Eye ball consists of three layers. Sclera, choroid and retina.
1. Sclera: The outer layer of the eyeball is called sclera. It is colorless, opaque, hard and elastic. It is composed of cartilage and connective tissue. At the front of the eye, the sclera changes to form a very thin and transparent cornea. The cornea is called the window of the eye. There is no blood supply to the cornea. It receives nutrients from the aqueous humor. Outside the cornea is a thin and transparent conjunctiva. The part behind the sclera through which the optic nerve enters is called the lamina cribosa.
Sclera work
(i) Sclera gives shape to the eye.
(ii) Light enters through the cornea.
(iii) Conjunctiva protects the eye from dust and germs.
2. Choroid: The second layer of eyeball is choroid. It is composed of blood vessels, muscle and melanin pigment. The parts of choroid are:
(i) Ciliary body: The thick part of the ring at the junction of iris and choroid is called ciliary body. It consists of ciliary ring, ciliary process and ciliary muscle.
Function of the ciliary body
* The ciliary muscle changes the shape of the lens by contraction-dilation and participates in accommodation.
* It produces aqueous humor.
(ii) Suspensory ligaments: The ligaments surrounding the lens are called suspensory ligaments. One end of the ligament is attached to the lens and the other end to the ciliary body. The suspensory ligament changes the curvature of the lens through contraction and extension.
(iii) Iris: The black screen formed by the choroid behind the cornea is called Iris. It is composed of the circular muscle and the areolar muscle. It dilates and dilates the pupil and ensures moderate light entry into the eye.
(iv) Pupil : The round hole in the middle of iris is called pupil. Its diameter is 3-4 mm. It is surrounded by circular and areolar muscles. The pupil becomes smaller or larger due to muscle contraction and expansion. The pupil enlarges in dim light and shrinks in bright light. It controls light entering the eye.
3. Retina : The innermost layer of the eyeball is called retina. It is the only light sensitive part. It consists of 10 sub-layers. The outer layers are composed of pigment cells and the inner layers are composed of nerve cells. It contains two types of pigment cells. Rod cells and angle cells. Rod cells are elongated and cylindrical and suitable for dim light vision. Corneal cells are convex and suitable for vision in bright light.
(i) Blind spot: There is a circular area in the retina devoid of rod cells and angle cells, called the optic disc. It is not light sensitive. It is called blind spot because no reflection is formed.
(ii) Fovea centralis: Above the optic disc in the retina is an oval and yellow area, called the macula lutea. The small hole in the center of the macula lutea is called the phobia centralis or yellow spot. It has very few rod cells and a lot of angle cells, so the best reflection is formed.
(iii) Optic nerve: Retina contains optic nerve. It sends signals to the brain.
Retina function
* Retina produces reflections.
* The bile duct forms a clear reflex.
* The optic nerve sends messages to the brain.

The human eye is like a spherical ball, it is called an eyeball. Each eyeball consists of three parts. Eye ball layer, lens and chamber.
1. Eye ball layer: Eye ball consists of three layers. Sclera, choroid and retina.
(i) Sclera: The outer layer of the eyeball is called sclera. It is colorless, opaque, hard and elastic. It is composed of cartilage and connective tissue. At the front of the eye, the sclera changes to form a very thin and transparent cornea. The cornea is called the window of the eye. There is no blood supply to the cornea. It receives nutrients from the aqueous humor. Outside the cornea is a thin and transparent conjunctiva. The part behind the sclera through which the optic nerve enters is called the lamina cribosa.
Sclera work
* Sclera gives shape to the eye.
* Light enters through the cornea.
* Conjunctiva protects the eye from dust and germs.
(ii) Choroid: The second layer of eyeball is choroid. It is composed of blood vessels, muscle and melanin pigment. The parts of choroid are:
(a) Ciliary body: The thick part of the ring at the junction of iris and choroid is called ciliary body. It consists of ciliary ring, ciliary process and ciliary muscle.
Function of the ciliary body
* The ciliary muscle changes the shape of the lens by contraction-dilation and participates in accommodation.
* It produces aqueous humor.
(b) Suspensory ligaments: The ligaments surrounding the lens are called suspensory ligaments. One end of the ligament is attached to the lens and the other end to the ciliary body. The suspensory ligament changes the curvature of the lens through contraction and extension.
(c) Iris: The black screen formed by the choroid behind the cornea is called Iris. It is composed of the circular muscle and the areolar muscle. It dilates and dilates the pupil and ensures moderate light entry into the eye.
(d) Pupil : The round hole in the middle of iris is called pupil. Its diameter is 3-4 mm. It is surrounded by circular and areolar muscles. The pupil becomes smaller or larger due to muscle contraction and expansion. The pupil enlarges in dim light and shrinks in bright light. It controls light entering the eye.

