The various organs which are connected to the human body with the external world are called sense organs. The eye is one of the important organs of the human body. Itis located in the deep socket of the skull called an orbit. It lies inside the lacrimal bones. Itis made up of three layers. They are sclera, choroid and retina.:

Sclera

It is the outermost layer of the human eye which is white, strong and made up of connective tissues. Its front part is transparent and is called the cornea. .and in the back of the eye with the external sheath of the optic, The cornea is covered by an extra layer called the conjunctiva. It gives a shape to the human eye and also protects the inner parts.

Choroid

It is the second layer of the eve consisting of connective tissue richly supplied with blood vessels. As it is a black layer, absorbs radiations. and prevents reflection. It provides food and oxygen to the eye. The choroid is an integral part of the nutrient and oxygen exchange with the outer third of the retina, retinal pigment epithelium, a portion of the optic nerve, and it acts as the sole source of metabolic exchange for the fovea.

Retina

It is the third and innermost layer of the human eye. The image of an object is formed on this layer. There are light-sensitive cells like rods and cones present in this layer. Rhodopsin present in the rod cells help to see in dim light and iodopsin found in the cones help to see the things in bright light and help to distinguish the colour. Colour blindness and the defect of vision are heredítary, Likewise, night blindness is another disease of the human eye. A person suffering from this disease cannot see clearly at night.

These are other important parts of eyes which can be explained as below:

Cornea: It is the front and transparent layer of the sclera. It lies directly in front of the iris and pupil, and it allows light to enter the eye.

Iris: It is present in between eye lens and cornea. It is a colourful part of the human eye. It allows only essential rays to enter inside it controlling the
light rays that fall upon the eyes.

Pupil: A small aperture at the centre of the iris is called the pupil. The size of the pupil, small or large, is determined by iris muscle. The pupil becomes smaller on viewing bright light or near objects but it becomes larger on viewing distant objects. It passes the light rays inside.

Lens: An object which is transparent, biconvex, crystalline present behind the iris is a lens. It helps to form real, inverted and diminished image by refracting the light rays on the retina.

Ciliary Muscles: Eye lens becomes thin and thick by the contraction and expansion of ciliary muscles or its focal length changes, ciliary muscle contracts while looking at the distant objects and the lens becomes thinner but it expands while looking at the nearby objects and lens becomes thicker.

Optic nerves: This nerve is connected to the brain. It links the image-formation of an object at the retina to the brain and makes the image upright or erect. The optic nerve, also known as cranial nerve II. 

Aqueous Humour: Aqueous humour is the fluid produced by the eye. The watery fluid present in the chamber between cornea and lens is called aqueous humour. It provides nutrition to the eye, as well as maintains the eye in a pressurized state.

Vitreous Humour: The chamber in-between lens and retina is filled with thick jelly substances called the vitreous humour. It maintains the round shape of the eye and helps in the refraction of light.

Functioning of Eye

Eye lens focuses the light rays coming from any object at the retina to form the image. The image is real and inverted. The image formed in this way passes to the brain through the optic nerve. In the brain, the upright or erect image is formed and see the object.

Care and hygiene of the human eye

It is essential to care for our eyes as they are very important organs. To care the eyes, we should take vitamin A containing yellow fruits and green vegetables. We should avoid reading in very bright and dim light. We should always wash eyes with clean water and the medicine should not be used unnecessarily in the eyes. Similarly, the care and protection of eyes can be done with-out rubbing eyes with hands, consulting an eye specialist
time to time, using glass and other eye-protecting materials while working in the factories and not playing dangerous games like 'Dandibiyo', etc.