EYE REFRACTION AND ACCOMMODATION 111 



well (Sivak 1974a). However, this failure is tempered by the fact that 

 the drugs chosen were those employed successfully with teleosts (Myer 

 and Schwassmann 1970; Sivak 1973). 



In contrast to the above studies, current work (Sivak and Gilbert, 1976) 

 supports the view that sharks can accommodate. This is based on the record- 

 ing of refractive error differences in nurse and brown sharks (Gingly mo stoma 

 cirratum and Carcharinus milberti) before and after tricaine anaesthesia 

 (Table 1). The direction of the changes suggests movement of the lens to- 

 ward the retina under anaesthesia. Furthermore, it is possible to reverse the 

 direction of refractive change by electrically stimulating the root of the 

 oculomotor nerve of anaesthetized nurse sharks. Histological study indicates 

 possible contractile elements in the ciliarybody of both species. A report on 

 the accommodative responses of the eye of the bluntnosed stingray (Dasyatis 

 sayi) is discussed below. 



The Ramp Retina of the Stingray Eye 



While a static accommodative mechanism is said to exist in horses and rays, 

 a recent study of the horse eye indicates an almost symmetrical relationship 

 between the lens and retina (Sivak and Allen 1975). However, additional 

 study (Sivak 19756) of the lens-retina relationship in three species of sting- 

 rays (dasyatidae) confirms the asymmetry reported by Franz (Fig. 2). In 

 each case the dorsal portion of the retina is further away from the lens than 

 the distance along the geometric axis of the eye. The accommodative effect 

 of this lengthening of the visual axis is about 6 diopters (as determined for 

 Dasyatis sayi by calculation and by retinoscopic measurements of refrac- 

 tive error along the appropriate directions). 



The same study notes that refractive error varies in D. sayi when the 

 unrestrained specimen is presented with flashing targets at varying distances 



Table 1. Refractive error changes (right or left eye) in two species of sharks 

 under unanaesthetized and anaesthetized (MS 222) conditions and following 

 electrical simulation of the oculomotor nerve. (+) indicates hyperopia and 

 (-) indicates myopia.* 



Oculomotor 

 Species Unanaesthetized Anaesthetized stimulation 



Carcharinus milberti 



Ginglymostoma cirratum 



*Sivak and Gilbert, unpublished. 



