RETINAL STRUCTURE AND VISUAL ACUITY 105 



collect into a single bipolar cell (Figure 24). The former 

 condition is supposed to present a structural situation pro- 

 viding for isolated conduction of the cones — thus favoring 

 greater visual acuity at high intensities. The latter is said 

 to enhance summated conduction of the rods. This concept, 

 however, has become subject to modifications as a result of 

 recent researches of Polyak (v. Chapter II, p. 30). In con- 

 nection with the structure of the retina it is of interest to 

 compare the width of the external nuclear and internal 

 nuclear layers. These have been measured in twenty-seven 

 different forms (see also measurements by Menner, 1929) 

 and are given in Table 1. Photomicrographs showing some 

 of the differences are seen in Figure 16. From an examination 

 of the table and figure, it becomes obvious that in birds, 

 where visual acuity is known to be great, the internal nuclear 

 layer is extremely wide as compared with the width of the 

 external nuclear layer. This is especially striking in the 

 hawk, the sparrow and the goldfinch, where the internal 

 nuclear layer is from five to six times as wdde as the external. 

 This great difference must mean the presence of many asso- 

 ciation cells, be they amacrine or horizontal cells. In the 

 human retina (paramacular region) the internal nuclear 

 layer is narrower than the external. Diurnal reptiles, which 

 are likewise known for their creditable visual acuity, show 

 the increased thickness of the internal nuclear layer, but 

 the difference, in general, is not as great as in birds. 



It is interesting to note also that in the dimlight fish 

 (Ameiurus) where behavior is dominated largely by taste 

 buds, the internal nuclear layer is very poorly developed. 

 It measures 7.8 /a in width as compared with 122 ^u in the 

 sparrow. 



Among the mammals, conditions vary considerably. In 

 those which are primarily nocturnal, the internal nuclear 

 layer is considerably narrower than the external. Whereas 

 the contrast in thickness of the tw^o layers is considerable 

 in both the hamster and the cat, the bat shows almost a 

 1 : 1 relation. This form, however, which is nocturnal, has 



