What is human eye and is its main function?

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              Introduction:

                             Human eye is present in anterior medial wall of skull bone and there is a hollow structure called orbit which contain several bone e.g lacrimal bone, ethmoid bone, sphenoid bone, zygomatic bone and also contain opening called optic foramen which is way for the entrance of blood vessels and nerves. It is one  of the special organ which present on the skull bone it is responsible for vision ( the ability to see every  thing) which convert light energy into electrical impulse and give us understand the shape of everything. only 1/6 of human eye is present outside the skull remaining all part is present inside the skull which is not visible to everyone .


Structure of human eye:

                             In the structure of Human eye it has three layers

1-       outermost layer

2-       Middle layer

3-      Inner layer.

There are three layers of human eye and there is one another layer which is not a part of this layer that just  cover the superficial surface of eye and eyelid that is called conjunctiva.

What is conjunctiva?

              It is the outer most covering of human eye which surround human eye and the lower surface of eyelid which provide protection, lubrication and defense from foreign particles. The layer of conjunctiva which  cover eye is called Bulbar conjunctiva and the conjunctiva which cover eyelid is called palperbral conjunctiva.



Outermost layer

              The outermost layer of human eye is formed by the tunica fibrous. Which has two major layer.

·       Cornea

It is the outer and transparent layer of human eye which cover the anterior surface of eye its transparent nature allow the entrance of light into human eye and also refract the rays of light for the purpose of its focusing on the cornea. It covers the 1/6 region of human eye. It is the only layer which lacks blood vessels as a result they are not able to receive nutrients directly from the blood.

Here is a general question how cornea receive nutrient?

               Due to its avascular nature there is another transparent fluid present at posterior of the cornea that is called  aqueous humor which provide nutrients to cornea and also from tear.

It is made up of five layers.

1.      Corneal epithelium

It is the outermost protective layer of  human cornea which give physical protection to human eye.

2.      Bowman’s membrane

It is second layer of cornea which is acellular in nature which also give protection to human eye.

3.      Stroma

It is the  third layer of cornea which is made up of collagen  fibers ,keratocytes, water and extracellular matrix. it is one of the thickest layer of cornea. This help in the maintenance of cornea transparency and in the refraction of light.

4.      Descemet’s membrane

It is a layer which is present between stroma and endothelium. It is the basement membrane of corneal endothelium which is made up of collagen fiber. 

It maintain corneal integrity and also resistant to damage.

5.      Endothelium

It is  the inner most layer of cornea which present toward aqueous humor and those cells which are present in this side are able to regenerate well after getting damage. This layer pump out excess water from the stroma layer into aqueous layer and also play a vital role in the exchange of nutrients between cornea and aqueous layer. If this layer doesn’t work properly also doesn’t allow excess water into aqueous layer from stroma region this may cause cloudy vision.

 

·       Sclera

It is the part of fibrous  layer or outermost layer of human eye that is white in nature when we see it from externally it is visible to us which cover 5/6 region of human external layer. Which mostly cover the posterior layer of human eye.

It is the strong and thick in nature. It is opaque and white in color their blood supply is poor.


It is made up of

·       Elastic fibers

·       Fibroblast

·       Collagen fibers

·       Dense irregular connective tissue

It is made up of 3 layers

a.      Lamina fusca

It is the innermost layer of sclera which is thin in nature and also a pigmented layer that contain melanocytes. It is the layer of sclera which is present between sclera and choroid layer. It reduce the scattering of light by absorption of reflecting of light through this process it clear.

b.      Sclera proper

It is the middle layer of cornea which is thickest  in nature , it is made up of elastic fiber  and collagen fibers. This layer provides toughness to the eye ball.

c.       Episcleral

 It is the outermost layer of sclera which is made up of connective tissues which connect sclera with conjunctiva and tendon’s capsule and also provide nutrient to sclera.

 

2. Middle layer

              The middle layer of human eye is called vascular layer. for about blood vessels https://healthduhs.blogspot.com/2026/05/what-is-veinous-system-of-human-body.html

              It is the layer  which is present between the outermost layer  and innermost layer. It has three main parts.

·       Iris

·       Ciliary body

·       Choroid

It is the part of middle layer which contain blood vessels and supply nutrients and oxygen to eye. It also contain melanin pigment which absorb stray light. This layer contain autonomic nervous  which control iris and ciliary muscle. From sympathetic division and from cervical chain which dilate pupil which dilates the pupil. From oculomotor nerve it works as parasympathetic nerve which function is in the contraction of ciliary muscle which close the pupil.

Iris :

               It is the colored area of human eye which is present infront of the lens and around the pupil which is responsible for the accommodation and also which allow how much light should have to  inter into the eye it will adjust itself eitherly contraction or relaxation through ciliary muscle.

There are three type of muscles are present inside the human eye.

·       Posterior epithelium

It is highly pigmented area which prevent from light passing through the iris in this layer there are melanocytes cell which produce this color.

When melanin is high in  amount the color will be brown, when the amount of melanin will be medium in amount the should be green eyes and when the amount of melanin will be low in amount  the color should be blue.

When there is extreme amount of melanin is present in the iris the color should be black.

·       Sphincter pupillae

This type of muscle is circular in nature which will get activation when it exposed to bright light.

·       Dilator pupillae

This type of muscle Is made up of  radial muscle which dilates pupil and get activation in dim light.

              Ciliary body

                Ciliary muscle origin from the sclera and attached with the lens through suspensory ligament.

When ciliary muscle become contract the lens become more curved and  when the ciliary muscle become relax the lens become less curved.

Choroid

              It the second layer of human eye which is rich in blood vessels and pigments.  The pigment presence in this layer is responsible for absorption of extra light and prevent it from reflection.  There are four layer of choroid from outer to inner side.

1.      Suprachoroid layer

It is the outermost layer of choroid which connect sclera with choroid, also contain connective tissues and melanocytes.

2.      Vascular layer

This layer of choroid contain arteries and veins both type of blood vessels are present small and large.

3.      Choriocapillaris

It is a layer which is made up of network of capillary which is closest to retina this layer nourish outer retina and photoreceptor cells.

4.      Bruch’s membrane

This is the inner most layer of choroid which is between retina and choroid layer and responsible for the exchange waste.

 

Inner most layer ( Retina )

       It is inner most layer of human eye which is responsible for receiving light and convert them into electrical energy later on that will become an impulse and send to brain as a stimulus through optic nerve. for  about optic nerve https://healthduhs.blogspot.com/2026/05/how-part-of-central-nervous-system.html

It has two main parts.

1.      Pigmented layer

This layer is also called retinal pigment epithelium which absorb extra light and  prevent from reflection. This layer nourish photoreceptors,  remove waste product and store vitamin A.

2.      Nervous layer

This layer contain neuron and photoreceptor cell.

 There are two type of photoreceptor cells are present here.

a.      Cones

The function of cones in retina is detect sharp light land color and its more active in bright light. Human eye contain 6million cones.

b.      Rod

The function of cones in retina is to detect dark light which is more active in dim light . human eye contain 120million rods.

pupil

    it is not a part of this layer but there is an empty space between the lens and cornea which is called pupil that is black in structure due to empty structure 

aqueous humor

      there is liquid between lens and cornea around the iris that maintain the structure of human eye is called aqueous humor.

 

vitrous humor 

    there is another liquid present between the posterior side of lens and till optic disc that is responsible for maintain the shape of human eye.

reference

WHO -World Health Organization

NIH- National Institute Of Health)

 CDC- Centre for Disease Control and prevention


 

 

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