It is the iris that gives the visible eye its color and individuality. The iris consists of pigmented bands of muscle that surround the pupil. It also contracts or expands the pupil in response to light, extreme emotion, and, occasionally, pharmacologic substances
 

The lens is a dense, transparent structure made of proteins, located just behind the iris and pupil. Its curvature and density cause light waves to bend as they travel through the lens. After passing through the lens, the light waves converge and project an inverted image on the retina. The retina changes the light waves to electric impulses that then travel along the optic nerve to the brain. This entire series of events results in a visual and understandable representation of light and color.
The lens also changes shape in order to focus on objects at various distances. The older one becomes, the stiffer and more inflexible the lens becomes. Oftentimes, the lens becomes so stiff that it cannot change its shape sufficiently to accommodate both images of objects up close and far away. This is why the need for bifocals or reading glasses are common as people age.
New, thin layers continually grow over old layers of the lens. These layers of cells grow inward from the edges but never form in the center of the lens. If the cells at the center of the lens grow cloudy, cataracts develop. Natural lenses can be replaced with artificial lenses in a relatively fast and easy surgical procedure, making cataracts an easily treatable disease.
 

The vitreous is a clear, gel-like substance that fills the area between the lens and the retina. The pressure of the vitreous allows the eye to hold its shape. It also keeps structures like the retina in place.
With age, the composition of the vitreous changes. It may liquefy, allowing small bits of debris to float freely. Sometimes, miniscule collagen fibers clump together within the vitreous. Both create black, spidery flecks called floaters seen in the field of vision. Floaters are generally harmless but will remain within the vitreous unless surgically removed. The changing composition of the vitreous may also cause it to pull on the retina, sometimes resulting in a macular hole or a detached retina.

The two types of light-sensitive cells of the retina are rods and cones. Named because of their cylindrical shape, rods are interspersed throughout the retina. They are highly sensitive to low light and shadow and allow for at least some degree of nighttime vision.
Cones are present throughout the retina also, but are very highly concentrated in the fovea of the macula. Different cones are sensitive to different wavelengths and allow one to see vivid colors and sharp detail in conditions of bright light. There are 20 times more rods than cones.  

The opening at the center of the iris through which light enters the eye. The iris causes the pupil to dilate and contract in response to stimuli such as light, extreme emotion, and certain drugs.
The pupil is in direct line with the macula, allowing light to be concentrated where vision is clearest. The iris will contract the pupil in conditions of bright light and will widen the pupil in low light in order to optimize vision.
 

The choroid is the nourishing layer of the eye located between the retina and the sclera, and is filled with tiny blood vessels. The choroid supplies the outer layer of the retina with blood. In the womb, the choroid gives rise to all other structures of the eye.  

At the back of the eye, directly in line with the pupil, is an area of the retina called the macula. In the center of the macula is an indentation called the fovea that is packed with cones, cells that require high levels of light in order to function. As light enters the eye through the pupil, it is concentrated into the area of the macula and the fovea. Because of the cones that comprise the macula, central vision is crisp, clear, detailed and in color.
The macula, an area just 5mm in diameter (roughly the thickness of a fingernail), is responsible for the vision that is necessary for many daily activities. Any condition that impairs this area will adversely affect central vision and the ability to carry out many activities that are important to daily life.

The white, fibrous, protective outer layer of the eye, composed primarily of collagen and elastic fibers. The sclera varies in thinkness from about 0.3 mm to 1 mm, and is covered by a thin layer of tinnue called the conjunctiva.  

In ophthalmology, the fundus describes the back interior surface of the eye, opposite the lens, and includes the anatomical structures of the retina, optic disc, macula and fovea.

Diagrams and descriptions of the parts of the eye.
Our eyes allow us to perceive light, dark, and color in astonishing detail, revealing the beauty and complexity of the world around us. Perhaps even more extraordinary is the fact that this organ of vision measures just 24mm (0.95 inches) and weighs 7.5 grams (0.25 ounces)!
The eye's many complex structures, some more obvious than others, are mostly unknown outside of the medical field. In an attempt to eliminate any mysteries, our encyclopedia offers easy to understand definitions of most of the terms you're likely to hear in your ophthalmologist's office, and cross-references them with other relevant terms when appropriate.