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Anatomy and Physiology of the Eye L1
Eye Anatomy
Vision is by far the most used of the five senses and is one of the primary
means that we use to gather information from our surroundings. More than
75% of the information we receive about the world around us consists of visual
information.
The eye is often compared to a camera. Each gathers light and then transforms
that light into a "picture." Both also have lenses to focus the incoming light.
Just as a camera focuses light onto the film to create a picture, the eye focuses
light onto a specialized layer of cells, called the retina, to produce an image.
Although the eye is small, only about 1 inch in diameter, each part plays an
important role in allowing people to see the world.
The eye is surrounded by the orbital bones and is cushioned by pads of fat
within the orbital socket. Extraocular muscles help move the eye in different
directions. Nerve signals that contain visual information are transmitted
through the optic nerve to the brain.
Orbit
The orbit is the bony eye socket of the skull. The orbit is formed by the
cheekbone, the forehead, the temple, and the side of the nose. The eye is
cushioned within the orbit by pads of fat. In addition to the eyeball itself, the
orbit contains the muscles that move the eye, blood vessels, and nerves.
The orbit also contains the lacrimal gland that is located underneath the outer
portion of the upper eyelid. The lacrimal gland produces tears that help
lubricate and moisten the eye, as well as flush away any foreign matter that
may enter the eye. The tears drain away from the eye through the nasolacrimal
duct, which is located at the inner corner of the eye.
Eyelids and Eyelashes
The eyelids serve to protect the eye from foreign matter, such as dust, dirt, and
other debris, as well as bright light that might damage the eye. When you
blink, the eyelids also help spread tears over the surface of your eye, keeping
the eye moist and comfortable.
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The eyelashes help filter out foreign matter, including dust and debris, and
prevent these from getting into the eye.
Conjunctiva
The conjunctiva is a thin, transparent layer of tissues covering the front of the
eye, including the sclera and the inside of the eyelids. The conjunctiva keeps
bacteria and foreign material from getting behind the eye. The conjunctiva
contains visible blood vessels that are visible against the white background of
the sclera.
Sclera
The white part of the eye that one sees when looking at oneself in the mirror is
the front part of the sclera. However, the sclera, a tough, leather-like tissue,
also extends around the eye. Just like an eggshell surrounds an egg and gives
an egg its shape, the sclera surrounds the eye and gives the eye its shape.
The extraocular muscles attach to the sclera. These muscles pull on the sclera
causing the eye to look left or right, up or down, and diagonally.
Cornea
The cornea is the transparent, clear layer at the front and center of the eye.
In fact, the cornea is so clear that one may not even realize it is there. The
cornea is located just in front of the iris, which is the colored part of the
eye. The main purpose of the cornea is to help focus light as it enters the
eye. If one wears contact lenses, the contact lens rests on the cornea.
Anterior Chamber
The anterior chamber is the fluid-filled space immediately behind the cornea
and in front of the iris. The fluid that fills this chamber is called the aqueous
humor. The aqueous humor helps to nourish the cornea and the lens.
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Iris and Pupil
The iris, which is the colored part of the eye, controls the amount of light that
enters the eye. The iris is a ring shaped tissue with a central opening, which is
called the pupil.
The iris has a ring of muscle fibers around the pupil, which, when they
contract, causes the pupil to constrict (become smaller). This occurs in bright
light. A second set of muscle fibers radiate outward from the pupil. When these
muscles contract, the pupil dilates (becomes larger). This occurs under reduced
illumination or in darkness.
Anterior Chamber Angle and Trabecular Meshwork
The anterior chamber angle and the trabecular meshwork are located where the
cornea meets the iris. The trabecular meshwork is important because it is the
area where the aqueous humor drains out of the eye. If the aqueous humor
cannot properly drain out of the eye, the pressure can build up inside the eye,
causing optic nerve damage and eventually vision loss, a condition known
as glaucoma.
Posterior Chamber
The posterior chamber is the fluid-filled space immediately behind the iris but
in front of the lens. The fluid that fills this chamber is the aqueous humor. The
aqueous humor helps to nourish the cornea and the lens.
Lens
The lens is a clear, flexible structure that is located just behind the iris and the
pupil. A ring of muscular tissue, called the ciliary body, surrounds the lens and
is connected to the lens by fine fibers, called zonules. Together, the lens and
the ciliary body help control fine focusing of light as it passes through the eye.
The lens, together with the cornea, functions to focus light onto 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.
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Retina/Macula/Choroid
The retina acts like the film in a camera to create an image. When focused light
strikes the retina, chemical reactions occur within specialized layers of cells.
These chemical reactions cause electrical signals, which are transmitted
through nerve cells into the optic nerve, which carries these signals to the
brain, where the electrical signals are converted into recognizable images.
Visual association areas of the brain further process the signals to make them
understandable within the correct context.
The retina has two types of cells that initiate these chemical reactions. These
cells are termed photoreceptors and the two distinct types of cells are the rods
and cones. Rods are more sensitive to light; therefore, they allow one to see in
low light situations but do not allow one to see color. Cones, on the other hand,
allow people to see color, but require more light.
The macula is located in the central part of the retina and has the highest
concentration of cones. It is the area of the retina that is responsible for
providing sharp central vision.
The choroid is a layer of tissue that lies between the retina and the sclera. It is
mostly made up of blood vessels. The choroid helps to nourish the retina.
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 for processing by the brain. The front surface of the optic
nerve, which is visible on the retina, is called the optic disk or optic nerve
head.
Extraocular Muscles
Six extraocular muscles are attached to each eye to move the eye left and right,
up and down, and diagonally, or even around in circles when one wishes.
Eye development (embryo)
Eye formation in the human embryo begins at approximately three weeks into
embryonic development and continues through the tenth week.
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Cells from both the mesodermal and the ectodermal tissues contribute to the
formation of the eye. Specifically, the eye is derived from the neuroepithelium,
surface ectoderm, and the extracellular mesenchyme which consists of both
the neural crest and mesoderm.
Neuroepithelium forms the retina, ciliary body, iris, and optic nerves. Surface
ectoderm forms the lens, corneal epithelium and eyelid. The extracellular
mesenchyme forms the sclera, the corneal endothelium and stroma, blood
vessels, muscles, and vitreous
The eye is a complex optical system – very similar to a camera.
Vision begins when light enters the eye through the cornea, a powerful
focusing surface. The cornea is what gives us clear vision. From there, it
travels through clear aqueous fluid, and passes through a small aperture in the
iris called the pupil.
As muscles in the iris relax or constrict, the pupil changes size to adjust the
amount of light entering the eye. Light rays are bent and focused through the
lens, and proceed through a clear jelly-like substance in the center of the eye
called vitreous humor, which helps give the eye form and shape. When light
rays finally land on the retina, the part of the eye similar to film in a camera,
they form an upside-down image. The retina converts the image into an
electrical impulse that travels along the optic nerve to the brain, where it is
interpreted as an upright image.