92 The Physiology of Sense Organs 



is identical (fig. 41), The contact chemoreceptors of Phormia 

 and many other insects are found in hair-like sensilla-tricoidea 

 which are perforated at their tip. The dendrites of one or more 

 chemosensory neurons run within the shank of these sensilla to 

 the openings at the tip (fig. 42). It is presumed that stimulating 

 substances make contact with the primary sense cells at the tip. 



300 



O 200 - 



■t-l 

 10 



100 - 



Fig. 41 . Plots of the adsorption equation using data from a blowfly 

 neuron in response to potassium chloride and sodium chloride 

 solutions. Note that the slopes of the two curves are very similar. 

 (From Evans and Mellon,** Fig. 5.) 



Now the composition of the extracellular fluid within the hair 

 canal is unknown; however, since impulses can propagate for 

 some distance out along the distal process, ^°^ there is good reason 

 to suppose that the concentration of extracellular sodium is close 

 to normal. If this is so, it is a puzzling fact that sodium ions 

 themselves appear to constitute an adequate stimulus for the 

 neurons, for the latter are known to remain electrically silent 

 unless an externally applied solution of this ion (or other stimulat- 

 ing substance) comes in contact with the tip of the sensillum. 

 Now, the stimulating effects of concentrated potassium solutions 

 would not be hard to understand, in view of the depolarization 

 caused by high extracellular concentrations of this ion species in 

 other nervous tissues. At high concentrations, however, potassium 

 is no more effective than — indeed, is identical to — sodium in this 



