Tumors grow (l) by direct extension from the periphery, and (2) by a growth of the tumor at its centre and a consequent pushing outward of its peripheral parts, or 'central growth.' (3) Tumors also extend by what is known as metas tasis. This with the formation of what are known as metastatic tumors occurs by the dis semination of cancer cells by means of the blood and lymph. In this way hits of cancer tissue are carried to remote parts of the 'body. This met astasis from a single primary focus may occur in any tissue or organ, and quite commonly occurs in many tissues and organs either in succession or at approximately the same time. It is one of the surest evidences of the malignancy of the original tumor. These cancer cells lodging in the tissues set up new centres of growth, of the same nature as the mother growth. As the process of growth goes on in tumors so also does the pro cess of degeneration. Tumors may ulcerate or become gangrenous. abscesses may form in them, and they may undergo fatty or calcareous de generation. Degenerative changes are more fre quent in the softer more rapidly growing forms. Various classifications of tumors have been suggested, all of which are more or less un satisfactory. Clinically, tumors are frequently simply divided into two classes: (1) benign tumors, or tumors which are in themselves harm less. and (2) malignant tumors, as cancer and sarcoma. The most common and as vet most satisfactory classification of tumors is morpho logical, based upon their structure relative to the normal body tissues. The following is the
classification given in Delaficld and Prudden's Handbook of Pathological Anatomy and Histol ogy (1902) Classification according to histogenesis (de velopmental history of the tissues) has been at tempted; and while there are reasons for believ ing that embryology may ultimately furnish the proper basis for tumor classification, that which it has thus far furnished is less satisfactory from a practical standpoint than the morphologi cal classification. When, as is not infrequently the case, more than one kind of tissue is present in a tumor, a eomb)nation of terms is used to designate its nature. Thus a eombination of a tumor composed of embryonal connective tissue with a bony tumor is known as an osteo-sarcoma; a muscle-tissue tuuno• in which vouch more than the usual amount of connective tissue is present, as a fibro-myoma, etc. In addition to the new growths included in the classification given there are other forms of tumors or of structures some times called tumors which do not fall within any of the subdivisions of the classification. Among these may be mentioned cysts of various kinds, including the so-called `dermoid cyst' and teratoma; such congenital tumors as moles, noevi, and angiomata, etc. The peculiar placental tis sue tumor known as deciduoma may also he men tioned.