Anatomy of the Eye

How the Eye Functions

The eye functions much like a camera. Suppose you are going to take a picture of something. Light rays bounce off of the object. The image which is composed of those light rays travels through the camera and is focused by the lens on the camera.

If the image is in focus, a sharp image is produced. If for some reason the lens is dirty, scratched, or is out of focus, a blurred or distorted image will result.

Suppose that you are looking at an object. Light bounces off the object and passes through the cornea, through the pupil in the center of the iris and then through the lens inside your eye. The iris opens and closes to adjust the total amount of light entering the eye. The human lens, acting just like the camera lens, focuses the light onto the retina.

In young people (under the age of 40), the human lens can adjust to add focusing power to bring near objects into better focus. The cornea contributes some of the focusing power of the eye, but it is not able to adjust like the human lens. The retina translates the light into electrical impulses which then travel to the brain via the optic nerve. The brain converts these impulses into the picture of the object.


Vitreous

The vitreous is a transparent gel-like substance that fills the center of the eye between the lens and the retina. The space that it fills is called the vitreous body. The vitreous comprises about 2/3 of the eye’s volume, giving it form and shape especially in fetal development. As we age, the vitreous goes through a normal degenerative process, where it loses its gel-like form and becomes more liquefied. This process replaces the gel with its own natural fluid as the structure of the eye is able to withstand its form and structure.


Optic Nerve

optic%20nerve

The optic nerve is a bundle of nerve fibers, about the diameter of a pencil, which passes through the back of the eyeball and connects to the nerve fiber layer of the retina. The optic nerve carries the visual messages from the photoreceptors of the retina to the brain through a meshwork of nerves known as the optic chiasm. The optic nerve is the final passageway of light and images to the brain, thus allowing us to complete the final stage of how the eye works.


Macula

macula

The center most part of the retina is the macula. The macula is the “bull’s-eye” of the retina, where the highest concentration of photoreceptors are located. The macula allows us to see images straight ahead, recognize faces and read. The macula is the only part of the retina that can see detail. If this area is diseased, images will not be processed in full before being sent to the brain.


Fovea

fovea

The fovea is a tiny pit at the center most part of the macula. This small area contains the highest concentration of photoreceptors, thus providing the clearest, most detailed vision. The fovea is the part of the retina that you use when you look directly at something.


Choroid

choroid

The choroid is the layer of blood vessels and connective tissue between the sclera (white of the eye) and retina. It is attached at the front of the eye and in the back along the edges of the optic nerve.

The choroid provides nourishment to the outer layers of the retina. It also acts as a barrier between the transparent retina, and the sclera.


Retina

retina

The retina is a neurological membrane that lines the most inner wall of the eye. It is comprised of different layers – some of which are nerve fibers, photoreceptors and cells. These layers all work together to capture an image much like the film in a camera does. The photoreceptors capture the images, and allow them to be transported through the nerve fibers to the optic nerve, and on to the brain for processing.

The retina is classified by two parts: the peripheral retina and the macula. The peripheral retina comprises 95% of the retinal surface, and this area allows us to see images “out of the corner” of our eye. The macula comprises the center most area of the retina and is responsible for detail vision.


Iris

iris

The Iris is the “colored part” of the eye. It divides the front of the eye (anterior chamber) from the back of the eye (posterior chamber). The black opening in the center of the Iris is called the Pupil. The size of this opening is controlled by the muscles of the Iris which act in much the same way as the shutter of a camera. This action of the Iris is what controls the amount of light that enters the eye.


Cornea

cornea

The Cornea is the transparent tissue covering the front of the eye. Because there are no blood vessels in the cornea, it is normally clear and has a shiny surface. It provides a physical barrier that shields the inside of the eye from germs, dust, and other harmful matter.

The Cornea is also a powerful refracting surface which provides 2/3 of the eye’s focusing power. When light enters the eye, it first passes through the cornea which bends it to pass through the lens. The lens then focuses the light onto the retina. That starts the translation of light into vision.


Pupil

pupil

The Pupil is the “black part” of the eye. Rather than being an actual part of the eye’s anatomy, the Pupil is merely an opening in the center of the iris. The muscles of the iris control the size of this opening through which light must pass to reach the retina. The iris allows the Pupil to become larger or smaller according to the amount of light available in any particular situation. This process acts much like the shutter on a camera.


Lenslens

The lens is a transparent structure behind the iris. It is held in place within the eye by hair-like fibers called zonules. The function of the lens of the eye is much the same as the function of the lens of a camera – to achieve a clear image.

The lens of the eye is able to change its shape as needed to focus light and images onto the retina. As we age, the lens loses its transparency and flexibility and becomes opaque. This is known as a cataract.