EYE MOVEMENTS IN FISHES 303 



the classification given above. These differences find their explanation 

 in the presence and absence, and the location, of special retinal regions 

 of particularly high resolving power. Table III (p. 187) lists these areae 

 and foveas and should be constantly consulted while reading the ensuing 

 discussion of eye movements in the various classes of vertebrates. 



Fishes — The fishes reveal plainly that the original, primitive function 

 of the eye muscles was not to aim the eye at objects at all. Their original 

 actions were all reflex and involuntary, and were designed to give the 

 eyeball the attributes of a gyroscopically-stabilized ship, for the purpose 

 of maintaining a constancy of the visual field despite chance buffetings 

 and twistings of the animal's body by water currents and so on. We will 

 see later, when we consider the subject of movement-perception, just 

 how and why this constancy of field is important. 



The vast majority of fishes have only the reflex, involuntary, eye move- 

 ments.* Except in such forms as the rays and flatfishes, these are chiefly 

 in the horizontal plane. The bottom-hugging rays look mostly up and 

 down rather than from side to side, and in them the superior and inferior 

 rectus muscles are better developed than the lateral ones, whereas in their 

 pelagic relatives the sharks, the lateral recti are the heavier. In fishes 

 whose eyes sit laterally, every turn of the head is accompanied by a com- 

 pensatory turning of the eyes. A moving object is never followed by an 

 eye movement — instead, the fish (having, ordinarily, no neck) bends or 

 turns the whole body so as to face the interesting object and keep it in 

 the binocular field. In aquarium specimens, 'wheel' movements of the 

 eyes can often be clearly observed : as the fish tilts his body in starting 

 to swim upward or downward, the eyeball makes a compensatory ro- 

 tation in the plane of its equator. This movement, obviously carried out 

 by the two oblique muscles, suggests that this was the primitive function 

 of those muscles. 



In a number of species, spontaneous movements are known to occur. 

 All of these forms which have undergone histological examination (ex- 

 cept Cory dor as! — see p. 387) have been found to be provided with a 

 fovea, and there is an excellent correlation between the degree of per- 

 fection of the construction of the fovea — (in regard to visual-cell con- 

 centration, exclusion of rods, depth of depression, etc.) and the extent 



'••^Retraaive movements of the eyeball, which may perhaps be voluntary, are common enough 

 in fishes and other vertebrates; but such movements have of course nothing to do with 

 space-perception. 



