Eyes - the outer shell of the eyeball is made up of three coats: the fibrous tunic, the uveal tunic and the retina. The fibrous tunic is tough and elastic. It is opaque at the back, where it is called the sclera, and transparent at the front where it is known as the cornea. The uveal tunic is so called because of its similarity to a grape (Latin: uva). It contains the choroid membrane which supplies blood to the eye. The retina, or innermost coat, is an extremely delicate tissue and is only a fraction of a millimetre thick. Its inner layers carry blood from the central artery to the central vein. At the centre of the eye is a circular opening, the pupil, and immediately behind it is the crystalline lens, which consists of a great many transparent fibres arranged in sheets like the layers of an onion and enclosed in a clear, elastic membrane, the capsule. The perfectly smooth surface of the lens is curved and it is suspended by a number of delicate threads.
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