PROLIFERATION OF HEALING 169 



occur as the result of a bruise or disease; and chronic 

 irritation, which is an invariable predisposing factor in 

 cancer, may give rise to exuberant multiplication of 

 the cells in the neighbourhood of the irritated part. 

 A common instance of the proliferation due to chronic 

 irritation is shown in the case of a "corn." An ill- 

 fitting boot irritates a certain portion of the foot by 

 pressing unduly on a certain portion of the skin. The 

 skin becomes hardened, and a small tumour may even 

 be formed on the irritated spot. This hardening is 

 due to excessive proliferation of the living cells in 

 and immediately underneath the skin. A wart is 

 an example of the proliferation due to irritation; 

 but although this irritation leads to proliferation, we 

 do not know exactly w r hy the cells proliferate in 

 response to it. If we think the problem out care- 

 fully, w r e can picture a group of living cells multiplying 

 by division, and then try to grasp how irritation of 

 that group by mechanical pressure can possibly make 

 the individual cells reproduce themselves; for this is 

 what they do. At the time when these cancer re- 

 searches w r ere started we could not grasp this point. 

 It seemed incredible that a cell would reproduce itself 

 because its cell-wall was "tickled" or pressed upon. 1 

 Why should a living cell undergo the complex phe- 

 nomenon of mitosis for a reason of this nature ? . Be- 

 sides, living cells are very delicate, and we know that 

 they will not stand much handling or pressure without 

 dying. No; it was necessary to find some better 

 explanation of the cause of cell-division than mere 

 mechanical irritation, and we appreciated that irritation 



1 The pressure of a cover-glass does not cause cell-division. 



