14 VISION 



Gilbert and Oren (1964) discussed the relation of the nictitating mem- 

 brane to the so-called subocular fold found, for example, in the Triakidae. 

 Based on the insertion of a muscle homologous to levator palpebrae nicti- 

 tans, they contended that the subocular fold is a poorly developed nicti- 

 tating membrane. They thus recommended dropping the term "subocular 

 fold." 



Movement of eyelids, including the nictitating membrane, has been de- 

 scribed by Bell and Satchell (1963). Using mainly Squalus, but also with 

 Cephaloscy Ilium and Galeorhinus, they stimulated the skin around the eye 

 by various means. Such mechanical or electrical stimulation elicited an im- 

 mediate unilateral and reflexive movement of the eyelids. Closure was slight 

 in Squalus but complete in the other species. Bell and Satchell thus mapped 

 the reflexogenic zone and measured such response properties as latency facil- 

 itation and rate of response. They presented excellent photographs of the 

 response. In addition, they recorded from, cranial nerves with micropipets 

 and attempted to follow the neural pathway of the response. 



Bell and Satchell concluded that the response may be interpreted as pro- 

 tective of the cornea. Clearly it is not visually mediated since it persists in a 

 preparation with eyes removed. Neither is it part of a labyrinthine reflex. 

 Gilbert (1963) and Harris (1965) also believed that eyelid closure was a 

 protective mechanism. 



Walls (1942) mentioned that blue sharks (Prionace) were seen to blink in 

 bright light, but we have not been able to confirm this either in personal 

 observations on the blue shark or by intensely illuminating the eyes of a 

 lemon shark. On the other hand, movements of the nictitating membrane 

 during feeding have been observed repeatedly in many species. Thus it seems 

 clear that eye closure in sharks functions neither to lubricate the cornea 

 which is bathed in an aquatic medium nor to reduce the amount of light 

 entering the eye. 



Agalides (1969) attempted to measure the sensitivity of the lemon shark 

 (Negaprion) to electric stimuli by using a reflexive movement of the nictitat- 

 ing membrane similar to that described previously. Few details were given; 

 however, Agalides estimated that the nictitating membrane will move if each 

 ampulla of Lorenzini receives an electric stimulus of about 2 X 10~ 4 A. 



Gruber and Schneiderman (1975), using the same response, i.e., uncondi- 

 tioned movement of the nictitating membrane to externally applied electric 

 stimuli, studied acquisition, extinction, and other parameters of classical 

 conditioning in the lemon shark (Negaprion). This response had been used 

 before in psychophysical studies by Gruber (1966, 1967, 1969), and details 

 on reliability of training as well as the precise form of conditioning were 

 required to enhance the validity of the visual data. The basic conditioning 

 trial consisted of a 500-ms flash of white light, the final 100 ms accompanied 

 by an electric shock. The interval between trials averaged 30 s and 100 trials 

 were given per session. All trials were videotaped and replayed in stop-frame 

 mode for temporal analysis. 



The experiment demonstrated that Negaprion could be reliably and 

 quickly conditioned to move its nictitating membrane in response to a flash 



