It has been popularly supposed that cancer is very abundant in certain districts or villages or houses, and that this fact pointed to the con tagiousness of the disease; but when the popu lation of these districts is studied, it is usually found that all the young people have gone to the neighboring large towns or cities, leaving only those whose age is such that they are especially liable to cancer. Consequently, the number of cases seems very large and the percentage is increased for that particular locality; but if the cancer incidence is considered in conjunction with the age of the population, it will be found that no greater number of cases has occurred, for example, between the ages of 45 and 50, than has occurred in the general population elsewhere in the country within the same age limits. It follows that °cancer villages* do not exist, although old-age villages do.
Human cancer is not derived from animal cancer, for it has been shown that it is impos sible to transplant, even with the greatest care, a tumor from a mouse to a rat or a tumor from a rat to a mouse; in other words, cancer of one animal will not grow in any other ani mal except it be one of the same species; that is, no animal cancer can be transplanted to a human being and remain alive and grow.
The theory of the origin of cancer from the so-called cancers of vegetables, which are really not cancers but only inflammations, seems too foolish to have any wide credence; but oc casionally the newspapers take up some such wild story and people are led to believe that there may be something in it. It is, however, not true.
Cause of Cancer.— The cause of cancer is not known, though there are two popular superstitions regarding it. One is, that cancers may be produced by a blow; and the other, that cancer is due to a germ. To uphold either of these there is not the slightest evidence. A large number of cases in which it was claimed that a blow had caused the growth of the can cer have been investigated, and it has been found that the disease existed before the blow was received. And when we compare the large numbers of people who in their lifetime have received blows in various portions of their bodies, with the small numbers relatively who have developed cancers, it is evident that there can be no relationship between the blow and the beginning of the tumor. It is true that a blow may stimulate a cancer which is already exist ing, but usually on investigation of such cases it will be found that the growth had reached a considerable size before the injury occurred.
The same thing is true of the growth of cancer in a broken bone; the presence of the tumor in the bone makes it liable to break, and often the fracture is the first evidence of the tumor. But the fracture did not cause the tumor, even though it may appear to have grown after the break occurred or was not discovered until after the injury.
The reasons for believing that bacteria have nothing to do with cancer are based upon ex perimental work on animals. If a cancer is transplanted from one animal to another of the same species, it will grow and ultimately kill the second animal; some cancers are so very virulent that they will, when properly transplanted, grow in every mouse in which they are implanted and cause the ultimate death of the host. But if some of this cancer tissue is crushed or frozen so that all the cells present are broken up, it will not produce a tumor when grafted on to another anunal. Now we know that bacteria will live in liquid air which is about F, below freezing, and that they are not killed merely by being crushed; and for these very good reasons we believe that cancer is not produced by bacteria.
While we do not know the cause of cancer, we do know the conditions under which it is liable to arise, and these are any chronic inflam mation or ulceration of the skin or of any organ of the body. If, for instance, the skin is burned, and the burn remains for a long time, there is always risk that a cancer may develop in the site. The burns due to X-rays are very chronic; and cancers frequently de velop in them. Cancers frequently follow irri tation of the lip, tongue or cheek, by smoking or by a rough tooth or an ill-fitting plate.
This relationship between chronic irritation and cancer is well shown by the fact that women of the white race very rarely suffer from cancer of the cheek, while in Ceylon and the Philippine Islands where women chew a highly irritating substance known as the betel nut, and keep the chewed mass in the cheek over night, they very frequently have cancer arising on the inside of the mouth. Chronic ulcer of the stomach in persons over 40 is another condition which has the same relation ship to cancer, for it has been shown that most of the cases of cancer of the stomach arise in one of these ulcers. Chronic ulceration or irritation of the intestine, also, is a cause of the beginning of cancer; and cancers of the womb often result from tears or injuries following childbirth.