THE ELASMOBRANCH EYE 



565 



relax all together. This action, having the effect of a levator bulbi muscle, 

 was perhaps its ancient, original function. Apart from the pedicel, the 

 eyeball in various elasmobranchs is supported and cushioned in the orbit 

 by masses of gelatinous connective tissue, lymph- or blood-sinuses (cf. 

 lampreys) , or combinations of these. 



In sharks and chimaeras the eyeball is regular in shape and usually is 

 strongly ellipsoidal, with its longest diameter horizontal and its shortest 



Fig. 165 — Hypothetical primitive arrangement of 

 the extra-ocular muscles in gnathostome fishes. 



(The diagram shows the eyeball and the muscles as 

 seen from the dorsal side, and emphasizes the favor- 

 able orientation of the obliques for the production 

 of simple wheel movements of the eyeball in the 

 plane of its equator). 



ext- external rectus; inf- inferior rectus (revealed 

 through gap in superior rectus ) ; int- internal rectus; 

 io- inferior oblique (revealed through gap in supe- 

 rior oblique); n- optic nerve; so- superior oblique; 

 sup- superior rectus. 



Fig. 166 — Anterior segment of 



Mustelus mustelus. x5. Combined 



from figures of Franz. 



/- lens; Im- lens muscle (black); 

 p- lens-muscle papilla; r- retina; 

 s- suspensorium of lens; sc- scleral 

 cartilage. 



diameter the antero-posterior axis (Fig. 164). The eye is quite homog- 

 eneous structurally in these forms, with its greatest variations occurring 

 in the sclera, which may be very thin as in chimaeras and some deep-sea 

 sharks {e.g., Etmopterus) , or extremely thick as in the largest sharks. 

 In one deep-sea shark, Lcemargus, perhaps as a mark of degeneracy, the 

 sclera sends massive cartilaginous diverticula into the chorioid. In most 

 rays the depression of the body has involved the eyeball, producing a 



