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Our topic for today is Human Eye. The human eye is one of the most valuable sense organs that enables us to see the world around us. Let us study about the different parts of an eye. Sclera is a tough outer coat that protects the entire eyeball. Choroid is a vascular layer of the eye. It contains blood vessels that nourish the inner parts of the eye. Cornea is the front portion of the eye. It is convex in shape and bulges outside. Light enters the eye through the cornea. Iris is a dark muscular diaphragm located just behind the cornea. It regulates the amount of light entering the eye by adjusting the size of the pupil. Pupil is a hole in the middle of the iris. Size of the pupil decreases or increases depending upon the intensity of light entering the eye. It is made up of a transparent jelly like substance, which is made up of proteins. It is held in position by the ciliary muscles and suspensory ligaments (ciliary zonules). The ciliary muscles and suspensory ligaments adjust the focal length of the eye so that we can see distant and nearby objects clearly. The ability of the eye to focus on distant and nearby objects by changing the focal length is called accommodation. Aqueous humor is a viscous liquid filled in the area between the cornea and eye lens. It prevents the eye from collapsing due to changes in the atmospheric pressure. Retina is a delicate membrane having a large number of light sensitive cells. The light sensitive cells get activated upon illumination and generate electrical signals. The light sensitive cells are are of two types. Rods: Cells that respond to the intensity of light. Cones: Cells that respond to the color of objects. An inverted real image of the object is formed on the retina. The electrical signals generated by the cells are sent to the brain through the optic nerve. The brain reassembles the signals into a meaningful image and gives rise to the sensation of vision. Vitreous humor is a jelly like substance filled in the space between the retina and the eye lens. It helps in maintaining the shape of the eye. Blind spot is a small region in the retina where the optic nerve enters the eye. It is insensitive to light. Hence, it is called the blind spot. Macula is a tiny area in the retina which is responsible for clear and sharp vision. Fovea centralis is a small area inside the macula. Here, maximum number of cones are present.
