No. 3-] ACUTE VISION IN VERTEBRATES. 485 



fovea, where only cones exist, colors are most easily perceived, 

 while in the periphery, where there are few cones, it is difficult 

 to distinguish them. 



M. H. Parinaud (43) has found by experiment on the excised 

 retina that the visual purple (hence the rods) cannot be demon- 

 strated nearer the center of the fovea than two millimeters. 

 From this place the rods are found to increase in number 

 toward the periphery and the cones to decrease. 



Again, the retina being four times as sensitive to faint 

 impressions 25° from the fovea as at the fovea, and since at 

 this distance the rods are far more numerous than the cones, 

 we can consider the functions of the rods fairly well determined 

 to be the perception of diffuse and gray lights. 



To sum up: (i) the rods and cones are the sensitive elements 

 of sight; (2) the rods give us the sensation of gray, while the 

 cones give us the sensation of color and gray; (3) the rods are 

 more sensitive to faint impressions than the cones; (4) the 

 elements of the other layers form the connection with the optic 

 nerve. 



With this in view concerning the functions of the retinal 

 elements in man, and supposing the functions to be the same 

 in the other vertebrates, a physiological comparison may be 

 attempted. 



Quite a difference is noticed in the relative thickness of the 

 layers of the retina of the different vertebrates. This is shown 

 diagrammatically in PI. XXVII, Fig. 22. The layers which 

 exhibit the greatest difference are the inner and outer nuclear 

 layers and the rod and cone layer. In mammals the outer 

 nuclear layer is much thicker than the inner, while in birds, 

 reptiles, amphibians, and fishes the reverse is true. 



The layer which shows the greatest diversity, however, is 

 that of the rods and cones. A great difference exists in their 

 size, length, shape, and relative number. Fishes possess the 

 longest rods, while amphibians have not only long rods, but 

 also the thickest found in the vertebrates. The rods of mam- 

 mals are long but very slender, while in birds they are compar- 

 atively short and thick. The cones are the longest and most 

 slender in some of the reptiles (chameleon), and of greatest 



