of the epithelium is at first slow, but later metastases form in the lymph glands and a typical malignant carcinoma is now present (E. T. Nere).
Eelated processes—namely the gradual transformation of epithelium into cancerous growth—have been observed in cases of chronic ulceration of various kinds with incomplete healing of the epithelial surface as for instance in the case of the chronic ulcer of the leg, in the case of lupus, a tuberculous skin affection and in other similar conditions. Another class of cancers is produced directly or indirectly through the action of certain parasites, especially of certain worms (nematodes and trematodes). Thus in Egypt infection with bilharzia (Distoma haematobium) is relatively common. This trematod lives in the veins of the abdomen and lays its eggs in the mucosa of the bladder; they cause inflammation in the bladder, mostly in older persons, and later cancer develops in about 312—5 per cent, of patients infected with bilharzia. In Bosnia, after bites of sandvipers, chronic ulcers without a tendency to heal may develop, and they may become carcinomatous. Also in other cancers a causal connection between a metazoan parasite and the tumor has been suggested in man, but this connection can not yet be considered as proven. Thus Askanazy found in a case of primary carcinoma of the liver Opistorchis felineus, while in Japan in similar cases a Distoma has been observed. In cases of chronic trichinosis carcinoma was found in organs which were situated in the neighborhood of the infected muscles. The presence of certain mites were noticed by Borrel in the case of mammary cancers and considered as its cause. It appears however that these latter parasites are just as frequently found in the non-cancerous breast as in cases of mammary cancer. The importance of nematodes as the cause of certain cancers in animals has been recently demonstrated, as we shall later explain more in detail. Of especial interest is the fact that while in the large majority of cancers caused by external irritation carcinoma developed, a few sarcomata have been observed in the skin under the influence of Röntgen rays, in the bladder in aniline dye workers and in persons infected with hilharzia.
We see then that under the influence of a considerable variety of agencies, cancer can develop in the human body and that in many cases cancer is preceded by the formation of warty or papillomatous excrescences and in other cases by long-continued ulceration, which in itself seems to be able to lead to the formation of cancer. At present we leave out of consideration the mechanism through which these agencies produce cancer, especially the question whether their action consists in a direct specific stimulation of the epithelial cells by the light and Rontgen rays, chemical substances and parasites or whether all these agencies act indirectly causing primarily ulceration, which later is followed by the development of cancer. We know indeed that chronic ulceration due to various causes may in itself lead to the development of cancer. Later we shall have occasion to return to this question.
