The benign tumors, on the other band, are distinguished by usually being encapsulated so that they can be easily extirpated through the absence of any tendency to form metastases. They are dangerous to life only through com pression of important organs, but it must be remembered that their position may in this way give rise to most serious conditions. Many varieties are also extremely liable to become the seat of malignant degeneration.
Theories of Causation and the Occurrence of Tumors.— Comparatively little is yet known in regard to the actual causes of tumor forma tion. Many highly ingenious theories have been elaborated, but plausible as they may seem they are mere speculations and none of them can be said to be satisfactory from every point of view. Cohnheim in 1882 promulgated a doctrine which has many adherents and affords a rational ex planation at least for congenital tumors and for many benign new growths. Briefly stated, his view is that during the development of the embryo (see EMBRYOLOGY) small clusters of cells become displaced from their normal sur roundings and form little islets of tissue dif ferent in nature from their environment. These cells remain alive, but lie dormant until some stimulus stirs them into activity and they then form the nucleus for tumor formation. Having once started to proliferate they exhibit the in ten,e vital energy of embryonal tissue and de velop untrammeled by the restraints imposed upon normal cell growth. This theory has much in its favor, such as the tendency of many tumors to develop in regions which correspond to embryonal foldings, and to assume embryonal types of structure, but it is impossible to demon strate any such displaced aggregations of cells, and clinical observations in many cases are strongly against it. A somewhat broader and more general view is taken by Ribbert, who be lieves that through some means, perhaps as the result of injuries or inflammation, numbers of healthy cells become cut off by bands of con nective tissue and are thus isolated from the physiological restraints which ordinarily keep the cellular capacities within normal bounds. As a result of this liberty, excessive growth and perverted activity, in other words. tumor forma tion, occur. Hansemann denies that mere segre gation of cell groups can have this result and accounts for the great intensification of vitality necessary by assuming a change in the individual cells, termed anaplasia, consisting in a loss of differentiation and increased capacity for inde pendent existence. According to this writer two factors, anaplasia and a proliferative stimulus, are necessary for tumor formation. It will be
seen that these theories leave much to be de sired, but it is probable that the truth lies in this direction of cytological research and that as we gain a deeper insight into the physiology of the normal cell our understanding of these com plex perversions of growth will be correspond ingly increased.
Much has been written of late on the para sitic origin of tumors, particularly of carcinoma, and bacteria, yeasts and protozoa have all been described and figured in great detail as causa tive agents. None of these so-called cancer parasites has, however, been able to withstand the crucial tests of cultivation and of repro ducing the tumors on inoculation into animals, and it has also been impossible so far to infect animals of one species with tumor tissue de rived from another. Belief in the infectious nature of carcinoma is shared by very few ob servers at present, and it is probable that the instances in which carcinoma has occurred in people intimately associated, as husband and wife, are either due to coincidence or to direct transplantation of cancer cells. So-called %can cer and cancer districts are described, but such observations are too fragmentary and unreliable to be taken as the basis for general izations.
Climatic, racial and geographical features have been drawn into the discussion by some authors, as well as matters of diet; for example, habitual over-indulgence in meat. In connec tion with this latter point it is an interesting fact that carcinomata are said to be much less frequent in herbivorous than in carnivorous animals. On the other hand carcinoma is not uncommon among religious sects in the East that are strict vegetarians.
Injuries, both acute, such as blows and falls, and in the form of long-standing irritation, un doubtedly have some connection with tumor formation. Acute trauma is most often asso ciated with tumors of the connective tissue group, and chronic trauma with carcinomata. Typical examples are found in the cases in which sarcoma follows bone injuries, and car cinoma of the lower lip is apparently produced by the constant irritation of pipe smoking or of a roughened tootfi. Certain occupations which involve chronic irritation of the skin through chemicals, such as tar or paraffin products, also seem to predispose toward tumor formation. Too much importance must not be attached to this factor, however, tor it has never been possible to produce tumors in animals experi mentally in this way, and in the cases in which new growths have followed trauma there was probably some already existing predis position.