42 



ACTIONS OF DRUGS ON SURFACES 



Ascaris is Iplotted against the concentration of detergent 

 present in the same medium. At first the penetration 

 of the drug is increased by the presence of the detergent, 

 but as the concentration of the detergent is increased a 

 maximum is reached and the rate of penetration falls 



n 



Fig. 9. The relationship between physiological activity, micelle for- 

 mation and n, in a series CH3(CH2)nX, where the physiological acti- 

 vity is a function of the concentration of single molecules 



off practically to zero as the detergent concentration is 

 still further increased (Trim and Alexander). In the 

 same figure, the surface tension of the solutions is plotted 

 also, and it will be seen that the point at which the rate 

 of penetration begins to fall coincides roughly with the 

 onset of micelle formation. It therefore seems very prob- 

 able that the decline in rate of penetration of the drug 

 is due to its forming a complex with the detergent when 

 the latter is present as micelles, and that this complex is 

 unable to penetrate the cuticle of Ascaris. 



