ANATOMY 



competition may take place between the various 

 cells of the body, that under certain circumstances 

 some cells may grow and proliferate apparently 

 at the expense of others. Further, cells of one 

 kind may wander beyond the territory to which 

 they properly belong and encroach upon another 

 territory or tissue in other words, invade it. It 

 is known that such encroachment or invasion, to 

 the ultimate detriment both of the invaders and 

 the invaded tissue, takes place abnormally in 

 disease, and is, for instance, characteristic of that 

 fell scourge, cancer. 



Such invasion may be due to a peculiar 

 abnormal aggressiveness of the invading cells, 

 whereby not only do they multiply very rapidly, 

 but they acquire, in some way or another, the 

 power of breaking down the barriers by which, 

 under usual conditions, they are limited. 



The invasion may not, however, be entirely 

 due to the aggressiveness of the invading cells, 

 but may be due, partially at all events, to a dimi- 

 nished resistance of the invaded tissue, a resistance 

 which under normal circumstances is sufficient to 

 keep neighbouring cells of another kind in their 

 proper place. For aught we know, different types 

 of cells in the body may be constantly striving 

 to invade one another, but are kept in their 

 proper places by a mutual and evenly balanced 

 antagonism. 



221 



