THE OPHIDIAN RETINA 



639 



and the retina owes as much of its sensitivity to increased summation 

 as to the lowering of the thresholds of the individual visual cells. 



Fig. 187 — Representative visual cells 

 (transmuted rods) of a nocturnal colubrid, 

 Hypsiglena o. ochrorhynchus. xlOOO. 



Fig. 188 — Visual-cell types in the Cro- 



talidcP: single cone, double cone, and 



rod; from the copperhead, Agkistrodon 



mokasen. x 1000. 



a b 



Fig. 189 — Representative visual cells in the Viperid« (see text). xlOOO. 

 a, of Cape viper, Causus rhombeatus. b, of puff adder, Bitis arietans. 



