INHIBITORY INTERACTION IN THE RETINA 243 



individual ommatidia can sometimes be seen in living preparations ; a micro- 

 pipette thrust into one of them yields electrical records consisting of a slow 

 "generator potential" superimposed on which are large spikes which are 

 synchronous with the nerve impulses recorded in the strand of fibers dissected 

 from that ommatidium (Hartline et al., 1952). The uniformity of the action 

 potential spikes in the nerve bundle, and the regularity of their discharge, are 

 evidence that they represent impulses in just one axon, presumably that of the 

 impaled eccentric cell. It would appear then, that the trains of impulses that 

 are recorded in "single" fibers dissected from the optic nerve originate in the 

 eccentric cell of the particular ommatidium from which this fiber arises. 

 Signs of activity of the retinular cells have not been identified. Perhaps they 

 all fire impulses in exact synchrony with the eccentric cell; perhaps the action 

 potentials in their axons are too small to have been recognized. 



For the purposes of the present paper, each ommatidium may be con- 

 sidered a single functional unit, excited only by light entering its own corneal 

 facet; each ommatidium, when so excited, discharges trains of impulses in 

 one and only one optic nerve fiber. These receptor units (ommatidia) as we 

 have said, are not independent in their action: each one may be inhibited by 

 its near neighbors and in turn may inhibit them. Thus, the activity of any 

 given ommatidium, while principally determined by the light shining upon its 

 facet, may be modified by the activity generated in the neigliboring omma- 

 tidia when they are stimulated by the light falling on their facets. 



The anatomical basis for this interaction of the ommatidia of the Limulus 

 compound eye is a plexus of fine nerve fibers — an extensive three-dimensional 

 network of fiber bundles immediately proximal to the layer of ommatidia. 

 In Fig. 1, a photograph of a silver-impregnated section of the eye, axons are 

 seen emerging from the proximal ends of the ommatidia. Several inter- 

 connecting bundles of fibers are labeled "5". At higher magnification (Fig. 2) 

 the axons of the eccentric cells {E. ax.) may be distinguished from those of the 

 retinular cells {R. ax.) by their greater thickness and density and by the 

 presence of a juxtaposed substance that is lacey in appearance and continuous 

 with the interconnecting bundles (B). In silver-stained sections such as this 

 the interconnections appear to be composed of small branches of the axons 

 of the retinular and eccentric cells. Electron micrographs of these structures 

 have proved that this is the case (Ratfiff et al., 1958). This is illustrated by 

 Fig. 3 and Fig. 4, electron micrographs of a retinular cell axon and an eccen- 

 tric cell axon in cross-section. In Fig. 3 the arrow marks the point at which 

 a small branch is seen emerging from a retinular cell axon (R) and joining a 

 small bundle of similar branches {B). In Fig. 4 a well defined branch (B) 

 emanates from an eccentric cell axon and joins a large bundle which cor- 

 responds to the lacey substance seen in silver-impregnated sections. In silver- 

 impregnated sections such as Fig. 2, areas of relatively greater density (A'^) 

 are seen within the lacey substance. The electron microscope has shown that 



