THE NERVOUS SYSTEM OF AMPHIBIA 207 



in the manipulation of growth cultures of microorganisms. In 

 short, in testing the effect of cocaine upon the irritability to 

 the chloride, Cole applied stimuli which in both extent and in- 

 tensity are beyond comparison with the stimuli which he used 

 to test the effect of cocaine upon the irritability to tactile stimuli. 

 Furthermore, Cole's observation that abrasions in the skin increase 

 the irritaJbility to acid, or shorten the reaction time, is also in 

 harmony with the hypothesis that the action of the chloride is 

 destructive and not a normal physiological stimulus. 



When the epithelial cell itself, without the intervention of any 

 nervous structure, is sensitive to as high a dilution of hydro- 

 chloric acid as are the taste organs of man (Parker ('12) esti- 

 mates this at n/1000), it is not surprising that the oral surfaces 

 of the shark should be sensitive to n/75 (Sheldon) ; nor does a 

 special set of receptors seem necessary to enable Ammocoetes 

 to respond to n/40 (Parker) ; or Ameiurus, to n/2 (Parker) when 

 applied to the skin of the trunk. Such concentrations of acids 

 as this are beyond comparison with the degree of concentra- 

 tion which will act destructively upon epithelial cells. Such 

 strengths of chemical stimulation certainly have no place in the 

 normal environment of the animals upon which they were used 

 experimentally, and the idea that there is a special set of recep- 

 tors, or nerves (Sheldon), to take account of such stimuli even in 

 these animals should have absolutely unequivocal and uncon- 

 tradictory evidence in its favor before it is adopted as a general 

 biological principle. 



