Model of visual focussing involving extraocular muscles and the causes of myopia and glaucoma

The classical model of visual focus has increased curvature of the lens for accommodation and flattening of the lens for far vision, with both determined by ciliary muscle tension. This new model proposes that change in the focal length is a second modulator of acuity. This model also proposes that the extraocular muscles are the more important neuromuscular entities for both lens and focal length changes. At accommodation there would be relaxation of the recti muscles and contraction of the superior oblique muscle. This increases focal length while allowing the ciliary muscle ring to contract, releasing tension on the lens capsule. Far object focus is due to contraction of the recti muscles. These pull on the sclera, stretching the zonule fibers taut to give flattening tension of the lens capsule. The myopic eye, in addition to a highly curved lens, also has longer focal length and is misaligned in the orbit. Ageing glaucoma is hypothesized as due to: 1. excessive curvature of an ageing lens; 2. chronic contraction of the recti in compensatory effort to flatten the lens and shorten the focal length; 3. the overcontracted recti pull to excess, stretching the inner tissue and distorting the Schlemm canal and its access mesh area to inhibit efflux of aqueous fluid.