The Parts of Human Eye | Structure And Function of Eyeball

The Parts of Human Eye are the organs that enable us to see the objects around us. Eyes are, therefore, also called photoreceptor organs. Eye functions like a camera and its structure can also be compared with a good quality camera.

Structure of Human Eye

Eye is a hollow and spherical ball like structure with diameter about 2.3 cm. We are able to see only outer 1/4th part of the ball. The eye consists of following parts :

Cornea

The outer part of the eye ball is transparent and slightly bulged out. It is called cornea. It is a thin membranous structure, through which light enters the eye ball. Most of the refraction for the light rays
entering the eye occurs at the outer surface of the cornea.

Lens

Behind the cornea, (inside the eye ball) there is a transparent biconvex lens of very short focal
length. The lens is held in the centre of the eye ball with the help of ciliary muscles. The lens is made up of crystalline protein. It is used to focus the light rays (from cornea) on the retina (which is the screen of the eye). Lens provides the finer adjustment of focal length required to focus objects at different distances on the retina.

Ciliary Muscles

Ciliary muscles keep the lens in proper position. These muscles can slightly change the convexity (i.e.,
focal length) of the lens for the purpose of accommodation. Weakening of ciliary muscles leads to poor
accommodation. This is called Presbyopia.

Iris and Pupil

Behind the cornea but before the lens (i.e., between cornea and lens), there is a dark muscular
diaphragm. It is called Iris. Iris is made up of contractile muscle girdle. In the centre of iris, there is a small aperture for the entry of light. This aperture is called pupil. The size of the pupil is variable. It is controlled by the muscles of iris. The pupil size determines the amount of light entering the eye.

Retina

Retina is a light sensitive layer. It consists of two types of sensory cells : rods and cones. Rods help to see the objects in dim and diffused light. Cones are helpful in coloured and contrast vision. The point of the retina which is in line with the optical axis of the lens is called fovea (yellow spot). It is the point of best and sharpest vision. Below this point, there is another point, called blind spot. From blind spot arises the optic nerve.

Optic Nerve

It is a thick sensory nerve, that arises from the retina and goes upto the brain. So, retina sends the
impulses to the brain through optic nerve.

Function of Human Eye

• When light strikes the eye ball, it is refracted first by the cornea.
• It then enters the eye ball through pupil.
• The iris muscles guard the size of the pupil and regulate the amount of light that enters the eye.
• The entering light now passes through the lens. The lens focuses the light rays on the retina.
• The eye lens is a biconvex lens and forms a real and inverted image on the retina (as shown in following figure).
• When light rays fall on retina, the light-sensitive cells (rods and cones) get activated and generate
• electrical impulses (signals).
• These signals are sent to the brain via the optic nerves. The brain interprets these signals (make the image erect) and finally processes the information.

Why do We Need Two Eyes ?

Our two eyes are positioned on the front of our heads, It causes an overlapping of their field of vision and gives us an important property, called stereopsis or 3D vision. Our eyes are separated by a few centimeters, so each eye forms a slightly different image. Our brain combines the two images into one, using the extra information to tell us how close or far away things are. So, we are able to judge the depth, height of the object. This property is called stereopsis or 3D vision.

Each eye has its own field of vision. The area or field in which the fields of vision of two eyes overlap, a stereoscopic, or 3D vision is felt. 3D refers to three directions of measurements-length, width and
height/depth. So, a 3D vision helps us to judge the objects particularly their distance, height, depth etc.,
more precisely. With one eye, we can not have a perfect 3D vision.

So following are the advantages of having two eyes:

• It gives a wider field of view. A human being has a horizontal field of view of about 150° with one
• eye and of about 180° with two eyes.
• The ability to detect faint objects is enhanced.
• Better judgment of height/depth and distance of objects.