TUMORS. 287 



plication of the cells, which until that time have remained 

 dormant. 



With regard to the parasitic origin of malignant tumors, 

 the many points of similarity in their clinical history and 

 that of certain infectious diseases, such as tuberculosis, are at 

 least very suggestive. Should their parasitic origin be demon- 

 strated they would no longer be classed with tumors, but with 

 the infective granulomata, which they closely resemble in many 

 respects. The parasite of cancer is described as a protozoon. 

 The true significance of the bodies so described is as yet sub 

 judice. An epithelioma has been successfully transmitted 

 from one animal to another by artificial inoculation. 



The predisposing influence of age is an undoubted factor, 

 some tumors only occurring at an advanced age, while others 

 occur preferably in the young. 



Heredity plays a doubtful role, the importance of which has 

 been greatly overestimated. 



Classification : Tumors have been variously classified. 



Clinically they are divided into benign find malignant. The 

 former are not dangerous except by reason of pressure they 

 may exert on vital structures, or by mechanically interfering 

 with the function of a part, as, for instance, by a tumor of the 

 brain or larynx. 



The latter threaten life and are characterized by certain 

 well-recognized criteria of malignancy : 



First, a tendency of the rapidly multiplying cells of the 

 tumor to break through normal barriers and spread locally, 

 infiltrating surrounding tissues. This is especially true of the 

 malignant variety of epithelial tumors carcinoma and to a 

 less extent of sarcoma, the malignant variety of the con- 

 nective-tissue group. An adequate explanation of this charac- 

 teristic is entirely lacking. The imagination of one author 

 goes so far as to enable him to see in the infiltrating cells of a 

 carcinoma only a host of invading parasites. In a child 

 some irritant, as dirty hands, may excite the multiplication 

 of epithelial cells, which, piling up and cornifying, constitute 

 a benign wart ; while in one advanced in years a similar irri- 

 tant induces a rapid multiplication of epithelial cells, which, 

 instead of piling up on the surface, invade the underlying 

 tissues, thus constituting a most malignant tumor, a carci- 



