246 



H. K. HARTLINE, F. RATLIFF AND W. H. MILLER 



Fig. 3. Electron micrograph of retinular cell axon and branches. Retinular cell 



axons (7?); axonal branch (arrow); bundle of axonal branches (5); nuclei of 



Schwann's cells {Nii). 



in part, to the latent period of tlie action of light on the neighboring receptors. 

 There was also a pronounced dip in frequency at the beginning of the period 

 of inhibition, with partial recovery. This dip may be attributed, in part, to 

 the strong transient outburst of impulses discharged initially when the neigh- 

 boring receptors were illuminated after having been in darkness for some 

 time. 



The stronger the intensity of light on receptors that neighbor a given 

 receptor, the greater is the depression of frequency of that receptor (Fig. 7). 

 Similarly, the larger the area illuminated in the neighborhood of an omma- 

 tidium that is under observation, the greater is the depression of the fre- 

 quency of its discharge, showing that there is spatial summation of the in- 

 hibitory influences exerted on a particular ommatidium by its neighbors. The 

 nearer these neighbors are to a particular ommatidium, the stronger is their 

 inhibitory effect on it; ommatidia that are widely separated in the eye interact 

 little or not at all. A fixed degree of activity elicited in a given region of the eye 



