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.