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



neighbors in the retinal mosaic than for more widely separated ones, such 

 contrast effects will be greatest in the vicinity of sharp discontinuities in light 

 intensity in the retinal image. That is, the outlines of objects imaged on the 

 retina will tend to be emphasized. Thus patterns of neural activity generated 

 by the receptor mosaic may be distorted in a useful way by inhibitory inter- 

 action in the retina; such distortions constitute an early step in the integration 

 of nervous activity in the visual pathway. 



A simple inhibitory interaction has been found to exist in the lateral eye 

 of the horse-shoe "crab", Limulus. This interaction was first noticed about 

 twenty years ago when studies of the properties of single receptor units in the 

 Limulus eye showed that these elements were not, as had previously been 

 thought, independent of one another. The analysis of this interaction has 

 been presented in several papers in recent years and has also been reviewed 

 elsewhere (Harthne, 1959; KslXM et al., 1958; Ratliff, 1961). This paper will 

 give a brief description of the histology of the Limulus eye, and a synopsis 

 of the experimental studies will stress the principles on which the theory of 

 this interaction has been developed. 



The lateral eye of Limulus is a coarsely facetted compound eye containing 

 approximately 1000 ommatidia, each with a corneal facet approximately 0-1 

 mm in diameter and a crystalline cone which is the dioptric system for the 

 sensory structure of the ommatidium. This latter structure is composed of 

 about a dozen retinular cells and a bipolar neuron ("eccentric cell") which 

 sends a dendritic process up the axial canal in the center of the retinular 

 cluster (see insert Fig. 2). The contiguous inner surfaces of the retinular 

 cells near the axis of the ommatidium are elaborated into the rhabdom. This 

 has been shown by electron microscopy to be a "honeycomb"-like structure 

 composed of densely packed microvillous out-pouchings of the retinular cell 

 surfaces (Miller, 1957, 1958): the rhabdom presumably contains the visual 

 pigment. The visual pigment of Limulus has recently been isolated and studied 

 by Hubbard and Wald (1960): it is a retininei rhodopsin whose absorption 

 spectrum adequately accounts for the action spectrum of the ommatidium 

 as determined from measurements of single optic nerve fiber activity by 

 Graham and Harthne (1935). Both the retinular cells and the eccentric cell 

 give rise to axons which, emerging from the proximal tip of the ommatidium, 

 travel in small bundles and collect with those from the other ommatidia of 

 the eye to form the optic nerve. 



Each ommatidium of the Limulus eye appears to function as a single 

 "receptor unit". Bundles of nerve fibers can be dissected from the optic nerve 

 and subdivided until the electrical record has the characteristics of the action 

 of a single unit. By exploring the corneal facets one by one with a small spot 

 of fight, this unit can be identified with one and only one particular omma- 

 tidium of the eye ; in numerous experiments the nerve strand has been dissec- 

 ted free all the way up to its ommatidium of origin. The eccentric cells in 



