Eyes tell us more about the world around us than any other of our senses. They inform us of the size, shape, position, and color of objects from pinpoints a few inches from our nose to stars billions of miles away in space. This is possible because each eyeball contains a nerve net (the retina) that is sensitive to light waves, which it converts electrochemically to signals that can be interpreted by the brain. These nerve nets are enclosed by two roughly spherical organs, the eyes, which can focus light, control the amount of light entering the eye, and move to follow a light source.
Eyelid and eyeballs
The delicate eyeballs are protected by the bones of the skull and by the two eyelids. Each eyelid has three main layers: skin; muscles that make it shut and open; and the tarsi, made of tough fibrous tissue. Blinking protects eyes from injury and allows tears to bathe the eyes. Tears, composed of a saline bactericidal fluid, come from each upper eyelid’s lacrimal gland. The fluid drains away through a tear duct opening at each eyelid’s inner corner into the nasal cavity.
Eyeballs are jelly-filled spheres set in fat, supplied with muscles, and shielded by the bony orbits of the skull. Six ocular muscles coordinate each eye’s movements so that both eyes can follow moving objects together. The eyes’ overlapping fields of view produce the binocular vision that enables us to judge depth and distance.
Light falling on the eye passes through the transparent cornea the bulging, transparent front of the outer layer of the eyeball. The rest of the outer layer (the sclera, or “white” of the eye) is opaque to light and is covered with a layer of conjunctiva. Light rays then continue through a so-called anterior chamber filled with the watery fluid known as the aqueous humor. This fluid and the cornea refract incoming light and serve as the front lens of the eye.
Light refracted by this outer lens then enters the pupil a hole surrounded by a muscular diaphragm, the iris. The pupil appears black because light is not reflected out from the interior of the eye. Pigmentation of the iris gives the eye its color. It contracts or dilates in response to the intensity of light, contracting in bright light to prevent too much light from entering the eye and dilating in dim light to allow as much light as possible to enter the eye. Light next passes through a flexible, transparent crystalline lens. Ligaments connect this lens to ciliary muscles that can make it shorten and bulge, or lengthen and grow slimmer, thereby altering its focal length to bring near or distant objects into finer focus on the retina.
Refracted further by this lens, light rays reach the eye’s posterior chamber. This is filled with the jellylike vitreous humor. After passing through this fluid, light rays reach the retina a layered network of nerve cells lining the inside of the back of the eyeball and separated from its outer, scleral layer by the chorion, or choroid, a layer of blood vessels that brings nourishment and removes waste products.
The retina, or “net,” covering the rear four-fifths of the eyeball’s inner surface is a cupshaped extension of the brain linked to it by the second cranial, or optic, nerve. It seems to be back-to-front, for light rays must pass through the layers of nerve cells communicating with the optic nerve before they reach the retinal cells sensitive to light. Each retina has about 120 million rods and about 6 million cones. The long, thin structures known as rods are concentrated toward the rim of the retina. Rods are highly sensitive to low intensities of light but register only shades of gray. The six million cone photoreceptors are relatively short, thick cells that are most plentiful toward the back of the eye and concentrated especially at the fovea, a shallow retinal pit opposite the pupil. Cones work well only in good light but, between them, register green, red, and blue light, and so perceive the range of colors of the visible spectrum.
As light falls on both rods and cones, their light-sensitive pigments instantly decay and then re-form. This change sends electrochemical signals along the optic nerves to the brain, where the signals are interpreted as sight.
Inside the brain
We analyze and understand the images registered inside our eyes because each bit of the image travels in coded fashion from retina to visual cortex at the back of the brain. The journey through the fibers of the optic nerves is complicated. Nerves from each eye meet in the front of the brain at the optic chiasm. This is a partial crossing point where fibers from each eye’s inner (nasal) side switch over to join the nerve-carrying fibers from the outer (temporal) side of the other eye. From the chiasm, both sets of nerve fibers now known as optic tracts continue through the brain. After passing the lateral geniculate bodies (relay stations in the thalamus), these nerve fibers fan out in the so-called optic radiation, ending at the primary visual cortex at the back inner edge of each cerebral hemisphere.
The partial crossing of fibers at the optic chiasm ensures that signals from the right side of each retina reach the right visual cortex while signals from the left side of each retina reach the left visual cortex.
Research suggests that, in addition to coordinating countless individual signals almost instantly, some specialization of visual perception may also occur. For example, different cell columns in the cortex may deal with signals from different regions in each retina. And three neighboring areas of cortex of some mammals (designated visual areas I, II, and III) have cells that are sensitive to different stimuli. The “simple” cells of visual area I react to bright lines and dark bars at special angles. Areas II and 111 have so-called complex and hypercomplex cells: the first register edges and movement; the second detect corners.