Our eyes are the precious and important part of our health that work non stop. The eyeball is a bilateral and spherical organ, responsible for vision. Eyeball lies in a bony cavity within the facial skeleton, known as bony orbit. In this blog, we discuss The Human Eye Anatomy.

Dimension of adult eyeball 

  • Anteroposterior diameter       – 24mm 
  • Horizontal diameter                – 23.5mm
  • Vertical diameter                    – 23mm
  • Circumference                       – 75mm
  • Volume                                  – 6.5ml 
  • Weight                                   – 7gm 

Eyeball is divided into two segments, Anterior and posterior segment. Here in this blog post, we are explaining different parts or structures of eyes. 

Different parts of eyes

Cornea

The front clear, transparent portion of the eye which transmits and focuses the light rays that enter the eye. This structure does not contain any blood vessels, unlike other tissues in the human body. 

It also contributes to preventing foreign bodies from entering the eye. Tears and aqueous humor provides nutrients to the cornea. 

Aqueous humor

It is a clear, thin, transparent fluid that is found inside the front portion of the eye which is similar to plasma. This fluid constitutes 99.9% water and other 0.1% of sugar, vitamins, proteins, and others. 

 It is produced by the ciliary body. So as to work properly, the production must be balanced by drainage at an equal rate. Aqueous humor helps to nourish the cornea and lens as well as it helps to maintain Intraocular pressure(IOP). 

Conjunctiva

Conjunctiva is a thin, transparent membrane that covers the white portion of the eye(sclera) and the inner surface of eyelids. It secretes mucus which contributes to the protection and lubrication of our eyes. 

It has two portions. Bulbar and palpebral conjunctiva. Bulbar conjunctiva covers the anterior portion of sclera (does not cover cornea), whereas palpebral covers inner surface of both upper and lower eyelids. Protects our eyes from dust and infection causing agents. 

Iris

Thin, annular structure, colored portion of the eye which helps to regulate the amount of light entering the eye. It controls the diameter and size of the pupil in various lighting conditions.

 In bright light conditions, the iris closes the pupil to send less amount of light while in lower light condition, the iris opens up the pupil to let more light in eyes. 

Pupil

The dark aperture or hole which is present in the center of the iris is a pupil. It ranges from 3-7 mm in diameter. 

It contributes in regulating the amount of light that enters the eye. 

Ciliary body

The ciliary body is a circular structure present behind the iris and is made up of ciliary muscles and ciliary processes that are attached to the crystalline lens via zonular fibers. 

The ciliary body produces fluid known as aqueous humor. It does contain ciliary muscles which help in accommodation. Changes the shape of the lens while focusing objects at different distances. 

Crystalline lens

The transparent bi-convex structure located behind the iris and pupil which helps to refract and bend light rays so as to focus the image on the retina. Lens shape can be altered by the relaxation and contraction of ciliary muscles which helps to focus objects at various distances. 

Clouding or opacity of lens is Cataract, which occurs as we age. Cataract interferes with vision and is corrected with surgery and replaced with artificial lenses. 

Vitreous Humor

Vitreous is a clear, colorless gel-like substance that fills the space between the lens and the retina of the eye. It comprises a large portion of the eyeball which consists of water, sugar, salt, collagen, and hyaluronic acid.

In children, it is milky and has gel like structure but as we grow older, it becomes clear. Hence light passes and allows us to see. It also contributes in maintaining the shape of  the eye.

Retina

The retina is a thin, light-sensitive layer of tissue of the eye lining the back wall inside the eye. It receives the light and converts it into signals, thus transferring those signals to the brain and we can see. There are 10 layers of the retina.   

Optic nerve

The optic nerve is also known as the second cranial nerve or simply CN II transmits visual information from the retina to the brain. It is actually a part of both the eye and brain. It transfers impulses formed by the retina to the brain, which interprets them as images. 

Macula

The macula is a small central area present in the retina which contains special light-sensitive cells and allows us to see any objects in fine detail. As we age, we can develop macular degeneration which causes vision issues or often leads to vision loss.

Sclera

The white outer portion of the eye is the sclera. It acts as a supporting wall of the eyeball. The sclera, along with the IOP of the eye, maintains the shape of the eyeball. It is continuous with the cornea, the junction between sclera and cornea is limbus. 

Basically, sclera forms more than 80% of the surface area of the eyeball, extending from cornea to optic nerve( back of the eye). Tough and fibrous nature of sclera helps to protect eyes from serious conditions like laceration, rupture and so on. It also helps to provide strong attachments for extraocular muscles. These muscles help to control the eyes movement. 

Choroid

A vascular layer of the eye present in between the sclera and retina is choroid. It is filled with blood vessels that supply oxygen and nutrients to the eye. 

Choroid contributes in maintaining the temperature and volume of the eye. Also supplies the outer retina with nutrients.  

How the eye works?

First of all, rays of light enter our eyes through the cornea( front clear portion of eye). Cornea helps to bend the light rays so as to focus. Colored portions of eyes, iris and pupil both help to control how much light to let in. 

Furthermore, that light passes to the crystalline lens. This lens also works together with the cornea to focus light on the retina. Afterall, light reaches and hits the retina (light sensitive layer that enables vision), photoreceptors cells present in the retina turn that light into signals. 

Hence, those signals travel from the retina to the brain’s visual cortex via optic nerve. Thus the brain converts signals into images and we can see. 

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