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RESEARCH
THEORIES OF ACCOMMODATION
Spencer P Thornton MD, FACS reviews
T
the different theories of accommodation through the ages
he mechanism of accommodation has been one of the most studied aspects of visual physiology over the last two centuries. On accommodation the pupil constricts with contraction of the ciliary muscle, the lens becomes more spherical, and the vitreous pushes forward, moving the lens forward, increasing its effective power. Although lenticular-based focusing was first proposed by Descartes, it was Thomas Young who in 1793 demonstrated changes in the crystalline lens that occurred on changing focus from distance to near, and Hermann von Helmholtz who in 1856 advanced the first widely accepted explanation of the accommodative process.
MÜLLER In 1854 Heinrich Müller described the circular muscle of the ciliary body – theorising that the contraction of the ciliary muscle pulled vitreous forward, forcing the lens forward, with resulting power increase. Müller’s theory is described by Frans Donders, as follows: “Müller's theory is based on his anatomical
VON HELMHOLTZ The most widely held theory of accommodation was proposed by von Helmholtz in 1856: “When viewing a far object, the circularly arranged Müller’s ciliary muscle is relaxed, allowing the lens zonules and suspensory ligaments to pull on the lens, flattening it in the periphery. The source of the tension is the pressure that the vitreous and aqueous humours exert outwards onto the sclera.” According to von Helmholtz, when viewing a near object, the ciliary muscles contract (resisting the outward pull of the sclera) causing the lens zonules to relax which allows the lens to spring into a thicker form. EUROTIMES | APRIL 2016
Ciliary body with zonules and Müller’s muscle
investigations of the ciliary muscle… he sees in the action of the most external layers of the ciliary muscle a means of augmenting the pressure of the vitreous humour, of pushing the lens forwards, of diminishing the increased convexity of the posterior surface, and, by the resistance of the simultaneous contraction of the iris, of increasing that of the anterior surface.”