sclera

Meaning: The sclera is the "white of the eye." It is dense connective tissue rich in collagen, so it is strong and opaque. It holds the eyeball's shape, protects the delicate inner structures, and gives the extraocular muscles a place to attach so the eye can move. At the front of the eye it becomes continuous with the transparent cornea. Together, the sclera and cornea form the outer layer of the eye. The sclera takes no part in detecting light.

On the MCAT: Sensation and perception questions ask you to trace the path of light: cornea, aqueous humor, pupil (the opening in the iris), lens, vitreous humor, then retina, where rods and cones transduce light. The sclera is part of the wall of the eye, not part of this path. You may also be asked to sort structures by layer: outer (sclera, cornea), middle (choroid, ciliary body, iris), and inner (retina). Related comparisons: the cornea bends most of the incoming light, the lens fine-tunes focus with help from the ciliary muscles, and the iris controls pupil size.

Common confusion: Students mix up the sclera with the cornea or the iris. The sclera is white and opaque, the cornea is the clear front window, and the iris is the colored muscle that sets pupil size. Because the sclera is opaque, light cannot enter through it. It is also not sensory tissue. Only the retina contains photoreceptors.