94 INJURY, RECOVERY, AND DEATH 



If we use the term recovery for the rise of resistance 

 which occurs when tissue is transferred to sea water 

 from certain solutions (such as the first mixture) there 

 seems to be no good reason why it should not be applied 

 to the fall of resistance which occurs when tissue is trans- 

 ferred from certain other solutions (such as the second 

 mixture) to sea water. 19 The amount of recovery after 

 any given period of exposure is equal to the vertical 

 distance between Curves B and C, in the case of the first 

 mixture, and between Curves A and D in the case of the 

 second mixture. 



It may be asked whether Curves B and D are better 

 criteria of toxicity than Curves A and C. The question 

 involves the definition of toxicity. Since this term is used 

 in a variety of ways, it is desirable that it should always 

 have a precise quantitative significance. In the present 

 case it is evident that we need not only A and C but also 

 B and D for a complete description of the facts. It seems 

 possible that this may be generally true in the study of 

 toxicity, although at present we may be unable to con- 

 struct similar curves in many cases because suitable 

 methods of measurement are lacking. 



The fact that recovery is never complete except at 

 the beginning (as shown by Curves B and D) might also 

 be explained as due to the death of certain cells; for if 

 some of the cells are killed by exposure to a solution of 

 NaCl the complete recovery of the surviving cells cannot 

 restore the resistance to its normal value. This hypoth- 



19 Substances which cause increase of resistance commonly produce per- 

 manent injury; this is apparent when the tissues are replaced in sea water. 



It would therefore seem that any alteration of resistance (increase or 

 decrease) may produce permanent injury if sufficiently prolonged. In 

 spite of this it seems preferable to restrict the term temporary injury to 

 the fall of resistance observed in toxic solutions without coining a new 

 term to express the injurious action accompanying rise of resistance. 



