What is the appearance of the Optic Nerve Head in Glaucoma?
In the normal state, the optic nerve head looks much like a doughnut, with the outer ring consisting of the nerve fibres. The hole (called the optic cup) is the space which remains after the nerve fibres turn to fan out onto the retina.

In glaucoma, the nerve fibres are damaged and erode away, leaving a larger cup (or hole of the doughnut) (Figure 3).