Sense organs and the structure of the eye

The visual organ is the eyeball and its accessory parts: eyelids, eyelashes, eye muscles and tear glands. The eyeball has three coats: the outer fibrous coat (transparent cornea at the front, the rest is the sclera), the middle vascular coat (its front part is the coloured iris with the pupil in its centre) and the inner retina. Muscles in the iris change pupil size: it narrows in strong light and widens in the dark. Behind the pupil is a transparent biconvex lens, whose curvature is changed by ciliary-body muscles. The inside of the eye is filled with vitreous body. The retina has light-sensitive receptors: rods (vision in dim light) and cones (colour and sharp vision). The yellow spot has the most cones; the blind spot has no receptors – the optic nerve leaves there. The image on the retina is small and inverted; the brain turns it right. If the eye is injured: do not rub or touch it and do not try to remove a foreign body yourself; if a chemical gets in, immediately rinse with clean running water for 15 minutes and call 103; after a strong blow or an object in the eye, cover it loosely with a clean cloth without pressing and take the person quickly to a doctor. Do not rub eyes with dirty hands and do not use another person’s towel.