the nose may follow a disease, ozæna, while in the larynx cancer originates most frequently in the vocal chords which, through their situation and function, are most exposed to various external injurious influences.
This may suffice to prove the great significance of the continuous action of external stimuli in the production of the majority of the typical cancers of more advanced age.
While in general we can very well determine the dividing lines between the fields of the typical tumors caused through the action of external stimuli and those due to embryonic disturbances, there may in individual cases be some doubt. A number of pathologists extended it seems unduly the field of the tumors belonging to the latter class. They believed that microscopic studies of early tumors frequently demonstrated that they took their origin not from cells attached in a normal manner to the rest of the organs but from detached small fragments of organs. There would have been good reason to interpret the existence of islands of disconnected cells as an indication of imperfect embryonic development. Careful microscopic investigations show however that in the typical cancers of the stomach and intestines (Hauser, Versé)—with the possible exception of certain atypical so-called carcinoid tumors of the small intestines—and even in the multiple tumors of the skin (Janeway, Loeb and Sweek) the cancers originate through a direct downgrowth of the surface epithelium into the deeper tissues. Conditions preceding the development of carcinoma cause primarily an increased proliferative power of the epithelium which, as the result of this change, in the large majority of cases grows down into the deeper tissues and destroys them, but in some cases the resistance of the deeper tissues is so great that it successfully counteracts the invasion or dissolving power of the epithelium and the latter may instead of growing downwards be forced to grow towards the outside of the skin, as we could observe in a case of multiple carcinoma of the skin. This increased proliferative and infiltrative power of the epithelium is therefore the principal characteristic of cancer.
This change in the proliferative power of the epithelium may be accompanied by a change in the structure of the affected cells, which appear often less differentiated. This loss in the complexity of proliferating cancerous cells has been called anaplasia. In other cases, however, this morphological change in the proliferating cells may be entirely absent, as for instance in some beginning Röntgen ray cancers.
Still furthergoing structural modifications of the proliferating cells may take place in the secondary (metastatic) growths, although on the whole metastases repeat more or less the structure of the primary tumor. Thus we may in the case of a primary carcinoma of the liver find metastases in which the tumor cells continue to produce bile just as the normal liver cells do, and especially primary as well as metastatic tumors
