Fovea centralis is a tiny pit in the retina that contains only cones. It lies in the back of the retina in line with the pupil. It is the area of sharpest vision. Anything we want to view critically is viewed on the fovea centralis.
Blind spot is the optic disk where optic nerve leaves the eye. No rods or cones there.
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When the ciliary muscle is relaxed, the choroid acts like a spring pulling on the lens via the zonule fibers causing the lens to become flat. When the ciliary muscle contracts, it stretches the choroid, releasing the tension on the lens and the lens becomes thicker.