The Human 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.

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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

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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.

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Fundus photo (photography of the inside of the back of the eye) showing scatter laser surgery for diabetic retinopathy.

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Proliferative retinopathy, an advanced form of diabetic retinopathy, occurs when
abnormal new blood vessels and scar tissue form on the surface of the retina.

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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.

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Diabetic macular edema

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Eye disease simulations such as this
can be viewed by clicking here.

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