tubercle, nævi, polypi, etc. Melanosis is commoner in horses than in the human subject, and chiefly in white or gray horses. Various tumors are inter-thoracic, affecting the heart, the lungs, etc. There are also tumors of the brain, of the liver, the rectum, etc.; and in women the uterus and the vagina are specially liable to be affected with tumor.