TUMOR (Lat. tumor, from tumeo, to swell). Terms frequently used in the same sense are new growth, neoplasm, malignant disease. A neoplasm or tumor in the narrower sense is a new formation of tissue, apparently arising and developing independently, atypical in structure, inserted uselessly into the body, possessing no function of service to the organ ism and showing no typical termination to its growth. (Ziegler).
The frequency with which tumors occur, the resistlessness with which many of them carry their victims on to a painful death and our ignorance in regard to their true nature com bine to render their study one of the most in teresting and important departments of pa thology. In spite of the enormous amount of research which has been devoted to this field we must admit that though the histogenesis or structure of tumors is now fairly well under stood, their pathogenesis or underlying prin ciple of causation is still shrouded in mystery. We have learned that the ultimate cells and tissues which make up the substance of these new growths are the same in type as those nor mally occurring in the body, and that each structural element of the tumor is derived from a pre-existing element of similar nature, but what was the force or stimulus which enabled these cells to break the laws of inter-relation ship which ensure the normal development and function of each Dart of the body is still un known. See CELL ; DEVELOPMENT HYPOTHE STS; HEREDITY; HISTOLOGY.
It is this tendency to assume an independ ence of existence and to flourish in opposition to the physiological restraints to which normal cells are subject in their growth and function that characterizes tumors and forms the chief point of difference between them and certain inflammatory and other tissue proliferations. In short, a tumor is an overgrowth of tissues beginning locally, but frequently, by various methods of extension, invading near or remote regions, which is never of use to the body and frequently is directly hostile to it. The lower animals, particularly mammals, are subject to the same aberrancies of cell growth, and tumors comparable to those occurring in man are ob served in all the vertebrates. Plants also ex hibit analogous formations. See VETERINARY MEntaxx.
General Characters of Tumors.— As has been indicated, the integral structural units of tumors may always be traced back to normal types, and each class of new growths takes origin from corresponding normal tissues. Thus epithelial tumors spring from epithelial tissues, connective tumors from connective tissues, etc. Viewed as a whole a tumor is a true parasite, since it leads an independent existence, deriv ing nourishment from its host without in any way contributing to the latter's welfare. The processes of growth, nutrition and cellular re production go on in tumors much as in normal tissues and they are provided with a connective tissue framework, blood and lymphatic vessels and nerves to provide for their vital needs. In
terference with nutrition or other causes, lead to degenerative changes and necrosis, and in flammation, cicatrization, ulceration. etc., take place in their customary manner. A tumor may continue to increase indefinitely in size, or it may in some instances become quiescent in growth and remain without change for longer or shorter periods of time. The growth of the tumor goes on independently of the rest of the body and often is at its expense; thus a lipoma or fatty tumor may continue to extend in size even after the body's reserve of fat has become exhausted. Growth is effected by three different means: (1) By central or expansile growth due to increase of elements within the tumor, so that the surrounding tissues are pushed aside. This has been likened to the increase in size produced by inflating a rubber balloon. (2) By infiltration, that is, the outly ing portions of the tumor push their way into the surrounding tissues as do the roots of a growing plant. (3) By metastasis. The im portance of this method of growth lies in the fact that it represents the means of dissemina tion by which remote parts of the body may be invaded. The blood or lymph vessels are broken into and bits of tumor tissue travel to a greater or less distance till they lodge in some tissue or organ and form initial foci of second ary or daughter tumors. In carcinoma the ad joining lymphatic glands through their close connection with the lymph current are usually the first structures to be secondarily invaded. From a clinical standpoint it is usual to divide tumors into two great classes: (1) Malignant; (2) non-malignant. There is also a small group of neoplasms which stand between these two and are sometimes malignant and sometimes behave like benign growths. The malignant tumors (or what in popular parlance are called embrace the carcinomata and sar comata and possess certain invariable character, istics: (1) Their growth is by infiltration and destruction of the surrounding tissues; (2) the form secondary growths by metastasis; (3) they are difficult to extirpate surgically and tend to recur locally; (4) they induce a gen eral disturbance of health known as cachexia. By this is meant a condition of defective nutri tion due wholly or in part to the demands on the body made by the growth of the tumor, to its interference with digestion or food ab sorption if situated in certain parts, as the oesophagus or stomach, to the loss of albumi nous material through the constant discharge from ulcerating areas, to the absorption of deleterious substances arising from putrefactive changes in the tumor, to hemorrhage, to the pain and resulting loss of sleep, anxiety, etc. It is possible that the tumor itself produces a poison which contributes in producing cachexia, but portions of human carcinomata introduced into the tissues of experiment animals if free from bacterial contamination do not appear to exert any toxic effects.