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Structure of the Eye

The cornea is the transparent but alive, curved structure at the front of the eye, with embedded nerve endings for pain, touch and thermal sensation. It has 80% of the focusing power of the eye but is not flexible. The sclera is the white, hard outside of the eyeball. The aqueous humor is the low viscosity fluid behind the cornea, in front of the lens. The iris is the muscular, colored tissue that surrounds (and shapes) the pupil. The pupil is the hole through which light enters the eye. The lens is the flexible, clear substance that provides 20% of the focusing power of the eye. The ciliary (or lens) muscles are the muscles that control the shape (curvature) of the lens. The vitreous humor is the high viscosity fluid filling the eyeball. The retina is the sheet of neurons at the back of the eye. The fovea is where light from the center of gaze lands on the retina. The pigment epithelium is the black layer behind the retina where visual pigments are replenished.

20% of the focusing power of the eye is in the lens. Although it is only 20% of the focusing power, it is important because it is flexible and gives us the ability to accommodate. Accommodation is the ability to focus on things that are near or far away. Presbyopia is when the lens gets hard and can’t be squished by the ciliary muscles to focus on things that are near. Eventually, your arms won’t be long enough to hold reading material far enough away for your eyes to focus on it. Cataracts are the crystallization of the lens that scatters light, making it hard to perceive detail.

The blind spot is located on the retina where there are no photoreceptors (because axons heading for the optic nerve occupy that space). Your brain fills in the hole with a copy of what’s around the blind spot. The axons pass through the retina where the optic nerve begins and where there are no photoreceptors at the very back of the eye. This is what creates a “blind spot” in the retina and a corresponding blind spot in our visual field.