W. J. V. OSTERHOUT 19 



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. 



If we use the term injury to denote loss of resistance, we may define 

 temporary injury^ as the loss which occurs in the solution and perma- 

 nent injury as the loss which is observed after the tissue has been 

 returned to sea water and the resistance has risen or fallen to a 

 stationary condition. Curves B and D are therefore the curves of 

 permanent injury. 



It m.ay 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 desira- 

 ble 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 construct 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. The chief objections to 

 this explanation are that during the death process the cells seem to 

 act alike and to die at about the same time ; moreover this assumption 

 does not lend itself readily to quantitative treatment. It does not 

 seem to be necessary to discuss this point more fully at present; but 

 it may be pointed out that this hypothesis would in no way invalidate 

 the conception developed above, that an individual cell may lose 

 part of its resistance and subsequently regain it, either partially or 

 completely. 



^ Substances which cause increase of resistance commonly produce permanent 

 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 of rise of 

 resistance. 