(iii) Retina : The innermost layer of the eyeball is called retina. It is the only light sensitive part. It consists of 10 sub-layers. The outer layers are composed of pigment cells and the inner layers are composed of nerve cells. It contains two types of pigment cells. Rod cells and angle cells. Rod cells are elongated and cylindrical and suitable for dim light vision. Corneal cells are convex and suitable for vision in bright light.
(a) Blind spot: There is a circular area in the retina devoid of rod cells and angle cells, called the optic disc. It is not light sensitive. It is called blind spot because no reflection is formed.
(b) Fovea centralis: Above the optic disc in the retina is an oval and yellow area, called the macula lutea. The small hole in the center of the macula lutea is called the phobia centralis or yellow spot. It has very few rod cells and a lot of angle cells, so the best reflection is formed.
(c) Optic nerve: Retina contains optic nerve. It sends signals to the brain.
Retina function
* Retina produces reflections.
* The bile duct forms a clear reflex.
* The optic nerve sends messages to the brain.

2. Lens: A lens is attached to the ciliary body by the suspensory ligament behind the pupil. It is transparent, elastic and bi-convex. Its diameter is 10 mm and thickness is 3.7-4.5 mm. There is no blood supply to the lens. Contraction-dilation of the ciliary muscle causes the lens to contract-dilate. With the help of the lens, the light rays coming from the object are reflected to certain parts of the retina.
3. Chambers: The eyeball has three cavities filled with fluid.
(i) Aqueous humor chamber: The cavity between the cornea and the lens is called the aqueous humor chamber. It is filled with a fluid called aqueous humor.
(ii) Posterior chamber: The cavity between iris and lens is called posterior chamber. It is also filled with aqueous humor.
(iii) Vitreous humor chamber: The large chamber between the lens and the retina is called the vitreous chamber. It is 80% of the eyeball. It is filled with a jelly-like fluid called vitreous humor. Vitreous humor is like egg white. Vitreous humor consists of 99% water, 1% collagen and hyaluronic acid. Vitreous humor helps in refraction of light.
Lens function
* Helps light rays enter the eye.
* Regulates internal and external eye pressure.
* Through it the light rays are refracted and fall on the retina.

The sensory organ that stores vision in the presence of light is called the eye. Two eyes are embedded in the eye socket of the crown. 1/6 of the eye can be seen. The remaining 5/6 is in Kotor.
Anatomy of Eye
Human eye mainly consists of two types of parts. Eye ball and accessories.
A. Eye ball
The human eye is like a spherical ball, it is called an eyeball. Each eyeball consists of three parts. Eye ball layer, lens and chamber.
1. Eye ball layer: Eye ball consists of three layers. Sclera, choroid and retina.
(i) Sclera: The outer layer of the eyeball is called sclera. It is colorless, opaque, hard and elastic. It is composed of cartilage and connective tissue. At the front of the eye, the sclera changes to form a very thin and transparent cornea. The cornea is called the window of the eye. There is no blood supply to the cornea. It receives nutrients from the aqueous humor. Outside the cornea is a thin and transparent conjunctiva. The part behind the sclera through which the optic nerve enters is called the lamina cribosa.
Sclera work
* Sclera gives shape to the eye.
* Light enters through the cornea.
* Conjunctiva protects the eye from dust and germs.
(ii) Choroid: The second layer of eyeball is choroid. It is composed of blood vessels, muscle and melanin pigment. The parts of choroid are:
(a) Ciliary body: The thick part of the ring at the junction of iris and choroid is called ciliary body. It consists of ciliary ring, ciliary process and ciliary muscle.
Function of the ciliary body
* The ciliary muscle changes the shape of the lens by contraction-dilation and participates in accommodation.
* It produces aqueous humor.
(b) Suspensory ligaments: The ligaments surrounding the lens are called suspensory ligaments. One end of the ligament is attached to the lens and the other end to the ciliary body. The suspensory ligament changes the curvature of the lens through contraction and extension.
(c) Iris: The black screen formed by the choroid behind the cornea is called Iris. It is composed of the circular muscle and the areolar muscle. It dilates and dilates the pupil and ensures moderate light entry into the eye.
(d) Pupil : The round hole in the middle of iris is called pupil. Its diameter is 3-4 mm. It is surrounded by circular and areolar muscles. The pupil becomes smaller or larger due to muscle contraction and expansion. The pupil enlarges in dim light and shrinks in bright light. It controls light entering the eye.

(iii) Retina : The innermost layer of the eyeball is called retina. It is the only light sensitive part. It consists of 10 sub-layers. The outer layers are composed of pigment cells and the inner layers are composed of nerve cells. It contains two types of pigment cells. Rod cells and angle cells. Rod cells are elongated and cylindrical and suitable for dim light vision. Corneal cells are convex and suitable for vision in bright light.
(a) Blind spot: There is a circular area in the retina devoid of rod cells and angle cells, called the optic disc. It is not light sensitive. It is called blind spot because no reflection is formed.
(b) Fovea centralis: Above the optic disc in the retina is an oval and yellow area, called the macula lutea. The small hole in the center of the macula lutea is called the phobia centralis or yellow spot. It has very few rod cells and a lot of angle cells, so the best reflection is formed.
(c) Optic nerve: Retina contains optic nerve. It sends signals to the brain.
Retina function
* Retina produces reflections.
* The bile duct forms a clear reflex.
* The optic nerve sends messages to the brain.

