The cornea
The cornea is the transparent "front window", through which light enters the eye. It has a very powerful ability to focus light onto the retina of the eye. It is clear, spherical, and consists of 5 different layers.

The most superficial layer, the epithelium, is highly regenerative and heals quickly following superficial injuries. The second layer of tissue is known as Bowman’s layer, followed by the stroma, Descemet’s membrane, and the endothelium on the internal aspect. The three central layers provide rigidity and is constructed in such a way that it transmits light. The internal layer of endothelium is a monolayer of cells with a highly specialized function to maintain the dehydrated status of the cornea, which among other unique properties thereof, is responsible for the clarity of the cornea.

The optical function of the cornea is to transmit and refract light (like a lens). It is the most powerful part of the refractive system of the eye, and provides more than two thirds of the eye's total refractive power. Light is focused by it onto the retina at the back of the eye, thereby creating an image of the original object, although inverted and smaller.

Iris
This is the colored part of the eye, visible through the cornea. The central dark part in the iris is named the pupil. The pupil regulates the amount of light that enters the eye. The muscles of the iris cause the pupil to constrict or dilate, in order to control the amount of light entering the eye for optimal vision.

The crystalline lens
This is a biconvex, transparent structure, located behind the pupil, that finally focuses light onto the retina.  It roughly contributes one third of the total refractive power of the eye. It has the ability to change it's shape, and therefore makes the finer adjustments to the focus of the image perceived. This is what is understood by the term "accommodation".

With advancing age, the lens becomes rigid and increases in size, with the result that the extent of accommodation deteriorates.  Focusing on near objects therefore tends to become increasingly difficult as the person approaches the age of 45 - 50 years.  This phenomenon is known as "presbyopia".

When the lens becomes cloudy or opaque, it is named a "cataract".

The retina
This is the light sensitive membrane that lines the inner wall of the eye. It contains photoreceptors (small, light sensitive cells) that transform light energy into electrical impulses that can be translated into images by the brain.

The "macula" is located at the central part of the retina, and has the sharpest resolution power. The macula is responsible for vision during daytime, as well as perceiving color. This part of the retina is critical for reading and the discrimination of fine detail.

The optic nerve
The eye is connected to the brain by means of the optic nerve. It collects all the electrical activity from the retina, and transmits it to the visual cortex in the brain, where these impulses are processed and perceived as visual images.

The visual cortex
This is the final destination of visual impulses. It is located at the back of the brain, in the occipital lobe, and handles the processing of electrical impulses from the eye. It is here where visual images are perceived and the process of “vision” occurs. It is interesting to note that you don’t actually “see” with your eyes, but rather with your brain!