Presbyopia in light of accommodation.

The "dual, indirect, active" mechanisms of accommodation proposed by Helmholtz is reviewed. New supporting evidence shows that the insertion of the ciliary muscle is onto the span fibrils of the zonule in the interciliary process regions of the ciliary body. Thus, the peripheral zonule is the elastic antagonist of a unified ciliary muscle. A biomechanical model illustrating accommodation is put forward that makes consistent and explicit the force and length changes that the six mechanical component elements undergo. The Hess-Gullstrand lenticular theory of presbyopia is compatible with this model.