The Normal Eye - Crystaline Lens
Lens
The lens is a thick retractile body bathed on one side by the aqueous humour and supported on the other side by the vitreous. The entire structure is maintained within a lens capsule. Normally the lens is transparent and transmits light onto the photosensitive retina and it can vary (or accommodate) the focus of that light by the action of ciliary muscle contraction or relaxation.
The lens is suspended by suspensory ligaments anchored at the ciliary body. When the ciliary muscle relaxes, the ligaments are taut and stretch the lens into an ellipsoidal shape. Ciliary muscle contraction via parasympathetic stimulation causes relaxation of the suspensory ligaments and induces a more spherical shape of the lens and focuses near objects to the retina. The prolonged contraction of the ciliary muscles can cause fatigue and this can be illustrated by tiredness of our eyes when we read for prolonged periods.
As the lens increases in weight and thickness with age it becomes less elastic and this is accompanied by a loss of near vision (presbyopia). Also with ageing the crystalline proteins may form aggregates resulting in altered solubility of these proteins. This can result in formation of lens opacity or cataract.