+Definition.+ A tumor is a circumscribed mass of tissue made up of cells of the same kind as the tissue from which it grows. There are two distinct types of tissue in the body: epithelial and connective, and therefore two types of tumors: the _epithelial tissue tumors_ and the _connective tissue tumors_. Tumors may also be classified as _typical_, and _atypical_. A typical tumor is one in which the cells are identical to those in the tissue from which it springs, and also has the same arrangement of cells. They may be of epithelial or connective tissue origin. The tissue is identical in all respects and the growth is benign. An atypical tumor is one of epithelial or connective tissue origin in which, though the cells are the same as those in the tissue from which it grows, their arrangement is quite different. They are malignant. The most important classification of tumors is that into _benign_ and _malignant_. A _benign tumor_ is one in which there is no tendency to rapid growth; the symptoms are purely local, and the general health is not affected, except indirectly. On the other hand a _malignant tumor_ is one which takes on a rapid growth with a tendency to infiltrate or adhere to surrounding tissues; recurs when removed, and is accompanied by great pain and a rapid loss of weight and strength. These are commonly known as cancerous. Malignant growths are of two types, carcinomatous and sarcomatous, dependent upon the tissue from which they emanate. The _carcinomata_ spring from the epithelial type of tissue while the _sarcomata_ emanate from the connective tissue type. +Origin.+ Tumors originate from many causes. Some are congenital and others grow in later life from an inherited tendency. Any continued irritation which acts mechanically or chemically so as to maintain a constant, though slight, degree of undue vascularity of a part, such as the hot, rough stem of a clay pipe or a jagged tooth, favors the development of a malignant growth. Certain benign growths, such as warts or moles, are especially prone to malignant change. Age and sex also predispose to tumor formation. Thus carcinoma is a rarity under thirty years of age; the mammary gland of the female is more liable to carcinoma than the male; while on the other hand the esophagus, lip and tongue of the male are more liable to attack. The possibility of certain malignant growths being of germ origin is thought to be evident (though not yet proven) from many facts. The fact that where there are malignant growths present, lymphatic glands, quite distant from the original growth, become secondarily infected, through the lymphatic vessels, seems to carry out this view. Particles of a carcinoma (metastasis) floating in the blood stream, finding lodgment elsewhere also establish new growths (metastatic). Tumors are named according to the tissues from which they arise, thus: CONNECTIVE TISSUE TUMORS