30 THE ORIGIN OF THE NERVOUS SYSTEM 



Comparison of the data on susceptibility with data 

 obtained by other methods has shown that a general 

 nonspecific relation between susceptibility and physio- 

 logical condition exists, at least in the simple organisms 

 and early developmental stages, where the differentia- 

 tion of organs is not so far advanced that a more or less 

 specific action of particular agents or particular organs 

 is involved. This relation is as follows: to a certain 

 range of concentrations or intensities of the agents 

 used, which is experimentally determined to be above 

 the limit of acclimation or tolerance for the particular 

 species examined, susceptibility varies directly with, 

 though not necessarily proportionally to, the general 

 physiological activity of the protoplasm. To a certain 



by Hyman (1919). Notwithstanding the differences of opinion con- 

 cerning the way in which the effects are produced, and it is not improb- 

 able that all the views are correct, practically all investigators agree 

 that cyanides decrease intracellular respiration to a marked extent, 

 except in very low concentrations, which may produce some accelera- 

 tion. As regards Planaria, for example, Allen (1919) and Hyman 

 (1919) have shown that KNC decreases oxygen consumption 80-90 per 

 cent and that this decrease is reversible. I have found that it also 

 decreases CO 2 production and its action is to some extent additive to 

 that of lack of oxygen (Child, I9i9c). Lund (1918) has reported that 

 KNC does not decrease oxygen consumption in Paramecium, but his 

 data are not conclusive, because the concentrations used by him are 

 highly alkaline and produce extreme stimulation and intense motor 

 activity, often continuing for hours, before the inhibiting action of the 

 cyanide appears. Since he failed to reduce the alkalinity to a point 

 approximately that of the normal environment of Paramecium, at 

 which the solutions produce no stimulation, the total oxygen con- 

 sumption observed by him does not represent the action of the cyanide 

 alone, but includes the stimulating effect of the alkali. There is then 

 at present no reason to alter the conclusion that susceptibility to cya- 

 nides is to some extent a measure of physiological condition, so far as 

 intracellular respiration is concerned, 



