Human eye structure and function with diagram.

HUMAN EYE

structure and function.

human eye

• Eye 

Human eyes are located in small portions of the skull known as the orbits or eye sockets. 

Eyelids wipe eyes and prevent dehydration. 

They spread tears on eyes, which contain substances for fighting bacterial infections.

 Eyelashes prevent fine particles from entering the eye.

 The structure of the eye can be divided into three main layers.

 

The outer layer of the eyeball.

The outer layer of the eyeball consists of sclera and cornea. The sclera gives the eye most of its white colour. It consists of dense connective tissue and protects the inner components of the eye and maintains its shape. 

In the front, the sclera forms the transparent cornea. The cornea admits light to the interior of the eye and bends light rays so that they can be brought to focus.

The middle layer.

The middle layer is called the choroid. It contains blood vessels and gives the inner eye a dark colour. The dark colour prevents disruptive reflections within the eye. Behind the cornea, the choroid bends to form a muscular ring, called the iris. There is a round hole, called a pupil, in the centre of the iris. After striking the cornea, light passes through the pupil. The size of the pupil is adjusted by the muscles of the iris. The pupil constricts in bright light when the circular muscles of the iris contract. Similarly, the pupil dilates in dim light when the radial muscles of the iris contract.

intensity of light.

Behind the iris, there is a convex lens, which focuses light on the retina. The lens is attached to the ciliary muscles of the eye via a ring of the suspensory ligament. To clearly see an object far away, ciliary muscles are relaxed and the lens becomes less convex. When ciliary muscles contract, the lens becomes more convex and round. The inner layer is sensory and is called the retina. It contains photosensitive cells called rods and cones and associated neurons.

The iris divides the cavity of the eye into two chambers. 

The anterior chamber is in front of the iris i.e. between the cornea and iris; whereas the posterior chamber is between the iris and retina. The anterior chamber contains a clear fluid known as aqueous humour while the posterior chamber contains a jelly-like fluid known as vitreous humour. It helps maintain the shape of the eye and suspends the delicate lens. Light from objects enters the eye and is refracted when it passes through the cornea, aqueous humour, lens and vitreous humour. The lens also focuses light on the retina. As a result, the image falls on the retina. Rods and cones generate nerve impulses in the optic nerve. These impulses are carried to the brain, which makes the sensation of vision. Rods contain a pigment called rhodopsin. When light falls on rhodopsin, it breaks generating a nerve impulse. In the absence of light, the breakdown products are again converted into rhodopsin. The body synthesizes rhodopsin from vitamin A.

 The deficiency of vitamin A causes poor night vision. 

This problem is called night blindness.

Cones also contain a pigment, known as iodopsin. 

There are three main types of cones and each type has a specific iodopsin. Each type of cone recognizes one of the three primary colours i.e. blue, green and red. If any type of cone is not working well, it becomes difficult to recognize that colour. 

Colour blindness.

Such a person is also not able to distinguish different colours. This disease is called colour blindness and it is a genetic problem.

Disorders of the Eye.

The following are the types of eye disorders:

1. Myopia (Short sight)

2. Hypermetropia (Long sight)

1. Myopia (Short sight).

The elongation of the eyeball results in myopia. Such persons are not able to see distant objects clearly

2. Hypermetropia (Long sight).

Hypermetropia (Long sight): It happens when the eyeball shortens. Such persons are not able to see near objects clearly. 

Contributions of Muslims.  

Ali ibn Isa 

ali ibn isa

Contributions of Muslim Scientists Ali ibn Isa (950 - 1012) was a famous Arab scientist. He wrote three books on ophthalmology (study of diseases and surgery of eyes). He described 130 eye diseases and prescribed 143 drugs to treat these diseases.

Ibn al-Haytham 

ibn al haytham.

Ibn al-Haytham (965 - 1039), an Arab scientist, made significant contributions to the principles of eye and vision. He is regarded as the father of optics (study of the behaviour of light). His “Book of Optics” correctly explained and proved the modern theory of vision. He discussed the topics of medicine and eye surgery in his book. He made several improvements to eye surgery and accurately described the process of sight, the structure of the eye, image formation in the eye and the visual system. Ibn al-Haytham also described the principles of pinhole cameras.

Parts of Eye:

• Orbit: The eye is located in a bony socket within the skull, known as an orbit.Six extraocular muscles are located in the orbit, which attaches to the eye to enable movement. The muscles allow you to look side-to-side, up and down, and rotate the eye. 
• Sclera: The white, visible area of the eyeball is called the sclera. It is durable and acts as a protective layer, covering most of the eyeball surface. The extraocular muscles are attached to the sclera.  
• Conjunctiva: A thin layer of tissue covers most of the front surface of your eye. The conjunctiva also acts as a lining inside of your eyelid. When the eyes look red, it means that the blood vessels within the conjunctiva are bigger than normal.
• Cornea: The front of the eye holds the cornea, which is like a glass lens that moves light into the eye. This clear part of the eye focuses the light so the image can reach the back of the eye. When you look closely at an eye, the cornea is the clear, bulging surface that forms the shape for the front of the eye.
• Lens: This part of the eye works along with the cornea to focus the light on the retina (located in the back of the eye). The lens can change shape by getting thicker or thinner to optimize the clarity of the picture as it hits the retina.
• Ciliary Muscles: Around the lens, a circle of small muscles works to change the shape of the lens. These muscles relax or squeeze depending on the distance of the object. For example, if you are looking at something up close, then the ciliary muscles make the lens fat by squeezing the lens. The muscles relax to thin the lens when you are focusing on distant objects.
• Retina: The retina is located in the back section of the eye and contains rods and cones, helping you distinguish between colors and black and white. This area of the eye is actually a group of light-sensing cells, collectively referred to as the retina. When the image you are seeing is in the retina, it is turned into an electrical message that is sent to the brain.
• Macula: Within the retina is a small, specialized area known as the macula. This part of the eye helps with central vision so you can see details in the various images.
• Optic Nerve: When the electrical messages are received in the retina, they are sent along the optic nerve to reach the brain.
• Vitreous Humor: In the space between the retina and lens, this area is filled with a transparent, jelly-like substance. It helps to maintain the shape of the eye and protect the structure of the eye.
• Iris: Every person has a unique eye color that can be seen in the middle of the eye. This pigmented area is known as the iris, and the color is inherited genetically. Not only is the iris beautiful, but it is also functional to control the amount of light that can enter into the eye.
• Pupil: In the center of the iris is the pupil, which is where the light enters in. When you are in a dark environment, the pupils get bigger to optimize your vision. In brighter environments, the pupils get smaller to manage the amount of light that comes inside. The diameter of the pupil usually ranges between 3 and 7 mm, depending on the light conditions.
• Eyelids: The skin covering that moves over the eye is important as a protective mechanism. Eyelids block the light when you are sleeping and have an instinctual mechanism to blink, so the eyes stay moisturized.
• Tear Glands: In the upper eyelid, small glands are there to provide the tears that keep the surface of the eyes moist and clean. This moisture is important for protecting your eye from damage.
• Eyelashes and Eyebrows: The lashes and brows around your eye work to block dust and sweat from getting in your eyes.