CHAPTER XXI 



TUMORS 



A TUMOR may be described as an atypic non-inflam- 

 matory proliferation of a tissue of embryonal or post- 

 natal origin (Hektoen). 



Causes of Tumor Formation. Though many theories 

 have been advanced in explanation of the phenomena of 

 tumor growth, a definite etiology is as yet unknown. 

 Virchow held that tumors were due to external irrita- 

 tion, but while there occur frequent instances of tumors 

 following irritation, such as smokers' cancer of the lip 

 and carcinoma of the breast following irritation and in- 

 flammation of the nipple, there are too many instances 

 of tumors occurring without previous irritation for the 

 acceptance of this theory. Cohnheim held that all 

 tumors were due to misplaced islands of embryonic 

 tissue (embryonic rests), which under favorable condi- 

 tions set up growth and development independent of 

 the tissues in which they were planted. It is true that 

 certain tumors most certainly do originate in this way, 

 but, like the irritation theory, it cannot be held applic- 

 able to all tumor growths. 



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