66 SENESCENCE AND REJUVENESCENCE 



consists essentially in determining the length of life of different 

 individuals or lots under certain standardized conditions which kill 

 by making impossible in one way or another the continuation of 

 metabolism. 



The substances used in my determinations of susceptibility 

 include the cyanides, and ethyl alcohol, ethyl ether, chloroform, 

 chloretone, acetone-chloroform, and in some cases various other 

 narcotics. Carbon dioxide and water in which large stocks of the 

 species under examination have been kept and which therefore 

 contain soluble products of metabolism have also been used in a 

 few cases with essentially similar results. Certain conditions, such 

 as lack of oxygen, low temperature, and high temperature, act in 

 much the same way, at least in certain cases and when properly 

 controlled. In my experiments the cyanides have proved most 

 convenient and satisfactory, because the concentrations required 

 are very low and osmotic and other complications are negligible, 

 and because in the lower animals, which have been chiefly used, 

 irritability and movement persist to some extent almost to the 

 death point, while in alcohol, ether, and other narcotics they dis- 

 appear earlier. There is no doubt that a relation exists between 

 the general metabolic condition of organisms, or their parts, and 

 their susceptibility to a very large number of substances which act 

 as poisons, i.e., which in one way or another make metabolism 

 impossible, and that differences 'n susceptibility may be used with 

 certain precautions and within certain limits as a means of distin- 

 guishing differences in metabolic condition and, more specifically, 

 differences in metabolic rate. 



Concerning the nature of the action of poisons such as hydro- 

 cyanic acid, the cyanides, and the great group of substances com- 

 monly called narcotics, opinions at present differ widely. As 

 regards the cyanides, it has been very generally believed since 

 Geppert's experiments that they decrease or inhibit cell respiration 

 directly or indirectly. 1 Recent experiments by Vernon, Warburg, 



1 Carlson, '07; Gasser and Loevenhart, '14; Geppert, '89; Grove and Loevenhart, 

 'n; Kastle and Loevenhart, '01; Loeb and Wasteneys, '130, '136; Mathews and 

 Walker, '09; Richards and Wallace, '08; Vernon, '06, '09, '10; Warburg, 'ioc, 'i^c. 

 Further references will be found in these papers. 



