Describe the defects that affect the mammalian eye and how they could be corrected

Short-sightedness (myopia); a condition where light rays from a distant object are focused in front of the retina; while those from a near object are clearly focused on the retina; it is caused by an abnormally elongated eyeball; or too much refractive power of the eye lens; it is corrected by wearing concave/diverging lenses; which help to diverge light rays; or reduce the refractive power of the eye before they reach the eye lens; Long-sightedness (Hypermetropia); light rays from a near object are not focused by the time they reach the retina; or may be focused behind the retina; while the rays from a distant object are sharply focused; the defect is caused by an eyeball that is too short; or a weak lens system (distance between lens and the retina is short); corrected by wearing a convex/converging lens; which refracts light rays before reaching the eye lens; this enhances refraction resulting in rays being sharply focused onto the retina; Astigmatism; rays from an object are brought to focus on different planes; due to unequal curvature of the cornea/lens; causing unequal refraction of light entering the eye; this defect is corrected by wearing special cylindrical lens in front of the eye; the lens corrects the focus in the defective planes; Colour-blindness; a genetic defect; in which an animal is unable to distinguish between colours particularly within the red-green spectrum; the retina lacks cones; pigments that respond to colour vision; Squintedness; an eye defect in which extrinsic muscles of the eye; that controls the turning of the eyeball do not co-ordinate accordingly on stimulation; it affects the paired rectus muscles that move the eyeball up and down; and the lateral rectus muscles that move the eyeball left to right; the eyeballs therefore face different directions; making focusing and accommodation difficult to achieve; corrected by specialized surgery; Old sight (Presbyopia); caused by old age; when supplies of nutrients and oxygen to the lens is far much reduced; hence the cells of the lens die; the lens’ elasticity is reduced; and hence cannot change shape; and becomes fixed into a shape that is not suitable for distant vision; managed by use of ‘reading glasses’ that have converging lenses; to give the eyes an extra power to manage close work; Cataracts; associated with old age; but may also be caused by an eye injury due to a blow;  or complications of diabetes mellitus; the eye lens become cloudy; blocking transmission of light rays; protein fibres become denatured; and clump together making the lens opaque; corrected by surgery; to replace the defective lens with a normal one from a donor; or use of artificial lens;