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Anatomy of an Eye

RFS - EyesThe eye is an amazing and complicated part of our anatomy, one that we often take for granted. The eyes allow us to interpret the shapes, colors and dimensions of objects by processing the light they reflect or emit. The eye can work in bright or dim light, but it cannot sense an object when no light is present.

The eye is quite small, a mere 1 inch wide, 1 inch deep and .9 inches high. There is a lot of function packed into that tiny space!

Parts of the Eye

Orbit.The eye, itself, sits in a socket (also, called the orbit), which is formed by the cheekbone, forehead, temple and the sides of the nose. The eye is also cushioned within the orbit by pads of fat.

The orbit also contains glands that produce tears that help lubricate and moistens the eye, as well as flush away foreign matter that may enter the eye.

Eye lids and Eye lashes.  The eye lids and eye lashes serve to protect the eye from foreign matter, such as dust, dirt and other debris. The eye lids also protect the eye from bright light that might damage the eye. Finally, when you blink, the eye lids help spread tears over the surface of your eye, keeping the eye moist.

Sclera.  The white part of your eye is called the sclera. The sclera is a tough, leather-like tissue that extends around the eye and gives the eye its shape. The sclera is also attached to the six extra-ocular muscles, which move the eye left and right, up and down, and diagonally.

Muscle Primary Function

  • Medial rectus moves eye towards nose RFS - Eye muscles
  • Lateral rectus moves eye away from nose
  • Superior rectus raises eye
  • Inferior rectus lowers eye
  • Superior oblique rotates eye
  • Inferior oblique rotates eye

Conjunctiva.  The conjunctiva is a thin, clear layer of tissue that covers the front of theeye, including the sclera and the inside of the eyelids.The conjunctiva keeps bacteria and foreign material from getting behind the eye.

Cornea.  The cornea is a clear layer of tissue located just in front of the iris, which is the colored part of your eye. The cornea helps focus light as it enters the eye. If you wear contact lenses, the contact lens rests on your cornea.

Anterior and Posterior Chambers.  The anterior chamber is a fluid-filled space immediately behind the cornea and in front of the iris. The posterior chamber is the fluid-filled space immediately behind the iris but in front of the lens.

The fluid that fills these two chambers is called the aqueous humor and helps nourish the cornea and the lens. If the aqueous humor doesn’t properly drain out of the eye, pressure can build up, causing optic nerve damage and vision loss, a condition known as glaucoma.

RFS - Anatomy of an eyeIris and Pupil.  The iris, which is the colored part of your eye, controls the amount of light that enters the eye.The irisis ring shaped with a central opening called the pupil. The iris has a ring of muscle fibers around the pupil, which enable the pupil to constrict or dilate, depending on whether it is bright or dark.

Lens. The lens is a clear, flexible structure located just behind the iris and the pupil. A specialized ring of muscular tissue, called the ciliary body, surrounds the lens. Together, the lens and the ciliary body help control the focusing of light as it passes through the eye.

Retina/Macula/Choroid. The retina functions to create the images we see. The retina converts light signals into nerve signals and sends them to the optic nerve, which carries the signals to the brain. There, the brain processes the image.

The retina is made up of 2 types of cells: rods and cones. Rods are more sensitive to light; therefore, they allow you to see in low light situations but do not allow you to see color. Cones, on the other hand, allow you to see color but require more light.

The macula is located in the central part of the retina.It is the area of the retina that is responsible for giving you sharp central vision.

The choroid is a layer of tissue that separates the retina and the sclera. It is mostly made up of blood vessels. The choroid helps nourish the retina.

Vitreous Cavity. The vitreous cavity is located behind the lens and in front of the retina. It is filled with a gel-like fluid called the vitreous humor. The vitreous humor helps maintain the shape of the eye and makes up approximately 80% of the eyes mass.

Optic Nerve. The optic nerve, a bundle of over 1 million nerve fibers, is responsible for transmitting nerve signals from the eye to the brain. These nerve signals contain information on an image for processing by the brain. The front surface of the optic nerve, which is visible on the retina, is called the optic disk.

How the Eye Works

Light waves are reflected from an object (such as a tree) and enter the eye through the cornea and progresses through the pupil. The light waves are bent by the cornea, and then by the lens to a nodal point which is located behind the back surface of the lens. At that point, the image becomes reversed (turned backwards) and inverted (turned upside-down).

The light continues through the vitreous humor and then, ideally, back to a clear focus on the retina. Within the retina, light impulses are changed into electrical signals. Then, they are sent through the optic nerve to the occipital cortex at the back of the brain. Here, the electrical signals are interpreted or “seen” by the brain as a visual image.

The eye and how we see – neat little interactive video.

Sources

American Optometric Association

Foundation of American Academy of Ophthalmology

Prevent Blindness America