VISUAL ANGLES AND FIELDS 299 



Devices for Enlarging the Binocular Field — Aside from eye move- 

 ments (which we shall shortly consider) there have been evolved various 

 devices, both static and dynamic, for enlarging the binocular field despite 

 the handicap of ocular laterality imposed by the presence of an indis- 

 pensable snout or beak. These devices are of very diverse nature, but are 

 best described here under the only heading that unifies them. 



Two of them occur in fishes — the aphakic space, and the temporad 

 movement of the lens in accommodation. The aphakic (i. e., lensless) 

 space is widespread in teleosts, and often consists of an anterior exten- 

 sion of the basic circle of the pupil into an egg-shape, with the narrow 

 end of the egg pointing forward. One can see into the eye through the 

 narrow end of the egg, past the lens whose center is opposite the big end 

 of the egg (Fig. 105f, p. 261). It was long debated what the fish saw 

 outward through the aphakic space; but we now know that he looks 

 through it only with the temporal part of the retina, and thus through 

 the lens after all. Were it not for the aphakic space, the line of sight 

 could not be so nearly parallel to the body axis. Again, when the lens is 

 drawn backward by the retractor lentis muscle, there is a considerable 

 temporad component of the motion (Fig. 105). In those fishes which 

 have a fovea, the fovea is always temporal in location (Fig. 77b, p. 185), 

 and the lens in accommodation moves temporally more than it moves 

 backward toward the fundus. This shifts the effective visual axis more 

 nearly parallel to the axis of the body. 



The ungulates are conspicuous for their broad, horizontally oblong 

 pupils (Fig. 85c, p. 218), which extend the visual field somewhat (see 

 Fig. 90b, p. 225) in the horizontal meridian (v. re the horse, above) and 

 hence help to enlarge the binocular field. The frogs, the marmots, and 

 two carnivores (Cynictis and the Meerkat, Suricata) employ the same 

 trick, though not nearly so effectively. The snakes Dryophis and Dry- 

 ophiops, and the probably unrelated Thelotornis, not only have the key- 

 hole pupil with its aphakic portion lined up with the center of the lens 

 and the temporal fovea, but also have excavated a groove on the side of 

 the head in front of the eyes, along which the eye looks ahead (Fig. 79, 

 p. 186). In Dryophis at least, the lens during accommodation moves not 

 only forward but also more strongly nasally than in other snakes — a 

 device which accomplishes, in reverse, the same end as that attained by 

 the foveate fishes. The transversalis muscle of turtles and lizards (see 

 p. 279) likewise moves the accommodating lens nasally as well as slightly 

 ventrally. This is probably of especial help to the lizards, for their eyes 



