RETINAL STRUCTURE AND VISUAL ACUITY 101 



(Figure 73). According to Wood, this position is very 

 similar to that found in man, and in this respect the general 

 conformation in Strigiformes resembles the human eye more 

 closely than does any other order. 



Birds of prey and swift fliers show the best marked and 

 greatest variety of macular regions. The sparrow hawk 

 serves as a good illustration. In this bird each macular 

 region shows a well-marked nasal fovea and a temporal 

 fovea which are connected 

 by a short band-like area. 

 According to Wood, in such 

 forms the nasal fovea is in- 

 variably the deeper and 

 sharper of the two and is 

 probably used for distinct 

 and binocular vision. Al- 

 though Wood says that in 

 the bimacular fundus, the 

 nasal fovea is probably used 

 for binocular vision, it is ap- 

 parent from Figure 78 that 

 the temporal fovea is used 

 for this purpose. The nasal 

 fovea is surrounded by a 

 large macular area and is 

 situated at about the center 

 of the retina. The temporal 

 fovea is shallower, surrounded by a small area, and is placed 

 near the periphery of the retina, lower than the nasal fovea 

 but about the same distance from the optic disc. The terns 

 and swallows, which are swift fliers, present much the same 

 foveal configuration as the hawks. This disposition of the 

 ' areas ' for acute vision assumes this particular form in birds 

 that take their prey on the wing, or by darting upon it from 

 a distance. 



Wood has classified the various 'areas' for distinct vision 

 in birds as follows : 



Fig. 78. Section through the head of 

 the white-belhed swallow (Tachycineta 

 bicolor), showing nasal and temporal 

 foveae. NX, NH, visual axes of foveae 

 nasales. TR, TL, visual axes of fovea 

 temporales. (Redrawn from figure 5, 

 Casey Wood, 1917.) 



