TUMOR (Lat., swelling, state of being swol len). In medicine, an abnormal increase in size of an area of tissue or of an organ; a swelling, excluding swellings occurring in the course of simple inflammation. In pathology, a tumor is en overgrowth or hypertrophy, deviating in size and Miape from the normal. with an inherent power and method of growth, this growth being independent of the rest of the body. Pathologists include under the term tumor ulcerations and their boundaries, when such boundaries are eom posed in part, at least, of new tissue. Pathologi cally. then, a tumor is a growth of new tissue in the body which causes local increase in the size of the part or organ in which it occurs. Tumors are distinctly pathological, subserving no useful physiological purposes; in fact. usually growing at the expense of useful tissues and organs.
The tissues of which tumors are composed conform to the same general types as arc nor mally found in the body, with the exception that in the case of some tumors there is a tendency toward the reversion of the tissues to embryonic forms. Tumors usually originate in tissues which are of like structure with that of the tumor itself. Thffli connective-tissue tumors usually take their origin in connective tissue, epithelial tumors in epithelium, etc. When such tumors remain confined to the tissues in which they they are known as homologous tumors. When, however, they spread to surround ing tissues of a different type. or when, as oc curs in the case of malignant tumors, new tumors are started, in distant tissues and organs by means of the dissemination of hits of tissue from the primary growth through the blood and lymph, they are known as heterologous tumors.
Some tunuors are congenital, notably the inevi or vascular tumors. Ln some cases there appears to he a distinct hereditary tendency to tumor formation; yet ideas as to the heredity of can cer are grossly exaggerated in the minds of the laity, for. while the occurrence of calmer in sev eral generations has been observed, the absence of cancer in the children and grandchildren of persons who have been afflicted with the most malignant forms has been equally noted. In flammation resulting from injury, especially from long-continued slight irritation, seems to be a frequent cause of tumor formation. injuries to bone being quite frequently followed by the development of osteo-sarcoma. Also injury, as for example from a blow or from a fall, not in frequently precedes the development of cancer of that organ. Cancer of the tongue or lip is often apparently traceable to the constant irri tation of a broken clay pipe or of a rough tooth. In general it would seem that tumors of the con nective-tissue type (sarcoma) most frequently follow a single injury, as for example from a blow, while tumors of the epithelial type (epithe lionm, carcinoma) are more apt to occur as the result of frequently repeated slight irritations.
Chronic inflammation seems to bear some causa tive relation to tumor formation. This is evi dent in the frequent association of carcinoma of the liver with cirrhosis of that organ. Again, malignant tumors or cancer not infrequently find their starting point in such local malformations as skin 'nevi. Age and sex also play important roles among the predisposing causes of tumors. Thus epithelial cancer or carcinoma is rare in persons under thirty years of age. Cancer of the breast is far more common in the female, this being undoubtedly due to the greater functional activity of that organ in the female. Cancer of the tongue and lip is, on the other hand, much more common in the male. This is probably due to the fact that frequent irritation from smoking is much more common in that sex.
As to the primary causative factor in the pro duction of tumors we have as yet very imperfect knowledge. Of the many theories which have been advanced in explanation of tumor growth, probably the most ingenious and the one to which at present greatest credence is given is one advanced by Cohnheim. According to his theory. during embryonal development some cells stop developing. These cells may either remain in among tissues of their own kind or may be come displaced in the course of further develop ment of the surrounding tissues and so come to lie among other tissues. Such cells lie dor mant for long periods until something in their environment calls them into activity, when they proceed to develop with all the potential growth of young embryonal cells. Furthermore, being unlimited by the wear and tear of physiologieal function, as is the case with normal tissues and organs, they are free to devote their entire ener gies to growth. For these two reasons, the high potentiality for growth of embryonal tissue and the lack of any control of the growth by the re quirements of function, tumor tissue, especially of the embryonal type, is apt to be characterized by a sort of irregular, wild cell-growth (pike different from the orderly arrangement of cells in normal tissues. While Cohnheim's hypothesis otters a very satisfactory explanation of most of the phenomena connected with the growth of tumors, it must be remembered that it is as yet purely a theory, with absolutely no basis in known facts: that no one has ever been able to demonstrate any of these little islands of embry onal tissue lying dormant and waiting to be excited into tumor formation. Bacteria as the excitants of tumor formation have been exploited by some investigators. While some at present be lieve that the true cause of cancer will ultimately be found to be bacterial in nature, the majority tend to the opinion that the presence of bacteria in tumors is an external accidental occurrence rather than an internal causative factor.