THE PERCEPTION OF SPACE 



679 



Fig. 816. 



-The Binocular Visual Field of a Flat-fish with Upwardly 

 Directed Eyes. 



and extending a considerable distance dorsally (some 135° from the 

 horizontal) and considerably less ventrally (some 60°) (Fig. 815) ; the 

 area behind and below is often blind. The binocular field in front 

 varies in width considerably, from exceptionally small values of 10° or 

 less {Box) to 35° or greater in such active predators as the trout or 

 pike (Verrier, 1930 ; Kahmann. 1932). The smallest binocular field 

 yet measured in any Vertebrate is that of the gurnard {TrigJa) of 2° 

 (Verrier, 1928) ; that of the carp {Cyiyriyius) is very little more (Rochon- 

 Duvigneaud, 1922). Depending on the configuration of the body of 

 the fish a small overlap in the unilateral fields may occur posteriorly, 

 particularly in eel-shaped forms, but it is probabl}^ of little functional 

 value (Fig. 817). In bottom-living fishes such as the selachian skates 

 and rays and the teleostean flat-fishes, the binocular fields are increased 

 overhead but not so much as might be expected since the two eyes on 

 the upper side of the head preserve to a considerable extent their lateral 



Trout 



Ray 



Fig. 817. — The Deep-sea Snipe-eel, Bokodi.wla isFAys, 



Lateral (above) and dorsal (below) views. Owing to the narrowness of 

 its body and the protrusion of the eyes there is a small posterior binocular 

 field (after Bertin). 