2. Lens: A lens is attached to the ciliary body by the suspensory ligament behind the pupil. It is transparent, elastic and bi-convex. Its diameter is 10 mm and thickness is 3.7-4.5 mm. There is no blood supply to the lens. Contraction-dilation of the ciliary muscle causes the lens to contract-dilate. With the help of the lens, the light rays coming from the object are reflected to certain parts of the retina.
3. Chambers: The eyeball has three cavities filled with fluid.
(i) Aqueous humor chamber: The cavity between the cornea and the lens is called the aqueous humor chamber. It is filled with a fluid called aqueous humor.
(ii) Posterior chamber: The cavity between iris and lens is called posterior chamber. It is also filled with aqueous humor.
(iii) Vitreous humor chamber: The large chamber between the lens and the retina is called the vitreous chamber. It is 80% of the eyeball. It is filled with a jelly-like fluid called vitreous humor. Vitreous humor is like egg white. Vitreous humor consists of 99% water, 1% collagen and hyaluronic acid. Vitreous humor helps in refraction of light.
Lens function
* Helps light rays enter the eye.
* Regulates internal and external eye pressure.
* Through it the light rays are refracted and fall on the retina.

B. Special parts of the eye
1. Orbit: Orbit is a hollow hole. It houses the eyeball.
2. Eyelid : Eyelids are-
(i) Upper eyelid: The upper lid of the eye is called upper eyelid. Under it there is a line of eye lashes like hair. It is contagious. It controls the opening and closing of the eyes.
(ii) Inferior eyelid: The lower eyelid is called lower eyelid. Its free end has single eye lashes like hair. It is not contagious. It protects the eyes from dust and sand.
(iii) Nectitating screen: It is the hidden almost passive part. It is located in the inner corner of both eyes. It is a reddish flesh. Through this the lacrimal gland ducts are opened.
3. Conjunctiva: The transparent thin membrane covering the cornea is called conjunctiva. It protects the eyes from dust and germs. It keeps the eyes slippery and moist.
4. Eyelash: The eyelash is called Akshipaksha. It prevents dust and germs from entering the eyes.
5. Eye brow: The hair above the eyelid is called eye brow. It prevents sweat from the forehead from entering the eyes.
6. Orbital muscles: The orb is attached to the orbit by 6 muscles. Muscles are double. Rectus muscle and oblique muscle.

(a) Rectus muscle: 4 rectus muscles.
(i) Lateral Rectus Muscle : It helps rotate the eyeball outwards.
(ii) Medial rectus muscle : It helps in inward rotation of the eyeball.
(iii) Superior rectus muscle : It helps the eyeball to rotate upwards.
(iv) Inferior rectus muscle: It helps the eyeball to rotate downwards.

(b) Oblique muscles: 2 oblique muscles.
(i) Superior oblique muscle: It helps the eyeball to rotate along the axis between the optic nerve and the cornea.
(ii) Inferior Oblique Muscle: It acts opposite to the superior oblique muscle.

7. Eye glands: Human eyes have three types of glands.
(i) Lacrimal glands: It is located above and in front of the eyeball. Ashra (अभुद्थ) is secreted from this gland. Tears keep the eyes moist and sterile.
(ii) Harderian glands: It is located behind the eyeball at the base of the eye. The oily substance secreted by these glands lubricates the cornea and prevents evaporation of tears.
(iii) Meibomian glands: It is located in the corner of eyeball. The oily substance secreted by these glands lubricates the cornea and prevents evaporation of tears.

The organ through which the external world is perceived is called the sensory organ. Eyes, ears, nose, tongue and skin are human sensory organs. They are organs of vision, hearing, smell, taste and balance. The eyes, ears, nose, tongue and skin are collectively called the five senses.

Arises from the medulla oblongata and innervates the muscles of the tongue and throat. It is the optic nerve. It controls the movement of the tongue.

Arises from the medulla oblongata and extends to the muscles of the pharynx, larynx and larynx. It is the optic nerve. It controls the movement of the muscles of the larynx and pharynx.

It arises from the medulla oblongata and divides into four branches. It is mixed nerves.
(i) Laryngeal: It enters the larynx.
(ii) Cardiac: It extends to the heart.
(iii) Gastric : Carries nerves in the stomach.
(iv) Pulmonary: Extends to the lungs.

Arises from the medulla oblongata and enters the tongue. It is mixed nerves. It helps in the circulation of taste, tongue and pharynx.

It arises from the medulla oblongata and enters the inner ear. It carries sensory nerves and sensations of hearing and balance to the brain.

It penetrates the eardrum, lower jaw and mucous membrane of the mouth. It controls the conduction of mixed nerves and muscles of taste and throat.