Parts of the Eye

Parts of the eye © 2019 American Academy of Ophthalmology

Parts of the eye



The main parts of the eye are the cornea, sclera, choroid and the retina. The two main parts that are responsible for keeping the eye firm and on its shape are the sclera and the vitreous body.

We will divide the eye into three layers which are the outer, the middle and the inner layers.


1- The Outer Parts of the Eye

A- Cornea

It is the transparent part at the center of the front part of the eye. It is dome-shape with clear and smooth surface. This layer is transparent because it has no blood vessels and it has special types of collagen fibers. Cornea and lens are the two main parts of the eye that act as optical systems in human eye that refract lights toward the center of vision at the retina which is called macula.

B- Sclera

It is the thick white layer of the eye. This layer keeps the eye in its shape and protects the inner layers from trauma and injuries.

C- Conjunctiva

This is a thin transparent membrane which covers the sclera and it is attached at the junction of sclera and cornea which is called the limbus. Conjunctiva also covers the back surface of the eyelid.

D- Limbus

It is that part in which there is a junction between the cornea and sclera. At limbus, the cornea will change from transparent tissue to white opaque part to form the sclera. At this part the conjunctiva will also be attached.



2- The Middle Parts of the Eye

A- Iris

This layer lies in the front part of uvea and it is responsible for the color of the eye. Black colored eye has more melanocytes and melanosomes in the iris while blue and light colored eye has less melanocytes and melanosomes.

It has two muscles, one is called sphincter papilla that is responsible for constriction of the iris and the other is called dilutor papilla that is responsible for dilation of the iris. Iris has a hole or opening at its center which is called the pupil.

constriction or dilution of the iris will make the pupil smaller or larger. This will help to protect the eye in response to the brightness of lights. Iris lies behind the cornea and in front of the lens.

B- Anterior chamber

This chamber or space lies between the back surface of the cornea and the front surface of the iris. This chamber is filled with clear fluid called aqueous humor which is secreted from ciliary process.



C- Angle of the Eye

This is the part of the eye that lies at the corner of the anterior chamber at the junction between the iris and the cornea. It lies behind the limbus. This part has the trabecular meshwork which is responsible for the drainage of the aqueous humor outside the eye.

Closure of this angle will cause angle closure glaucoma. When this angle is widely opened but there are defects in trabecular meshwork cells, open angle glaucoma will occur.

D- Posterior chamber

This space is smaller than the anterior chamber and lies between the back surface of the iris and the anterior surface of the lens. At the corner of this chamber, there is ciliary body. This chamber also filled with aqueous humor.



E- Ciliary body

This part of the uvea lies behind the iris, at the corner of the posterior chamber. This part is covered by the aqueous humor in the posterior chamber and by the vitreous in the vitreous cavity. It consists of two parts, the first part which is called the pars plicata and the second part is the pars plana.

The front part which is called the pars plicata consists of two parts, the ciliary muscle and the ciliary process. Ciliary muscle is responsible for accommodation while the ciliary process responsible for secretion of aqueous humor.

The zonules also attached to ciliary process. Zonules are special fibers that attach the natural lens to the ciliary process and together with the lens and ciliary muscle; they will be responsible for accommodation.

Pars plana is the back part of the ciliary body and is covered by vitreous get. It consists of epithelial layer and stromal layer. Stromal layer has ciliary muscle and it is continuous with the choroid, while the epithelial layer is continuous with the retina at ora serrata.

F- Choroid

This part lies between the outer layer of the eye, the sclera and the inner layer of the eye, the retina. It starts from ciliary body in the front part of the eye and ends at the margin of the optic disc at the back part of the eye. This layer consists mainly of blood vessels of different sizes. This layer is responsible for nourishment of the retina layer.

The layer that separates choroid from retina is called bruchs membrane which has the retinal pigment epithelial layer (RPEs). In choroidal neovascularization, there will be invasion of abnormal blood vessels from choroid through bruchs membrane toward retina. This occurs in wet macular degeneration.



3- The Inner Parts of the Eye

The inner parts of the eye consist of retina and optic disc. Optic nerve that emerges of optic disc is consisting of nerve fibers from the inner layer of retina.

A- Retina

This layer consists of ten layers and the outer most layer is called the photoreceptor layer which lies near the retinal pigment epithelial layer and choroid.

Photoreceptor layer, is the one responsible for generation of specific electrical and chemical impulses when lights active the photoreceptors cells. These impulses will travel through the inner most layer which is called the ganglion layer towards the optic disc and then to the brain. This inner layer is covered by the vitreous body or gel.

Retina consists of three main parts. Ora serrata part which leis adjacent to the ciliary body at the front surface of the eye. The peripheral retina which lies between the macular and the ora serrata. This part is responsible for night vision.

The macula is the part that is responsible for the central vision and it lies between two major retinal arteries. At the center of the macula, there is the fovea, which is responsible for the main central vision and the highest visual acuity.

B- Optic disc

Optic disc is located at the posterior pole of the eye and from it, retinal blood vessels emerged to supply the retina. At the margin or the optic disc, the three main layers of the eye will end. Optic disc has no photoreceptors, and that is way it is called the blind spot. From Optic disc, the optic nerve will emerge toward the brain.



Other important Parts of the Eye

1- Crystalline lens

It lies behind the iris. It is separated from the iris by a space which is called the posterior chamber. The lens is hold in place by the zonules which are attached to the ciliary process. The lens is a clear, transparent tissue and it is devoid of blood vessels.

It is nourished by the aqueous humor. Lights that passed through this clear lens will be refracted toward the macula. Due to age and other factors, this lens becomes opaque and lights can’t pass through it, causing the visual acuity to drop. Lens opacity is called cataract. Behind the lens, there a clear gel called vitreous gel.

The most important parts of the eye that act as optical system to ensure the lights reach clearly to the macula are the cornea and lens. Any defects in one or both of them, will cause drop in vision.

2- Vitreous gel or Vitreous body

This part covers the inner parts of the eye such as layer of the retina, ciliary body and the back surface of the lens. 99% of vitreous gel is water and the remaining is collagen fibers.

Together with the sclera and aqueous humor, they will form the normal intraocular pressure and keep the eye in its firm shape. Opacities in vitreous gel will cause eye floaters.



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