292 ADAPTATIONS TO SPACE AND MOTION 



margin nor necessarily coincident with, or even close to, the visual axis 

 — the actual physiological line of sight in fixation (cf. Figs. 3, 16; 

 pp. 7, 37). 



Paradoxically, the optic axis can be considered to be the visual axis 

 only when there really is no visual axis — that is, where there is no area 

 of acute vision or fovea and hence no fixation or precise aiming of the 

 eye at objects. In mammals there is usually an area, but it is central 

 (except in ungulates) and here the orientation of the optic axis does 

 become a fair criterion of the direction and extent of the binocular visual 

 field. Lindsay Johnson's chart of mammalian inter-axial angles (Fig. 

 113, p. 297) is therefore acceptable; but a similar chart for fishes (whose 

 foveae are strongly temporal) would be worthless as indicating the direc- 

 tion of best vision with the eyes at rest. 



The best studies have been the recent ones of Rochon-Duvigneaud, 

 Kahmann, and Pisa, who have made direct determinations of the visual 

 fields by observing the trans-scleral images of a movable light, in dis- 

 sected heads clamped in a perimeter. Most of our accurate knowledge 

 of visual fields in animals has come from these investigations. 



In fishes, Kahmann found that the binocular field measured usually 

 from 20 to 30 in the horizontal plane. There were wider variations 

 among the marine forms, where the angle might be as small as 4 (Box, 

 Trigla) or greater than 30 (Trachurus, Cepola, Serranus, certain 

 labrids, and especially in flatfishes) . Among freshwater forms the widest 

 binocular fields, and thus the greatest degree of frontality, were in such 

 predators as the trout, perch, and pike, with values ranging from 30° 

 to 40 or more. But on the marine side the predaceous Julis revealed a 

 value of only 15° and the mackerel-like Lichia, 8°. A great surprise to 

 Kahmann was the low value of 14 for the archer-fish, Toxotes jaculator 

 — which, by analogy with the snakes which have the habit of striking 

 and hence have similar visual requirements, might be expected to have 

 as wide a field as Dryophis (v./.). One fish, Chlorophthalmus agassizH, 

 probably does rival Dryophis, for it is reported to have a strikingly 

 similar pupil (see Fig. 79, p. 186). In one type of chondrostean, the 

 spoonbill or paddlefish Polyodon spathula, the eyes are aimed forward 

 about as frankly as in some deep-sea fishes (see Fig. 138b, p. 403). But 

 since they are set on the dorsal side of the 'paddle' near its base, their 

 view downward is cut off. 



Because of their periscopy, nearly all fishes also have something of a 

 dorsal binocular field. Bottom-dwelling fishes have truly specialized such 



