What are eyes made of?

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.

Why do we have two eyes?

Two eyes - we have two eyes placed in the front of our heads because we need to be able to judge distances and to see in depth. The change in the position of the eyes from the side of the head in our remote ancestors probably came about because they needed to be able to judge distances accurately as they swung from branch to branch in the trees. With both eyes in front, their separate fields of vision overlap. We see to images superimposed one upon the other but, because of the space between our eyes, the image from each goes a little way around its own side of the object. This is called stereoscopic vision, or vision in depth, which we share with apes and monkeys. Most other animals and fish do not have this advantage. To them the world appears flat. One exception is the owl, who sees better than any of us, and has not only stereoscopic vision, but telescopic vision, too. Our judgement of distance depends, with near objects, upon our stereoscopic vision. As the distance increases, there is less difference between the left-eyed and the right-eyed view. So we depend upon other factors as well. Experience tells us that the farther away an object is, the smaller it looks. Its colour also changes, its details disappear, its outline softens. Nearer objects give us a measure against which to judge the distance of farther ones. Then there is perspective, the familiar illusion that parallel lines converge towards the horizon.