The human eye is remarkable. Even though it is only about 2.5cm in diameter, it serves a very important function: your sense of sight. Sight is arguably the most used of our 5 senses as we use our eyes in virtually everything we do. The eye allows us to see and interpret the shapes, colours, and dimensions of the world by processing the light they reflect or give off. It changes light rays into electrical signals, then sends them to the brain which interprets these electrical signals as visual images.
The eye's individual components work in a manner similar to a camera. Each part plays a vital role in providing clear vision. Light rays enter the eye through a transparent layer of tissue known as the cornea. As the eye's main focusing element, the cornea takes widely diverging rays of light and bends them through the pupil, the dark, round opening in the centre of the coloured iris. The eye accommodates changing light conditions by changing the size of the pupil.
The light rays then pass through the lens of the eye which completes refraction by making delicate adjustments in the path of the light rays in order to bring the light into focus upon the retina. The eye focuses light rays originating from various distances by changing the shape of its lens. The retina's photoreceptor nerve cells then change the light rays into electrical impulses and send them through the optic nerve to the brain where an image is perceived. A protective cavity in the skull, called the orbit or socket, houses the eye. The eye-socket is surrounded by layers of fatty tissue which protect the eye and enable it to turn easily with six muscles regulating the motion of the eye.
The cornea is the transparent, outer "window" of the eye and is the main focusing element when light enters the eye. The cornea is made up of 5 layers of tissue with the outer layer's (the epithelium) main job being to protect the eye. The epithelium is made up of transparent cells that have the ability to regenerate quickly, usually within 3 days to provide rapid healing to minor injuries. The inner layers of the cornea are also made up of transparent tissue, which allows light to pass and predominantly provide strength to the eye.
The black circle in the centre of the eye, surrounded by the coloured iris. The pupil controls how much light enters the eye. When you are in a bright environment, the pupil becomes smaller to allow less light through. When it is dark, the pupil expands to allow more light to reach the back of the eye.
This is the part of the eye which is responsible for one's eye colour. The main function of the iris is to control the size of the pupil by dilating or constricting its muscles, thereby allowing more or less light into the eye.
The lens is the part of the eye immediately behind the iris that performs fine tuning of light rays upon the retina for focusing. The lens performs this function by altering its shape to become thinner or thicker as necessary. In general, for people under 40, the lens is soft and pliable allowing for fine focusing from a wide variety of distances. For individuals over 40, the lens begins to become less pliable, making focusing upon objects near to the eye more difficult. This is known as presbyopia. As people reach their 60’s or 70’s, the lens sometimes becomes cloudy and hard, preventing light from entering the eye. This condition is known as cataracts.
The retina is the membrane containing photoreceptor nerve cells that line the inside back wall of the eye. It consists of fine nerve tissue which lines the inside wall of the eyes and acts like the film in a camera. Its primary function is to transmit images to the brain via the optic nerve. When your vision is perfect, the light rays coming into your eye focus precisely on this part of the eye.
The optic nerve takes information from the retina as electrical signals and delivers it to the brain where this information is interpreted as a visual image. The optic nerve consists of a bundle of about one million nerve fibres.
The white part of your eye that you see when you look at yourself in the mirror is the front part of the sclera. However, just like an eggshell surrounds an egg and gives an egg its shape, the sclera, a tough, leather-like tissue, also extends around the eye and gives the eye its shape.
The vitreous cavity is located behind the lens and in front of the retina and is filled with a clear gel-like fluid called the vitreous humour. The gel’s purpose is to provide a spherical shape to the eye. The vitreous may develop small clumps known as “floaters,” which are more common in shortsighted people.
