24 VERTEBRATE PHOTORECEPTORS 



especially in the lemurs; whereas in the diurnal lizards, and 

 particularly in birds, the internal nuclear layer is much 

 thicker. This is striking in the sparrow and goldfinch (Figure 

 16). The great thickness of this layer in birds is in all proba- 

 bility an anatomical expression of their great visual acuity. 

 In the nocturnal forms, the reverse is true, and many rods 

 are collected into fewer bipolar and association cells, so that 

 the external nuclear layer is much broader (thicker) than 

 the inner nuclear layer (Figure 16). 



It is apparent that the very thick internal nuclear layer 

 of birds must contain a great many association cells. In the 

 sparrow and rosefinch it is between five and six times as 

 thick as the external nuclear layer. There are probably less 

 bipolar cells than visual cells (since these forms have rods in 

 addition to cones). Hence the greater part of this layer 

 must be made up of other neurones (amacrine cells and 

 horizontal association cells). In this connection it is of 

 interest to quote from Casey Wood (1917) who says, '^Ac- 

 cording to Franz (Das Vogelauge, 1911) the layers of the 

 avian retina are more sharply defined than in any other 

 vertebrate; even more so than in Mammals. There are no 

 separate bipolar or optical ganglion cells in Birds, while we 

 find in Mammals isolated amacrine cells, and, in reptiles, 

 bipolar cells." This statement is difficult to understand for 

 Franz (1913) certainly shows bipolar cells in the avian 

 retina, and Wood himself (page 17) in speaking of the inter- 

 nal nuclear layer says, '^ Between, and further in than the 

 cells, which Schiefferdecker calls nucleated cells, are found 

 two kinds of bipolars; first, outer bipolars with highly de- 

 veloped dendrites, and internal, small or thin bipolars with 

 weaker dendrites." The presence of bipolar cells in all classes 

 of vertebrates is an established fact. 



Although the earlier anatomists did much to unravel the 

 detailed cellular architecture of the retina, it was not until 

 the introduction of the chrome-silver method of Golgi and 

 the methylene blue technique of Ehrlich that the finer 

 structures and their connections were adequately worked 



