The roughly spherical organ of vision, housed in the bony socket of the skull.
Structure
The eyeball is about 24 millimeters in diameter. Its wall has three layers: the tough outer sclera, the vascular choroid in the middle, and the inner retina that contains the photoreceptors.
Function
Light passes through the transparent cornea and lens, which together focus an inverted image on the retina. Cones detect color in bright light, while rods detect movement and shape in low light.
Movement
Six extraocular muscles in the orbit move the eyeball precisely so that both eyes can track a target together.
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