260 ADAPTATIONS TO SPACE AND MOTION 



there would seem to be no need whatever for such an action since the 

 resting eye is too long (that is, myopic) to begin with. Such an action 

 of the external muscles might however serve to combat any temporary 

 flattening effect of water pressure in lampreys which descend to consid- 

 erable depths, as ocean species are known to do. 



Elasmobranchs — In the elasmobranchs the lens is again spherical or 

 nearly so (Fig. 104). The iris does not commence at the ora terminalis 

 as in lampreys, for a ciliary body with many low radial folds intervenes. 

 A washer-shaped, gelatinous membrane, attached peripherally over the 

 whole surface of the ciliary body and centrally around a narrow equator- 

 ial zone on the lens surface, serves as a zonule. A dorsal, radial thicken- 

 ing in this membrane gives the lens most of its actual support. Diametri- 

 cally opposite, in the ventral meridian, there is an elaborate papilla on 

 the ciliary body which contains smooth muscle fibers. These fibers are so 

 oriented that when they contract, they swing the lens pendulum-fashion 

 toward the cornea. Accommodatory effort is thus exerted for near vision, 

 as in ourselves, and not for distant vision as in lampreys and teleosts. The 

 anterior chamber is very shallow, but there is always some space between 

 the relaxed lens and the cornea. The depth of this space represents the 

 range of accommodatory movement of which the lens is capable — unless, 

 as may be, the little protractor lentis muscle is strong enough to make the 

 lens bulge the cornea somewhat. The eyes of elasmobranchs have con- 

 siderable hypermetropia — ten to fifteen diopters in various species, 

 according to Franz; but they can accommodate from fifteen to twenty 

 diopters, hence may have very close near points. 



Teleosts — Teleost fishes have a mechanism which is superficially similar 

 to that of the elasmobranchs but actually can have no evolutionary con- 

 nection therewith. Here again the lens is spherical, and touches the cornea 

 as in the lampreys (Fig. 105). It is suspended by a dorso-nasal ligament 

 consisting of material essentially like tough vitreous, and running from 

 the pars caeca retinae to the surface of the lens a little anterior to its 

 equator. On this ligament the lens can swing — not freely as an undamped 

 pendulum however, since there is evidence for the presence of a diaphan- 

 ous zonule with radial and even circular fibers, anchoring the lens to the 

 pars caeca in all meridians. 



Approximately diametrically opposite the suspensory ligament, but 

 with much variation in location and structure from species to species, 

 is the tendinous insertion, in front of the lens equator, of a small ecto- 



