278 THE ACTION OF THE LIVING CELL 



Care must be taken not to press such analogies too 

 far, for while isolated cells or tissues are unable to 

 control such factors as oxygen tension, they are, in a 

 measure, able to control the cytost content of their 

 immediate environment, since, as we have seen, cytost 

 is an endocellular autosynthetic product. On the other 

 hand, our experimental results have shown that the 

 liberation of cytost in quantities sufficient to affect 

 the cells in one way or another is dependent upon an 

 injury of some sort; that is, although the living cell 

 contains cytost or its immediate precursor, mobiliza- 

 tion of this material in quantity sufficiently large to 

 cause an observable alteration in the activity of cells 

 does not take place until an injury of some sort occurs. 

 It should be noted in this connection that we use the 

 term injury in a very general sense, including any 

 response to various physical and chemical agencies 

 which we have discussed previously. It follows there- 

 fore that so-called "mild stimulation" as well as physi- 

 cal destruction falls in this category. 



Since such injury provokes the liberation of cytost 

 to a greater or lesser degree, and since, as our experi- 

 ments have shown, an optimal quantity of cytost in 

 the surrounding media appears to lead to maximal 

 cellular activity, we are enabled to see a definite rela- 

 tionship between cellular activity and alterations in 

 the physical environment. Assuming that such an 

 environment is well adapted as regards oxygen, sources 

 of nutriment, and so forth, the activity of a cell, tissue, 

 organ, or organism is to a considerable degree condi- 

 tioned by alterations in any environmental factor 

 which may lead to a liberation of cytost. 



Let us first consider the beneficial action of such 



