SENSITIVITY VS. ACUITY 67 



connect with the same ganglion cell. Other things being equal, the more 

 bipolar and ganglion cells in a retina, the higher its resolving power. 

 Two retinae may be about equal in this regard even when one has many 

 slender, tightly packed visual cells and the other has fewer, plumper, 

 more widely spaced ones; for in the first retina there might be many 

 bipolars but few ganglion cells, or fewer bipolars and more ganglion 

 cells, and the overall resolving power be no greater than that of the 

 second retina whose visual cells were scanty and large — provided they 

 had isolated bipolar and ganglion-cell connections. 



When sections of the retina are especially prepared so that its nerve 

 fibers and their connections are brought out, the retinal foundation for 

 the visual-acuity tenet of the Duplicity Theory is at once evident. Rods 

 are always connected in large numbers to single bipolar cells while cones 

 tend to have more isolated connections (Fig. 19, p. 43). Of the many 

 forms of bipolars in the human retina, the smallest (midget bipolars of 

 Polyak) each tend to be connected with a single cone and in turn to an 

 individual ganglion cell and optic nerve fiber, so that each such cone 

 has a 'private wire' to the brain; whereas, to extend the telephone anal- 

 ogy, other cones and especially rods are on the old-fashioned multiple 

 'party line'. 



This great difference in the degree of summation of rods and cones 

 is the most important single factor in making rod vision diffuse and 

 cone vision sharp. It is much more than enough to compensate for the 

 fact that in almost all retinae the rods are more slender than the cones, 

 which would give the rod-population the higher resolving power if the 

 degrees of rod- and cone-summation were made equal. Thus the chief 

 reason for the crude character of rod vision is outside of the rod itself; 

 and we should so state the Duplicity Theory that it attributes acuity 

 differences not to the rods and cones themselves but to the entire rod- 

 vision and cone-vision mechanisms, each including a set of visual cells 

 and their particular bipolars, ganglion cells, and optic nerve fibers. 

 Relatively few bipolars connect with both rods and cones and probably 

 a minority of ganglion cells embrace both rod- and cone-bipolars. Parin- 

 aud's 'theorie des deux retines' is thus really more expressive of the 

 facts than is 'Duplicity Theory'. The most recent and accurate esti- 

 mates of the number of rods and cones in one human retina are : rods, 

 110,000,000 to 125,000,000; cones, 6,300,000 to 6,800,000 (Osterberg). 

 There are about 1,000,000 fibers in the human optic nerve, not all of 

 which are sensory; and in a sizable group of these (the macular bundle) 



