A white part (the white of the eye, or “sclera”) and a colored part. The latter consists of a small black circle—the pupil—and a colored area—the iris—both covered by a transparent dome formed by the cornea and the aqueous humor. Behind the eyelids, which protect this visible part of the eye, lie, in succession, the lens, the vitreous humor, the retina, and the optic nerve.
The cornea is made up of very thin, uniform cells arranged in a honeycomb pattern. It is a delicate structure 11 millimeters in diameter and only 4/10 of a millimeter thick. It is 78% water. To maintain this level of hydration and its optical function, it is covered by tears that are continuously supplied by the lacrimal glands and distributed by the blinking of the eyelids.
The pupil, if compared to a camera, is an aperture through which light enters. Its diameter can vary from 1 to 2 millimeters in bright light, up to 8 mm in the dark. This variation can also be triggered by our emotions. For example, intense interest causes a reflexive widening of the pupil.
The iris determines the color of the eyes. This color depends on the thickness of the fan-shaped structure formed by the pigmented lamellae and its concentration of melanin. The thicker the fan and the more melanin it contains, the darker the eye. It is a membrane thinner than silk, which also regulates pupil dilation. The iris is nourished by the aqueous humor in which it is immersed and by a few small arterioles.
The lens is an optical lens located behind the iris. Light rays enter through the cornea, pass through the aqueous humor, and then through the pupil. There, the lens causes them to converge on the retina through its contractions. If the object is close, the lens bulges—this is called convergence. If it is far away, the lens flattens. This ability to focus is called accommodation. It works from about ten centimeters to infinity.
The vitreous humor is a gel-like fluid composed of 95% water, which accounts for 90% of the eye’s volume. Located in a sac behind the lens and extending to the back of the eye, the vitreous humor acts as a shock absorber. Its role is to maintain the rigidity of the eyeball and keep the retina firmly in place.
The retina is a membrane 1/10th to 4/10th of a millimeter thick, located at the back of the eye on its inner wall. Highly sensitive, it allows us to detect very faint light from a distance of 10 kilometers, even in total darkness! It is crisscrossed by numerous blood vessels, the pattern of which distinguishes us from one another even more clearly than fingerprints. It is a mosaic of hundreds of millions of photoreceptor nerve cells: the cones (6 to 7 million) and the rods (130 million). Their role is crucial. They enable us to perceive light, colors, shapes, and movement, and also provide night vision. The cones detect three primary colors that enable the reconstruction of colors: they can distinguish more than 100 different shades and 750 levels of brightness. Color perception changes depending on the light level: in daylight, the eyes are more sensitive to yellow, whereas in the evening, they are more sensitive to blue. The retina converts all these elements into nerve impulses, which are then transmitted to the brain via the optic nerve.
The optic nerve acts as a “conveyor belt” for information received by the eyes and transmitted to the brain. It is 4 mm in diameter and 5 cm long. Like a perfect computer, the brain records, interprets, and translates this information into images.