Chapter XX

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Vision starts when light rays enter the eye through the cornea, the hard, “-like” covering that forms the anterior part of the (Grk. “hard”), which is a tough, white outer coat that surrounds most of the eyeball. To the sclera are attached muscles that direct the eye’s movement, called “” muscles: these include the medial, lateral, superior, and inferior (Lat. “straight [muscles]”). Immediately exterior to the sclera is the (Lat. “connective [coating]”), a delicate membrane that lines the eyelids and protects the exposed surface of the sclera, in addition to aiding in lubrication for the movement of the eye. Immediately interior to the sclera is the (Grk. "like a membrane"), the vascular layer of the eye (i.e., the one containing blood vessels, by which oxygen is supplied).

Incoming light rays are refracted by the cornea into the pupil, officially called the pupilla, Latin for “little ” (both the Greeks and Romans referred to this anatomical feature in this way because of the small figure one sees reflected on this surface when gazing into someone else’s eyes). The pupil is expanded or contracted by the iris to let in more or less light, as needed. The is named after the Greek messenger-goddess, who descended from heaven on a rainbow. Her name became synonymous with the rainbow itself, and then to any brightly colored circle, hence the labeling of the colored portion of the eye.

Light passing through the pupil then goes through the (Latin for “lentil,” based on similarity in form), which refracts the light further. Its shape is modified by the ciliary body, which is connected to the lens by the zonule of Zinn, a “little zone” named after an 18th-century German anatomist (it is also called the “ of Zinn,” surely one of the most awe-inspiring names ever to be given to a human body part). In the center of the eyeball is the body, a transparent, gelatinous substance that maintains the eye’s spherical shape. Through this body runs the thin canal, which transports lymph to the lens of the eye to adjust its volume for changes in refraction. Both terms mean “glassy” in Latin and Greek respectively.

The is a thin layer of tissue along the back interior surface of the eyeball and gets its name from the post-classical Latin word for “net” because of its delicately interwoven fibers. It contains the photoreceptors known as the “rods and cones.” Rods are extremely light-sensitive and allow for vision in low-light scenarios, while cones allow for the perception of color. The fovea (or simply fovea, “pit”) is a small depression in the surface of the retina located in the center of the lutea (Lat. “yellow spot”), the location of the most acute vision. The photoreceptors on the retina then transmit a signal through the optic nerve to the brain, which interprets the signal and produces perception of an image.