The Mechanisms of Accommodation - Text 1
Physiologically, the angle of clear vision is very narrow and refers to the central part of the eye, the fovea, made up solely of cones. For an emmetrope, meaning someone who does not suffer from any ametropia, when a spot forms on several of the cells of the central foveal bouquet (the most precise part of the fovea), a complex mechanism, by means of the parasympathetic in the case of accommodation and the sympathetic for disaccommodation, informs the ciliary muscle, which contracts or relaxes accordingly. The fibres attached to it – the zonule of Zinn, or the crystalline zonule – makes the connection with the capsule, a sack in which the crystalline is enclosed, giving rise to the modification of its curvature in order to make focus possible. This operation is set in motion by a contraction of the pupil, or myosis, as if to soften this temporary state. More precisely, when the ciliary muscle is relaxed, it holds the radial fibres of the crystalline zonule in a state of tension. These fibres then exercise centrifugal traction on the equator and periphery of the crystalline capsule, thereby flattening its curvature, principally its anterior curvature. When, inversely, the ciliary muscle contracts, it relaxes its tension on the zonule fibres; the crystalline no longer being subject to their traction, it can relax, thanks to its elasticity and in particular the elasticity of its capsule.
