THE ELASMOBRANCH EYE 567 



along the retinal surface. This vessel is squeezed inexorably forward 

 during development, however, as the embryonic fissure heals itself pro- 

 gressively forward from the fundus toward the periphery of the optic 

 cup. The adult counterpart of this vessel may be the tiny one which 

 supplies the lens-muscle papilla (v./.). 



The thin, broad, amuscular ciliary body bears low folds anteriorly, 

 which may run up onto the back of the iris, and always leave a smooth 

 orbicular zone behind them, toward the ora terminalis. The folds are 

 meridional in sharks, but in rays are restricted to the dorsal and ventral 

 quadrants (like the ciliary folds of the few teleosts that have them; and 

 like the iris folds in many amphibians). A gelatinous zonule, shaped 

 like a washer with a thickened rim, is anchored to the coronal region 

 and to the lens near its equator. Further support is given the lens by a 

 median dorsal, downward extension of the ciliary body into the zonule, 

 forming a 'suspensory ligament'. Ventrally, the lens rests upon a cushion- 

 like protuberance of the ciliary body. This papilla is in turn supported 

 erect by a fin-like continuation of itself onto the back face of the iris. 

 It is along the crest of this fin that the protractor lentis, the muscle of 

 accommodation, is placed (Fig. 166). This little muscle is a derivative 

 of (and indeed remains intercalated in) the pars ciliaris retinae covering 

 the papilla; hence, it is ectodermal. In its gross anatomical relationships, 

 the lens-muscle papilla varies considerably from genus to genus of elas- 

 mobranchs; and as an extreme of this variation it may give a fair 

 imitation of a teleostean falciform process together with its campanula 

 (^.v., pp. 582-3) — with which some of the elder anatomists seem to have 

 confused it. 



The iris is bowed forward in its middle by the subspherical lens, mak- 

 ing the anterior chamber extremely shallow. Histologically, the iris is 

 much like that of the parasitic lampreys, but with considerable pigmented 

 stroma underlying the argentea (which, as in lampreys, is confined to 

 the iris and does not embrace the chorioid) . Some blood vessels are free 

 in the stroma, but most lie against the retinal layers at the back of the 

 iris. The posteriormost of the two retinal layers is devoid of pigment 

 toward the root of the iris (as also in the ciliary body, as usual), but 

 takes on more and more pigment toward the pupil until, for the pupil- 

 lary one-third or so of the radial width of the iris, both retinal layers 

 are heavily and about equally pigmented. The epithelio-muscular ele- 

 ments of the sphincter and dilatator (the former not so well separated 

 from the parent epithelium as in man) are spindle-shaped, pigmented. 



