Cancer Its Course and Its Causes 5 



To-day it would come nearer the truth if we would ac- 

 knowledge that while the external cause of tuberculosis is 

 known, much has still to be learned about the cause of 

 tuberculosis. That the external factor does not consti- 

 tute the only cause of tuberculosis; that many individuals 

 are equally exposed to or perhaps even invaded by the 

 tubercle bacillus, but that the result differs very much 

 in different cases. What part does the character and 

 constitution of the exposed or invaded organism play in 

 this infection? Does heredity play any part? Is the shape 

 of the thorax, the condition of the lymphatic apparatus, 

 of the circulatory organs of importance in determining 

 the character of this disease? Does it assume an espe- 

 cially virulent course in certain populations, because 

 they lack an acquired or perhaps inherited immunity? 

 How does the toxin of the tubercle bacillus produce its 

 specific effect on the organism? There are, then, multi- 

 ple causes of tuberculosis and the analysis of the inner 

 causes of tuberculosis still remains a large field for fur- 

 ther investigation. In the case of cancer, conditions are 

 somewhat analogous. We can speak very definitely 

 about some of the principal causes of cancer, but as in the 

 case of tuberculosis certain aspects of the problem need 

 still further investigation. 



In general, the causes of cancer in animals and man 

 are the same. In many respects, however, animal can- 

 cer lends itself much more readily to an analysis than 

 human cancer. One of the most completely analysed 

 kinds of cancer is that of mice. The common variety in 

 mice is cancer of the breast in female individuals. Mam- 

 mary cancer in mice is essentially caused by two factors : 

 1) a disposition to cancer which is inherited in certain 

 families and strains, and 2) a chemical stimulus which 

 emanates from the ovary and causes the disposition in- 

 herent in the mammary gland in certain families to as- 

 sert itself; thus cancer develops. The hereditary dis- 

 position to mammary cancer varies very greatly in dif- 



