72 INJURY, RECOVERY, AND DEATH 



point. If, on the other hand, the tissue be placed in a 

 solution of some substance which causes a rise, followed 

 by a fall in resistance, we may get a curve somewhat like 

 that shown at B. 



The most common method of measuring the toxicity of 

 a solution is to determine the time necessary to cause 

 death. But it is evident from an inspection of the curves 

 that it is impossible to determine the precise moment of 

 death, since the death curves approach the axis asymptot- 

 ically. This is doubtless true of death in all cases. It 



/ 



is therefore obvious that the death point does not offer 

 a perfectly satisfactory criterion of toxicity. 



We may avoid this difficulty by taking as a criterion 

 the time needed to reach any convenient point on the 

 curve, as, for example, 55% (half way between the normal 

 condition and the death point). This may be determined 

 with a good deal of precision by the measurement of elec- 

 trical resistance or by any method which permits us 

 to follow the reaction accurately from moment to moment. 

 But where this cannot be done, we may employ other 

 criteria. We may assume that as the reaction goes on, 

 certain phenomena appear at definite points on the curve, 

 such, for example, as changes in metabolism, cessation 

 of motion, or loss of irritability. The employment of 

 such criteria may give trustworthy results in many cases 

 if proper precautions be taken. 



In the employment of any of these criteria, except 

 that of death, we may meet the difficulty that the relative 

 toxicity of two substances may vary greatly according to 

 the point in the curve at which the comparison is made. 

 Let us suppose that two toxic substances are so chosen 

 that they produce death at about the same time, giving 

 curves as shown in Fig. 31. They must be regarded 



