RETINAL STRUCTURE AND VISUAL ACUITY 95 



small cones and large rods of two types, viz., the red rods 

 and the green rods (Figure 44). Anyone who has made 

 repeated examinations of the retina is well aware that the 

 visual cells of fishes and amphibians are much easier to 

 study than are those of birds and most mammals, by reason 

 of their larger size and greater distance apart. 



In the reptiles one meets with a variety of conditions. 

 In the predominantly cone retinae of diurnal turtles and 

 in the pure cone retinae of the diurnal lizards, the cones 

 near the ora serrata are short and stout with considerable 



Fig. 62. Photomicrograph 

 showing area centralis retinae 

 of the turtle, Chrysemys 

 picta. Note local thickening 

 of the external nuclear layer 

 caused by increased number 

 of cones. X 360. 



intervals between them. As one passes from the ora towards 

 the fundus, they become more numerous per unit area, and 

 gradually longer. In the turtles one sees a small region in 

 the visual axis of the eye where the cones are distinctly 

 smaller and more numerous than in the remainder of the 

 retina. The increased numbers of nuclei, therefore, cause 

 a bulging and thickening of the external nuclear layer. This 

 region is termed the area centralis retinae (Figure 62) and is 

 regarded as the forerunner of the appearance of the fovea. 

 The concentration of numerous thinner and more closely 

 packed elements in this region, no doubt presents a struc- 

 tural condition enhancing visual acuity. In the diurnal 

 lizards the fundus specialization has gone further and here 

 one meets a highly developed fovea centralis where the 

 cones become long, closely packed, and filamentous. This 

 is particularly striking in such forms as the horned toad 

 and the chameleon (Figure 68) . In the latter form the f oveal 

 cones reach a length of about 100 /z which surpasses that of 



