20 VERTEBRATE PHOTORECEPTORS 



Thickness of entire retina in ju 



Distance from lamina vitrea to external limiting mem- 

 brane in jjL 



Distance from external limiting membrane to distal tips 

 of visual cells (rods and cones) in fx 



Thickness of the external nuclear layer in jjl 



Thickness of the internal nuclear layer in jjl 



It is only by making such measurements that one can get a 

 true picture of the great variation in the thickness of the 

 various retinal layers in different forms. The thinnest 

 retinas in the region of the fundus of the forms studied were 

 found to exist in the guinea pig (130 m), the bat (133 jjl), the 

 hamster (182 jjl), and the catfish (182 fx). The thickest were 

 in the macular region of the chameleon (442 fx), the hawk 

 (400 m), the owl (389 m), and the human (390 m). The 

 diurnal lizards and birds in general were found to have 

 thicker retinas than mammals and the anthropoids. 



The longest cones were found in the chameleon where the 

 foveal cones measured 100 m (from distal tip to external 

 limiting membrane), and in the marmoset (91 m)- The 

 shortest cones were measured in the catfish (31 m, light- 

 adapted). The longest rods were encountered in the teleost 

 (Fundulus) where the light-adapted rods measured 99 m and 

 in the owl (83 m)- The shortest rods were found in the bat 

 (76 m) and the alligator (23 m)- 



Perhaps the most extreme differences were found to exist 

 in the thickness of the external and internal nuclear layers. 

 The external nuclear layer ranged from 10 m in Ameiurus 

 (catfish) to 104 m in the nocturnal monkey (Nyctipithecus), 

 although it was found to be almost as thick in the nocturnal 

 lemurs (Galago 78 m; Nycticebus 86 m)- The internal nuclear 

 layer was found to range from 21 m in the dogfish to 122 m in 

 the sparrow, and 166 m in the macular region of the hawk. 

 These extreme differences (and others) are seen in Figure 16 

 and in Table 1. 



The external nuclear layer of all nocturnal animals (ex- 

 cept geckos) is relatively much thicker than the internal, 



