Outside the Bishopsgate development near Liverpool Street Station in London is an abstract sculpture by Bruce McLean called ‘Eye-I’ (opposite). I don't know why Mr McLean chose this title but I misread it when I first saw it and wondered where ‘Eye–2’ might be! At least for me, the title of this dramatic, bold and colourful sculpture is an appropriate heading for this opening chapter. The chapter will deal with the structure and function of the human eye.

The External eye

The eye we see

Take a look at your eyes in the mirror and also refer to the first illustration (Figure 1.1). In painting classes, when drawing the face, we are told that the eyes come half-way down the head and that most of us when we first draw tend to make the error of positioning the eyes far too high.

The clear part at the front of the eye is the cornea and the white of the eye is the sclera. The coloured tissue behind the cornea and inside the eye is the iris and the dark opening in the centre is the pupil, which is wide in dim light and constricted in bright daylight. Not all of the iris circle can be seen when you look in the mirror because a segment is always covered by the open upper eyelid.

The eye we don't see

Pull back your eyelid gently and the hidden segment of iris can be seen. Look carefully and a transition region between the clear cornea and the white sclera may be evident. This is called the limbus. When the lid is pulled back, you will be able to see a large number of red blood vessels. These become even more obvious if there is dust or smoke in the eye, which then becomes red. Most of the vessels that can be seen are in a thin membrane of tissue on the surface of the sclera called the conjunctiva. The conjunctiva extends back and then over the inside of the upper lid. These blood vessels do not extend on to the clear cornea in a healthy eye. They are prevented from so doing by a number of biological and mechanical processes, because if vessels grew over the cornea, vision would be adversely affected.