The Eye
The human eye is the
organ which gives us the sense of sight, allowing us to learn more about the
surrounding world than we do with any of the other four senses. We use our eyes
in almost every activity we perform, whether reading, working, watching
television, writing a letter, driving a car, and in countless other ways. Most
people probably would agree that sight is the sense they value more than all the
rest.
The eye allows us to see and interpret the shapes, colors, and dimensions of
objects in the world by processing the light they reflect or emit. The eye is
able to see in bright light or in dim light, but it cannot see objects when
light is absent.
Glossary of Eye Terms
Anterior Chamber
The cavity in the front part of the eye between the lens and cornea is called
the Anterior Chamber. It is filled with Aqueous, a water-like fluid. This fluid
is produced by the ciliary body and drains back into the blood circulation
through channels in the chamber angle. It is turned over every 100 minutes.
Chamber Angle
Located at the junction of the cornea, iris, and sclera, the anterior chamber
angle extends 360 degrees at the perimeter of the iris. Channels here allow
aqueous fluid to drain back into the blood circulation from the eye. May be
obstructed in glaucoma.
Ciliary Body
A structure located behind the iris (rarely visible) which produces aqueous
fluid that fills the front part of the eye and thus maintains the eye pressure.
It also allows focusing of the lens.
Conjunctiva
A thin lining over the sclera, or white part of the eye. This also lines the
inside of the eyelids. Cells in the conjunctiva produce mucous, which helps to
lubricate the eye.
Cornea
The transparent, outer "window" and primary focusing element of the eye. The
outer layer of the cornea is known as epithelium. Its main job is to protect the
eye. The epithelium is made up of transparent cells that have the ability to
regenerate quickly. The inner layer of the cornea is also made up of transparent
tissue, which allows light to pass.
Hyaloid Canal
A narrow channel that runs from the optic disc to the back surface of the lens.
It serves an embryologic function prior to birth but none afterwards.
Iris
Inside the anterior chamber is the iris. This is the part of the eye which is
responsible for one's eye color. It acts like the diaphragm of a camera,
dilating and constricting the pupil to allow more or less light into the eye.
Pupil
The dark opening in the center of the colored iris that controls how much
light enters the eye. The colored iris functions like the iris of a camera,
opening and closing, to control the amount of light entering through the pupil.
Lens
The part of the eye immediately behind the iris that performs delicate focusing
of light rays upon the retina. In persons under 40, the lens is soft and
pliable, allowing for fine focusing from a wide variety of distances. For
individuals over 40, the lens begins to become less pliable, making focusing
upon objects near to the eye more difficult. This is known as presbyopia.
Macula
The part of the retina which is most sensitive, and is responsible for the
central (or reading) vision. It is located near the optic nerve directly at the
back of the eye (on the inside). This area is also responsible for color vision.
Optic Disc
The position in the back of the eye where the nerve (along with an artery and
vein) enters the eye corresponds to the "blind spot" since there are no rods or
cones in these location. Normally, a person does not notice this blind spot
since rapid movements of the eye and processing in the brain compensate for this
absent information. This is the area that the ophthalmologist studies when
evaluating a patient for glaucoma, a condition where the optic nerve becomes
damaged often due to high pressure within the eye. As it looks like a cup when
viewed with an ophthalmoscope, it is sometimes referred to as the Optic Cup.
Optic Nerve
The optic nerve is the structure which takes the information from the retina as
electrical signals and delivers it to the brain where this information is
interpreted as a visual image. The optic nerve consists of a bundle of about one
million nerve fibers.
Retina
The membrane lining the back of the eye that contains photoreceptor cells. These
photoreceptor nerve cells react to the presence and intensity of light by
sending an impulse to the brain via the optic nerve. In the brain, the multitude
of nerve impulses received from the photoreceptor cells in the retina are
assimilated into an image.
Sclera
The white, tough wall of the eye. Few diseases affect this layer. It is covered
by the episclera (a fibrous layer between the conjunctiva and sclera ) and
conjunctiva, and eye muscles are connected to this.
Vitreous
This is a jelly-like substance that fills the body of the eye. It is normally
clear. In early life, it is firmly attached to the retina behind it. With age,
the vitreous becomes more water-like and may detach from the retina. Often,
little clumps or strands of the jelly form and cast shadows which are perceived
as "floaters". While frequently benign, sometimes floaters can be a sign of a
more serious condition such as a retinal tear or detachment and should be
investigated with a thorough ophthalmologic examination.
The Visual Process
Light waves from an object (such as a tree in the figure below) enter the eye first through the cornea, which is the clear dome at the front of the eye. The light then progresses through the pupil, the circular opening in the center of the colored iris. Next, the light passes through the crystalline lens, which is located immediately behind the iris and the pupil.
Initially, the light waves are bent or
converged first by the cornea, and then further by the crystalline lens, to a
nodal point (N) located immediately 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, the clear gel that makes up
about 80% of the eye’s volume, and then, ideally, back to a clear focus on the
retina behind the vitreous. The small central area of the retina is the macula,
which provides the best vision of any location in the retina. If the eye is
considered to be a type of camera, the retina is equivalent to the film inside
of the camera, registering the tiny photons of light which interact with it.
Within the layers of the retina, light impulses are changed into electrical
signals and then sent through the optic nerve, along the visual pathway, to the
occipital cortex at the posterior or back of the brain. Here, the electrical
signals are interpreted or “seen” by the brain as a visual image. When the light
entering the eyes is bright enough, the pupils will constrict (get smaller), due
to the pupillary light response.
Actually, then, we do not “see” with our eyes but, rather, with our brains. Our
eyes merely are the beginnings of the visual process.
The Eye Examination
Eye Disease
Anatomy
Conventional surgery to treat glaucoma makes a new
opening in the meshwork.
This new opening helps fluid to leave the eye and
lowers intraocular pressure.
Fundus photo (photography of the inside of the back of the eye) showing scatter laser surgery for
diabetic retinopathy.
Proliferative retinopathy, an advanced form of
diabetic retinopathy, occurs when
abnormal new blood vessels and scar tissue
form on the surface of the retina.
In background retinopathy, a slight deterioration in
the small blood vessels of the retina,
portions of the vessels may swell and
leak fluid into the surrounding retinal tissue.
Diabetic macular edema.
Eye disease simulations such as this
can be viewed by
clicking here.
Some common eye conditions
are described here.