Anatomy of the eye.

The exterior of the eye is composed of a white sclera and a transparent cornea. Immediately below the cornea is the aqueous humor, a transparent fluid similar to blood plasma that circulates through the eye to deliver nutrients and oxygen. Behind the aqueous humor lies the iris, the colored ring of muscles that expand and contract to let in more or less light depending on lighting conditions. The opening in the middle of the iris is the pupil through which light enters the lens, which lies directly behind the iris.

The lens is a flexible, transparent substance which can be stretched by the ciliary muscles around its perimeter to alter its shape, and thus its refractive properties. Far away objects cause the ciliary muscles to contract, pulling the lens into a flatter shape, while nearer objects relax the muscles allowing the lens to return to a more round shape, bringing the close-up objects into focus.

The interior of the eye is filled with vitreous humor, a jellylike substance which gives structural support to the eye. It also serves to hold in place the retina, the location of the eye's photoreceptors. Photoreceptors are of two types: rods and cones, which perceive gray shades and colors, respectively. The fovea, a tiny pit in the center of the retina, has a very high concentration of color-sensing cones. When we are looking directly at something its image is projected on this area. The fovea sits in the middle of the macula lutea, a small area of the retina containing yellow pigment (which helps correct for blue chromatic aberration). From there, the number of cones decreases and the number of rods increases out to the periphery of the retina.

Rods are the more numerous of the photoreceptors. They are 500 times more sensitive to light than cones, but can only detect shades of gray. Rods are responsible for night vision. Because of their sparsity in the fovea, it is easier to see things in the dark via ones peripheral vision than by looking at it directly. Though more numerous, many rods share a single optic neuron, so the effective resolution is fairly poor.

Cones perceive color. There are three types of cones, each kind responding to a different wavelength of light: red, green, and blue. Blue cones are the most sensitive of the three, but also the most sparse. Cones are concentrated in the fovea, the center of our visual field. Each cone is connected to its own unique neuron, so the image sent to the brain is very clear. The cones are often refered to medically by the abbreviations S (blue) for "short", M (green) for "medium" and L (red) for "long" wavelengths of light.

Underneath the retina is the choroid, which contains vessels for supplying blood to the various parts of the eye. The iris is an extension of the choroid.

Just off from the center of the retina is where the optic nerve exits the eye. This produces a blind spot where there are no photoreceptors. The brain fills in this gap from nearby photoreceptors, and the blind spots in each eye do not overlap, so this blind spot is rarely noticed. The center of the optic nerve contains blood vessels which supply blood to the eye.

To see your own blind spot, close your left eye and look at the plus sign from about a foot or so away from your screen. Slowly move closer to the screen. At a certain distance you'll notice the green dot disappears from the corner of your eye, leaving nothing but a gray background.

Color is sent to the brain as brightness plus two color axes: red-versus-green and yellow-versus-blue. If both axes are neutral it is perceived as shades of gray.

As light enters the eye and falls on the cones, they cause neurons in the retina to fire faster or slower. Visual information is not sent as red, green, and blue signals, however. It is sent as brightness, red-versus-green, and yellow-versus-blue channels. Brightness is a sum of all photoreceptors. The more light overall, the faster the firing rate. The other two channels fire at a base rate and are sped up or slowed down by differences between the cones. The red-versus-green channel fires faster if red is detected, but slower if green is detected. If both are seen in the same area they cancel each other out and the firing rate is unchanged. The yellow-versus-blue channel fires faster if red or green are detected (which is interpreted as yellow) and slower if blue is detected. This method of color representation is the basis behind the YCrCb and CIELab color models.

Path of optic nerve through optic chiasma.

As an image from the eyes travels out through the optic nerve to the brain it is split at the optic chiasma (meaning cross). This point just behind the eyes acts like a railroad junction for the two optic nerves. The medial (closest to the nose) half of each eye's picture is swapped so that the right halves of both retinal images go to the left visual cortex and the left halves go to the right visual cortex.

This swapping of images assists in stereoscopic vision, the ability to perceive depth. Each eye looks at an object from a slightly different angle. By comparing the two pictures the brain is able to perceive which objects are closer or further away.

The visual cortex in the brain (located at the rear of the brain in the occipital lobe) makes "sense" of the information coming in from the eyes. It looks for edges and lines, then finds shapes, then figures out what those shapes are (e.g. an apple, a person). The imagination conceptually fills in hidden parts of an obstructed object. The environment is taken into consideration so that when we see white snow illuminated by colored Christmas lights we still know the snow is white, not red or blue. Also, motion is detected and individual moving objects are tracked. Finally, the objects are tied to memories and associations (e.g. apples are food, that apple belongs to Bob) and unique identities (e.g. that person is Susan).

Many visual disorders can result from damage to the eye, optic nerve, or brain, which are detailed here.

Sample image: detail of "Fragment 2 for Composition VII," painted by Wassily Kandinsky in 1913.