Question

Describe the structure of the human eye.

Answer 1

Hint: Our sense organs- the ear, eye, nose, tongue, and skin help us to take in information from our external environment. Our brain computes this information so that we can navigate the outside world. Our eyes not only help us see objects but also perceive various colours. It acts like a camera that collects these images, converts them into nerve impulses that are then read by the brain.

Complete answer:
The eyeball has a somewhat spherical shape. The human eye can be divided into an anterior segment and a posterior segment. The region between the cornea and the lens is called the anterior segment. The region between the lens and the retina is a part of the posterior segment. The anterior segment is filled with aqueous humour. The aqueous humour nourishes this region and also removes the waste products. The region between the lens and the retina is filled with vitreous humour. It is the vitreous humour that helps keep the eye shape.
Three layers, fibrous tunic, vascular tunic, and retina make up the walls of the eyeball. The cornea at the front and the posterior sclera makes up the fibrous tunic layer. The pigmented iris is covered by the cornea. The cornea helps focus light so that it can pass through the lens and fall on the retina. Connective tissues make up the sclera and it functions as a protective layer.
The second layer i.e. the vascular tunic also called ‘uvea’, consists of the posterior choroid, ciliary body (thickened part of the vascular tunic), and iris. The choroid is pigmented with melanin. This region helps with the nourishment of the eyeball as it is rich in blood supply. The ciliary body keeps a check on the eye pressure and the ligaments from the ciliary body hold the lens. Iris helps dilate or constrict the pupil (central part of the iris) thereby keeping a check on the amount of light that enters the eye. The biconcave lens focuses the light on the retina.
The final layer is the retina, which has a neural part and a pigmented part. The rods and cones (photoreceptors) are present in this layer. They absorb the light that falls on the retina, get activated, and produce an action potential. The electromagnetic wave (light) is converted to a nerve impulse that is then carried by the optic nerves to the brain.

Note: Rods are very light sensitive. Due to an injury, vitamin deficiency (vitamin A), etc. rods are unable to absorb light. This causes night blindness. Visual impairment caused by refractive error (inability to focus light on the retina) is the most common eye problem. Refractive error includes myopia (short-sightedness), hypermetropia (far-sightedness), astigmatism, etc.