PITHELIOMATA. 73 



occurs where there is long-continued irritation, consequently 

 epithelioma is most often seen on those spots where irritation 

 (and it must he remembered possibly also infection) is most 

 likely to occur. Hence any exposed portions of skin, such as 

 the hands and lips, are frequently the sites of epitheliomatous 

 deposits. 



Other common sites are those in which two skin surfaces tend 

 to come in contact, as for instance about the anus and vulva 

 where there is great liability to irritation, to small wounds and 

 possibly to infection. Also, where the skin is thrown into 

 rugae there is again the liability for squamo us- celled carcinoma 

 to make its appearance, as on the scrotum, forming the so-called 

 " chimney-sweep's cancer." It is often said that the line of 

 junction of skin and mucous membrane that is, where tran- 

 sition from one kind of epithelium to another takes place is a 

 region in which epithelioma commonly originates. While this is 

 undoubtedly true, there is no clear evidence that it is due to the 

 change from one form of epithelium to another, but it is of every- 

 day observance that these same spots, if on the surface, are those 

 commonly the site of slight injuries. 



Again, as in connection with warts, so the epithelium covering 

 the tongue, and also that forming the superficial layer of the true 

 vocal cord, may give rise to squamous-celled carcinoma. Hence 

 the connection between the simple papillomata and epitheliomata 

 is self-evident : both originate from epithelium, and both are 

 found frequently where irritation, and particularly prolonged 

 irritation, is common, and slight injuries such as a crack or a 

 fissure often occur. 



Epitheliomata, like other malignant growths, spread in one or 

 more of three ways. First, by direct extension into the surround- 

 ing tissues, which extension is termed infiltration. It is inter- 

 esting to note however that this infiltration by no means occurs 

 only in the line of least resistance, and so differs from the 

 extension of innocent tumours, which merely push the parts 

 aside, or of abcesses finding their way to the surface. Secondly, 

 epitheliomata spread to distant parts by means of the lymphatic 



