RETINAL RECEPTOR MECHANISM 123 



their outer segments pointing to the pigment cell layer. Protruding 

 beyond the external limiting membrane (8) is a rod or filament-like 

 structure divided into three segments : inner segment, fiber apparatus, 

 and outer segment. If the structure is a cone, it is also divided into 

 three sections: inner segment, lentiform bod} r , and outer segment. 

 The rod and cone elements are regarded as specialized neuroepithelial 

 cells and not as nerve cells. It has been suggested that the rods and 

 cones are not distinct elements because various characteristics of the 

 one are found in the other. Since in man the cones in the fovea resemble 

 the rods at the periphery of the retina, it might be concluded that the 

 cones have become specialized in one direction and the rods in the 

 opposite direction, having a common relatively neutral ancestor. 



The distribution of rods and cones is not uniform throughout the 

 retina. At the entrance of the optic nerve (optic disk), rods and cones 

 are absent; hence light incident upon this area (blind spot) gives no 

 visual sensation. The fovea centralis contains no rods. In the macula 

 lutea, and in a widening circle around the fovea, rods and cones are 

 present in approximately equal proportions. Towards the periphery 

 the cones decrease in number until at the very margin only rods remain. 



The mosaic of rods and cones is very regular. Near the macula the 

 cones are separated from each other by a single circle of rods; a short 

 distance from the macula each cone is surrounded by series of three 

 rows of rods, then four, and this pattern of increasing number of rods 

 continues even close to the ora serrata, where only rods are found. 



The macula lutea is about 0.6 mm in diameter, and in this minute 

 area a real image must be projected to produce distinct vision. The 

 inference is that the cone structure is identified with perception of 

 detail. The threshold for stimulation in this region is rather high and 

 it is not appreciably increased by dark adaptation. The cones, there- 

 fore, are not particularly adapted to perception of low intensities of 

 illumination. This fact can be verified experimentally by looking at a 

 night sky; the stars located at the border of the visual field appear 

 much brighter than those lying at the center. 



In the most peripheral zones of the retina, where only rods are present, 

 the threshold for vision is much less and can be decreased still more by 

 dark adaptation; hence, the inference is that the rods respond best in 

 dim illumination, and it has been found that they are particularly 

 capable of detecting movements of retinal images of low intensity. 



Color perception, being associated with the cones, is most highly 

 developed at the fovea. Here the inner layers of the retina become very 

 much thinned, until near the center the transparent nerve tissues are 

 represented merely by scattered cells of the inner nuclear and ganglion 



