DARK ADAPTATION AND NIGHT VISION 



however, the cones increase greatly in 

 number and reach an estimated maximum 

 density of 147,000 per square millimeter at 

 the center. Rods, on the other hand, are 

 entirely missing in the fovea (in 0sterberg's 

 specimen, in a region about 0.6° in diameter). 

 Outward in all directions from the fovea, 

 they increase in number and reach their 

 greatest density in a region about 20° from 

 the center. Beyond this region, the rod 

 population decreases out to the extreme 

 periphery of the eye. Very much the 



In the fovea, cones have their own individual 

 nervous connections into the optic nerve. 

 It is probable that many cones in the 

 periphery also have their own individual 

 lines of communication, although it is 

 more common to find several cones, or 

 several cones and rods, linked together on a 

 single nerve pathway in the outer regions of 

 the retina. Rods, however, are never 

 found to have individual nervous connec- 

 tions; they are always linked together in 

 groups to single nerve fibers. 



2000 



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2 . 



d 



CO 



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o 



o 

 oo 



o 

 tr 



UJ 

 CD 



3 



1600 



1200 



800 



400 



lOO 



NASAL RETINA 



FOVEA 



TEMPORAL RETINA 



DISTANCE FROM THE FOVEA 



Fig. 2. The density of cones and rods on or near the horizontal meridian through a human retina. 

 The inset is a schematic map of the retina showing F, the fovea, and B, the blind spot. The striped 

 area represents the regions of the retina which were sampled in obtaining the counts plotted here. 

 (Data from 0sterberg, 72) 



same pattern is obtained in other directions 

 from the fovea, e.g., above and below it. 

 It is also clear from Fig. 2 that the rods 

 outnumber the cones throughout most of 

 the retina. 0sterberg estimates that the 

 eye contains between 110,000,000 and 

 125,000,000 rods and between 6,300,000 

 and 6,800,000 cones. 



Nervous Connections to the Cones and Rods. 

 There are also some important differences 

 in the kinds of nervous connections which 

 Hnk the cones and rods with the optic nerve. 



The relative concentration of the cones 

 and rods in different parts of the retina, 

 and the kinds of nervous connections these 

 cells have with the optic nerve, determine in 

 part certain important visual characteristics. 

 As will be pointed out later (Fig. 17), visual 

 acuity is best when the image of an object 

 falls directly on the fovea. It is here, in the 

 fovea, that we find the greatest density of 

 cones. In addition, the pin-point, one-to- 

 one, transmission of nerve impulses from 

 individual foveal cones into the optic nerve 



