94 The Physiology of Sense Organs 



then apparently emerges from the region of the cell body, which 

 is screened from the incident light energy by a layer of pigment 

 cells (the region labeled P in figure 43). Experiments with a 

 confined light source simply demonstrate that the influence of the 

 stimulus is strictly localized to the directly affected areas of 

 membrane. These areas can apparently respond independently 

 of surrounding unexcited regions. These conclusions, concerning 

 the relation between response magnitude and per cent, of sensory 



S-L 

 -1 O 



100 



200 



+ 1 



mV 



\^-Va 



Fig. 43. Diagram of a single photoreceptor cell of a squid, and the 

 results of potential measurements along the outer segment of the 

 cell due to passage of a light stimulus along it. Each sub-unit of 

 the cell membrane responds to light independently of the rest of 

 the cell — a current sink occurs only in that region which is being 

 illuminated by the stimulus. Abscissa: Voltage difference between 

 the fixed electrodes, A and B. Ordinate: Position of the stimulus 

 (S) from the outer tip of the receptor cell (L). (From Hagins, 

 Zonana and Adams,*^ Fig. 2.) 



