<P> In cartilaginous fish, the zonular fibres are replaced by a membrane, including a small muscle at the underside of the lens . This muscle pulls the lens forward from its relaxed position when focusing on nearby objects . In teleosts, by contrast, a muscle projects from a vascular structure in the floor of the eye, called the falciform process, and serves to pull the lens backwards from the relaxed position to focus on distant objects . While amphibians move the lens forward, as do cartilaginous fish, the muscles involved are not homologous with those of either type of fish . In frogs, there are two muscles, one above and one below the lens, while other amphibians have only the lower muscle . </P> <P> In the most primitive vertebrates, the lampreys and hagfish, the lens is not attached to the outer surface of the eyeball at all . There is no aqueous humor in these fish, and the vitreous body simply presses the lens against the surface of the cornea . To focus its eyes, a lamprey flattens the cornea using muscles outside of the eye and pushes the lens backwards . </P> <P> The lens is flexible and its curvature is controlled by ciliary muscles through the zonules . By changing the curvature of the lens, one can focus the eye on objects at different distances from it . This process is called accommodation . At short focal distance the ciliary muscle contracts, zonule fibers loosen, and the lens thickens, resulting in a rounder shape and thus high refractive power . Changing focus to an object at a greater distance requires the relaxation of the lens and thus increasing the focal distance . </P> <P> The refractive index of human lens varies from approximately 1.406 in the central layers down to 1.386 in less dense layers of the lens . This index gradient enhances the optical power of the lens . </P>

When does the lens of the eye thicken
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