30 VISION 



nervous control. Yet neurons contacting muscular elements had not been 

 observed and only one investigator (Carrere 1922) actually described myo- 

 filaments from the irides of elasmobranchs. 



Kuchnow and Martin (19706) established that the elasmobranch iris 

 possesses contractile elements with the characteristics of smooth muscle. 

 These elements were composed of a single type of myofilament 4-10 nm in 

 diameter. Neurons and neuromuscular junctions were also identified in both 

 dilator and sphincter muscles. The finding of neuromuscular junctions in the 

 sphincter was unexpected since it is an independent effector. The ratio of 

 neurons to sphincter fibers was, surprisingly, very nearly 1:1. 



The existence of neural elements suggested CNS control, and the authors 

 cited evidence for a retinally mediated iridial reflex. In addition, they 

 claimed that the iris dilator is cholinergically innervated by cranial nerve III. 

 However, attempts to dilate the pupils of both Negaprion and Gingly- 

 mostoma (Plate IIA) with adrenergic and cholinergic agents topically applied 

 and even injected into the anterior chamber were unsuccessful (Gruber 

 1969). 



Kuchnow and Martin (1972) next investigated the unique iris of skates 

 (Raja). The upper margin of the iris in many rajaform fish is modified into a 

 so-called operculum pupillare (Plate IIB-D) which descends over the pupil 

 during light adaptation. The completely expanded operculum reduces the 

 pupil to a series of stenopaic ("pinhole") apertures. According to Carrere 

 (1922), the operculum is amuscular, and its mode of action is unknown. 

 On the basis of fine structure, Kuchnow and Martin postulated that the 

 operculum has no contractile mechanism and only a weakly dilatory mech- 

 anism, if any. They noted that if the top of a dilated eye is gently depressed 

 the operculum abruptly descends, unconstricted, and rises again when pres- 

 sure is released. Thus, according to Kuchnow and Martin, dilation results 

 from relaxation of the iris sphincter and contraction of the iris dilator. 

 When the spinchter contracts, opercular fibers that the authors call tonic 

 allow the operculum to spring into shape, occluding the pupil. 



Contraction of both pupils in response to illumination of only one eye is 

 known as the consensual pupillary reflex. Such a reflex was long ago reported 

 in Raja and then confirmed by Kuchnow and Martin. A consensual reflex is 

 unexpected in skates since it does not occur in other elasmobranchs. The 

 fine structure of the skate's iris did not elucidate the mechanism underlying 

 this reflex. 



Pupillary Activity— Pupillary kinetics of elasmobranchs have been re- 

 ported by several authors (Gruber 1967, Kuchnow and Gilbert 1967, 

 Kuchnow 1970, 1971). Gruber (1967) found that the pupil of Negaprion 

 dilates fully in about lh (Figure 6). Using infrared photography, he followed 

 the course of pupillary dark adaptation after intense white light adaptation. 

 At the onset of dark adaptation, the pupil was a vertical slit covering an area 

 of about 10 mm 2 . As the pupil dilated, the area doubled after 2 min in 

 darkness and again after 5-7 min. A final doubling was observed between 

 40 and 55 min. Dilation was essentially complete after lh. The maximum 



