74 The Physiology of Sense Organs 



membranes, it would be pointless to suggest that these results can 

 be explained purely on the basis of removal and replacement of 

 part of the receptor membrane e.m.f. 



lOOr 



75 



%50 



25 



30 



60 90 



[Na] (mM) 



120 



150 



Fig. 33. Effect of reduced external sodium ion concentration on 

 the amplitude of the receptor potential in the Pacinian corpuscle. 

 Abscissa: external sodium concentration. Ordinate, amplitude 

 of the receptor potential (open circles) or its rate of rise (closed 

 'circles) expressed as per cent of the value obtained to an identical 

 stimulus in normal Ringer solution. (From Diamond et al.,^'^ Fig. 8.) 



Certain species of fish, notably from the families Gymnotidae 

 and Mormyridae, are well known for their ability to produce fields 

 of electrical potential in the surrounding water. ^^ Many of these 

 animals generate such weak fields that there is little ground for 

 supposing that they take any direct part in offensive or defensive 

 activities of the animal. There is, however, a large body of 

 experimental evidence which indicates that the fish can use the 

 electric fields — or rather the distortions which occur in these 

 fields — to locate objects in the surrounding water.^^.*^, 46,67, 99 



