ACCOMMODATION IN SAUROPSIDA 



275 



The bones may be so dovetailed into each other that the ring is im- 

 mobile (Fig. 107b, c), or they may so overlap that they can slide on one 

 another; but there is no experimental evidence that they ever do so. 

 'Plus' and 'minus' plates are distinguished as to whether they overlap 

 both of their neighbors or are overlapped by both, and these exceptional 

 plates tend to occur in the vertical or the horizontal meridians, or both 

 (Fig. 107a), There is always an unusual situation mid-ventrally (where 

 either a '+' or '— ' plate or an edge-to-edge junction without overlap 

 occurs) — attributed to the disturbance of the formation of the plates 

 created by the embryonic fissure of the optic cup. 



THIS SIDE, 

 RELAXATION 



THIS SIDE, 

 ACCOMMODATION 



Fig. 109 — Diagram of generalized reptilian mechanism of accommodation. 



ap- annular pad of lens; bm- Briicke's muscle; bp- base plate of ciliary body; c- cornea; 

 ch- chorioid; cp- ciliary process; i- iris; lb- lens body; ot- ora terminalis; pi- pertinate liga- 

 ment; /, /- sclera; sc- scleral cartilage; scs- sclerocorneal sulcus; so- scleral ossicle; sr- sensory 

 retina; tbm- tendon of Briicke's muscle (continuous with inner layers of corneal substantia 

 propria); //- tenacular ligament; z- zonule. 



Accommodation in Sauropsida (Except Snakes) — The produc- 

 tion of a sulcus is the whole meaning, physiologically, of the sauropsidan 

 ossicular ring. It stiffens the concavity against the force of the intra- 

 ocular pressure which, if unresisted, would evaginate it. This pressure 

 rises slightly during accommodation, which it does not do in fishes, 

 amphibians, or mammals. On examining a sagittal section of a saur- 

 opsidan eye we see the internal result of the sclero-corneal sulcus: an 

 approximation of the ciliary body to the lens. 



