The main cavity of the eye, between the lens and retina, is filled with a jelly-like fluid – the vitreous humour. For this reason, the rear part of the eye is sometimes called the vitreous body.

The transparent lens is kept flattened by the normal tension of the eyeball, transmitted through the ligaments that hold the lens in position (the suspensory ligaments).

The shape of the lens is altered by the ciliary muscle, which is contained in the ciliary body. When the muscle contracts, the lens bulges. Light from near objects is then focused on the retina.

On the other hand, when the ciliary muscle relaxes, the lens flattens. Light from distant objects is then focused on the retina.