Revelation, Rationality, Knowledge & Truth — Page 541
ORGANIC SYSTEMS AND EVOLUTION extending from the inside of the socket to the eyeball. . These are the muscles which move the eye. . The eye (see plate 8) has an almost spherical shape. . The wall of the eyeball consists of three layers: 1. The sclera: the outermost layer made of tough white connective tissue, commonly known as the white of the eye. It bulges and is transparent at its front, forming the cornea. 2. The choroid layer: the middle layer made of a delicate network of connective tissue and richly supplied with blood vessels. This layer completely surrounds the eye except for the pupil which is a small opening at the front of the eye, directly behind the cornea. Around the pupil the choroid layer is pigmented, known as the iris, giving eyes their different colours, either brown, blue, green, hazel or a combination of these. It is the pupil which controls the amount of light entering the eye onto the convex crystalline lens attached to the choroid layer by ciliary muscles. These muscles, when they contract, allow the eye to focus on objects whether they are near or far. The aqueous humor is a watery fluid filling the area between the cornea and the lens and helps to maintain the forward curve of the cornea. . Behind the lens, the entire space is filled with a thicker transparent substance, the vitreous humor, which is necessary to keep the eyeball firm and in its spherical shape. ' 3. The retina: the innermost and perceptive layer less than a millimetre thick. It includes some 10 different layers of cells known as the receptors, ganglia and nerve fibres. 8 The receptors, better referred to as photoreceptors, are of two types: cones and rods. There are about 130 million rod cells for black and white vision and only 7 million cone cells for colour vision in 508