RETINAL STRUCTURE AND ANIMAL HABITS 63 



migrate during the daytime and settle down at night, whereas 

 many birds migrate only at night. Almost yearly we hear of 

 scores of migrating night birds which encounter death on the 

 wing, by flying into strongly illuminated tall buildings, 

 apparently dazzled by bright lights. 



Rochon-Duvigneaud (1919), who has done considerable 

 work on the avian retina, says that diurnal birds have a 

 predominance of cones, and 

 nocturnal birds have a pre- ^ Hrh __ H 



ponderance of rods. Krause ^^' 

 (1894), Hess (1913), and 

 others, also found that noc- 

 turnal birds possess some 

 cones and that the diurnal 

 forms possess some rods. 



Certainly it is within the 

 experience of anyone who 

 goes from the city into the 

 country in late spring and 

 early summer to realize 

 that, as dawn begins, birds 

 become so active as to be either a delight or a source of. 

 annoyance to the individual, depending largely upon his 

 temperament. Who also has not experienced the pleasure 

 of hearing the solemn call of the whippoorwill as nightfall 

 approaches? 



Because of the fineness of the visual elements and the 

 great abundance of epithelial pigment, the finer details of 

 the bird retina are made out with difficulty. By bleaching 

 out the pigment, and with successful staining, one can, in 

 most instances, obtain a satisfactory view of the visual cells. 

 A photomicrograph of the bleached chicken retina, stained 

 with iron-hematoxylin, is shown in Figure 45. This figure 

 shows the presence of a goodly number of rods, which is 

 somewhat disturbing because we all know that the fowl 

 apparently has a poor apparatus for night vision, for he goes 

 to roost long before man ceases to enjoy good vision. 



A 



Fig. 44, Drawing showing red rods 

 (RR) and green rods (GR) of the frog 

 from A, dark-adapted eye, and B, 

 light-adapted eye. X 850. (From Arey, 

 1916, J, Comp. Neur., v. 26.) 



