THE DIURNAL RETINA 



177 



gone to bed, and tend toward twenty-four-hour activity. The presence 

 of enough rods to make this possible would sometimes affect visual 

 acuity too adversely, except for the development of a small pure-cone 

 area, the 'area centralis', in the otherwise duplex retina. Such an area, 

 like the whole of a pure-cone retina, is necessarily blind in dim light. 

 The outer nuclear layer, formed by the rod and cone nuclei, tends to 

 have few rows in diurnal retinse. Cones being usually more plump than 

 rods, there is more room for their nuclei to lie directly against the ex- 



|Diurnal| 



receptors ' 



(many cones) 



summoted 



but little 



in: 



BIPOLAR CELLS-^ 



finally 

 sunnnnated 

 but little 

 * in: 



GANGLION CELLS* 



-•-BIPOLAR CELLS 



nally 



summated 



extensively 



in: 



GANGLION CELLS 



Fig. 72 — Diurnal and nocturnal retinae contrasted. 



The diagrams represent two related species, one of which is diurnal and the other noaurnal. 

 The characteristic differences in the relative thickness of the nuclear layers are the result of 

 the visual-cell patterns and the differing extents of summation in optic nerve fibers. 



ternal limiting membrane. Where rods are few or absent, this makes for 

 a thin outer nuclear layer. In some lampreys, there is but a single row 

 of nuclei. Turtles and squirrels have but a couple of rows, as do the 

 amphibians — in the latter it is the unusual bulk of the rods, and their 

 relatively small numbers, which is responsible. Where the rods are 

 slender and are as numerous as they are in man, the outer nuclear layer 

 becomes thick; and (Fig. 69a) it becomes far thicker still, of course, in 

 twenty-four-hour and nocturnal eyes (except, again, in amphibians) . 



