VISUAL SYSTEM: STATE OF THE ART 15 



of light by repeatedly giving such trials. The conditioned nictitating mem- 

 brane response was shown to be a reliable indicator of both learning and 

 detection of visual stimuli by the sharks. This study also demonstrated how 

 rapid the movement of the nictitating membrane is: latency between onset 

 of electrical stimulus and unconditioned movement of the nictitating mem- 

 brane was less than 34 ms. In a well-trained shark, the conditioned response 

 occurred only 190 ms after onset of light. 



Extraocular Muscles/Eye Movements— Oliva (1967) reported on the 

 topography of eye muscles in eight elasmobranch species. This ecological and 

 phylogenetic study attemped to show a relation between habitat, extrinsic 

 eye muscle, and external form of the eye. Oliva found that littoral sharks 

 and Rajaformes have smaller eyes and better developed extrinsic eye muscles 

 than pelagic elasmobranchs. The forms of eye muscles in various elasmo- 

 branch species, including origin and insertion, are clearly diagrammed; this 

 appears to be the main value of this paper. 



Bell and Satchell's (1963) study reported that stimulation of the snout of 

 Squalus causes a reflexive rolling of the eye backwards and inwards. The 

 function of this abduction, produced solely by contraction of the external 

 rectus, was analogous to eye closure in Mustelus and Cephaloscy Ilium. That 

 is, it serves to protect the shark's cornea. 



Harris (1965) studied the eye movements of Squalus in detail. Eye move- 

 ments of free-swimming sharks were recorded by cinematography after small 

 plexiglass rods were glued onto each cornea. The rods served to amplify 

 angular movements of the eye and acted as reference points. Studies made 

 on restrained animals involved measurement of visual fields including blind 

 areas and the relation of eye movements to passive body bending. 



Five categories of eye movements were thus identified: (1) compensatory 

 eye movements, i.e., those caused by static labyrinthine influences; (2) 

 swimming movements, an active process opposing compensatory movements; 



(3) turning eye movements, which predicted a change in swimming direction; 



(4) fine movements possibly similar to slow saccades; and (5) the protective 

 eye reflex just described. Other categories of eye movements were seen 

 under artificial restraint. One important finding was that no visual stimuli 

 had any immediate effect on eye position and no visual fixation of any type 

 was ever observed. This agrees with our casual observations on Negaprion. 



Harris showed that the visual field of an active shark contains a large 

 component of binocular overlap (45°), but a blind spot is created by the 

 bulge at the pectoral girdle which is enhanced by the 20° lateral misalign- 

 ment of the eyes. This blind spot, amounting to 60° of visual angle below 

 the fish, is completely eliminated by normal head and body movements 

 associated with swimming. Thus for each complete stroke cycle the shark has 

 nearly panoramic vision. This description casts some doubt on Hobson's 

 (1964) functional interpretation of exaggerated swimming modes in gray 

 sharks (Carcharhinidae). Hobson suggested that the highly serpentine move- 

 ments might aid in increasing the visual field. However, if Harris's calcula- 

 tions are correct, exaggerated head movements are not necessary; indeed, 



