36 



VISION 



Figure 6 Pupillary dilation in Negaprion breuirostris revealed by infrared photography. 

 The shark was intensely light-adapted, then placed in darkness and photographed after 

 various intervals. During the 60 min of dark adaptation the pupil increased tenfold in area 

 and changed shape from slit to circular. This shark's iris contracted somewhat less than 

 usual. Left number is min in darkness; right number is pupillary area in mm. (Taken from 

 Gruber 1967. P.W. Gilbert, R.F. Mathewson, and D.P. Rail, eds. © 1967 by permission 

 of the Johns Hopkins University Press.) 



the species tested, including Rajaformes (Dasyatis and Myliobatus), dis- 

 played a consensual reflex. 



Pupillary control mechanisms were also investigated by Kuchnow, using 

 electrical, pharmacological, and photic stimulation combined with denerva- 

 tion and extirpation. Injection of prostigmine speeded dilation, while 

 d-tubocurarine inhibited it. Similarly, sectioning of the occulomotorius also 

 inhibited dilation and speeded constriction in light. The illuminated eye 

 dilated upon electrical stimulation of the occulomotorius. Illumination of 

 the retina alone (1-mm spot through the pupil) caused rapid and complete 

 pupillary constriction, while illumination of the iris with this spot produced 

 only local constriction. From these results, Kuchnow suggested that pupil- 

 lary size is the result of two opposing forces: (1) the action of both light and 

 neural signals on the sphincter and (2) the tonus of the dilator. It was 

 originally thought that during dilation the iris was an independent effector 

 only (Young 1933), but Kuchnow has demonstrated a neural component in 

 the dilator system. Finally, results with spots of light suggest an iris-retina 

 reflex. Kuchnow thus proposed that the iris of diurnal sharks constricts 

 rapidly because of greater influence of nervous mechanisms. Nocturnal 

 sharks are intermediate and Rajaformes are slowest, probably reflecting a 

 decrease in importance or absence of nervous control. 



