AMPHIBIAN, REPTILIAN EYE MOVEMENTS 305 



a prominent aphakic space (Fig. 105f, p. 261) — so commonly associated 

 with a temporal fovea — and all of them should be studied histologically. 

 Examples are: Promicrops itaira, Stenotomus versicolor, Monacanthus cili- 

 atus, Centropristes striatus, Mycteroperca bonaci, Sphceroides maculatus. 

 The fishes thus illustrate clearly the universal principle that: where 

 there is no fovea, or at least a well-defined area of acute vision, there are 

 no spontaneous eye movements. For, unless one spot of the retina is 

 clearly superior to the rest in resolving power, there is no advantage 

 in aiming any one part of the retina at the object of interest, whether the 

 latter is still or in motion. Only when the object has moved close to the 

 edge of the visual field will any action be taken to maintain visibility of 

 it — and then it is by a turning movement of the whole body (or of the 

 head if a neck is present) , and not by a movement of the eye unless the 

 optomotor reaction is being evoked. The act of precise fixation, then, is 

 performed only by areate and foveate animals. 



Amphibians — No amphibian is known to perform any eye movements 

 other than retraction and elevation. ' Since retraction is usually, if not 

 always, elicited by a contact with the eye or used (by the Anura) as an 

 aid to swallowing, it is questionable whether it is ever spontaneous. In 

 turntable experiments, amphibians exhibit the usual compensatory move- 

 ments and also give an optomotor reaction to a rotating field; but in the 

 absence of a neck (anurans) these movements are of the whole body. 

 The eyes do not turn in the orbits at all. Frogs have an area centralis, 

 but this is a large, vaguely defined, horizontal crescent (in Hyla, a large 

 circle), whose superiority in resolving power, over the remainder of the 

 retina, is extremely slight. There is therefore no more need of any fix- 

 ative aiming of the eye than in the great majority of fishes. 



Reptiles — Reptiles may sit for hours without making spontaneous eye 

 movements; but most species are capable of them, as well as of the full 

 panoply of labyrinthine and optomotor reflexes involved in the gyro- 

 scopic maintenance of the visual field. 



The crocodilians have not been much studied; but, being nocturnal, 

 they are probably comparable to the amphibians in the matter of eye 

 movements. The turtles however, despite the absence of a fovea in all 

 but Amy da, have a good-enough area centralis to need the power of 

 fixation. Their lateral eye movements, particularly in carnivorous forms, 

 are coordinated for binocular observation; but vertical motions are made 

 independently by the two eyes, which are thus not truly conjugated. 



