The eye is our window to the world, and a vital organ of vision that responds to light making it possible for us to see the surrounding objects. Our eye allows us to interpret shapes and sizes, identify objects and people, read, appreciate colours, and enjoy the beauty around us. It also allows us to perform a plethora of activities that would be impossible to perform without vision.
The function of a human eye can be compared to that of a camera. A camera uses various lenses to focus on the object that has to be clicked, the aperture adjusts accordingly, and lets a specific amount of light pass through to capture the image on the film. However, this organ of vision is much more complex than a camera, and has many small parts that are interconnected and work in tandem with each other in order to provide vision. It not only allows the identification of shapes, sizes and colours but also allows us to gauge the depth and distance of an object (providing 3D vision), something the camera cannot do.
Parts of the eye and their specific functions
Sclera – Sclera is the white portion that is connected to the cornea. It is an opaque, fibrous outer layer that provides protection and attachment for intrinsic muscles of the eye.
Cornea – The cornea is a round and clear, dome shaped protective structure that covers the front of the eye, protecting it against dirt, germs, and other particles, which may harm the other delicate components. As light enters, it first falls on the cornea, and due to the dome shaped surface, gets refracted. It controls and focuses the entry of light into the eye. The cornea contributes 65-75 % of the eye’s total focusing power.
Pupil – Light, after passing through the cornea, enters the pupil. The pupil is a small aperture in the iris. The amount of light that will enter the eye depends on the size of the pupil. The dilator and sphincter muscles present in the iris control the size of the iris.
Iris – The iris is a thin diaphragm that imparts colour to the eye with the help of microscopic pigment cells called melanin, and the colour, texture, and pattern of every individual’s iris are as unique as a fingerprint. The iris is composed of smooth muscle fibres and connective tissues that relax and contract, controlling the size of the iris and amount of light that falls on the retina.
Lens – The crystalline lens, which lies behind the iris, provides further focus and clarity in vision. This lens is made up of a fibrous material, and can change its shape to fine tune the vision. The ciliary muscles are attached to this lens. These muscles relax and contract thus assisting the lens to change shape and produce an image at the back of the eyeball.
Retina – The retina is a multi-layered sensory tissue that contains millions of photoreceptors (rods and cones) that capture light rays and convert them into electrical impulses. These impulses travel along the optic nerve to the brain where they are turned into images. There are around 120 million rods and about 6 million cones, which detect the intensity and frequency of light.
Optic nerve – The optic nerve is responsible for transmitting the electric impulse from the retina to the brain. The optic nerves separately collect electric impulses from the right and left eye, and this information is received by the visual cortex in the brain, where the image is interpreted and a vision is created.
Protection mechanism of the eyes
The human eye is an extremely delicate organ. It is also an important organ of our body, as it provides vision, and hence, there is an elaborate protective mechanisms to safeguard them. The eyelids along with the eyelashes protect the eyes from any foreign bodies or debris. The lacrimal gland lies in the outer portion of the upper eye. The gland performs the function of continually secreting tears, which lubricate the eye, and flush out any dirt or foreign particles that enter the eye. Tears also contain natural antiseptic properties, and hence play a major role in preventing infections.