| Lens | |
|---|---|
&sq=Qlik&lang=&file=File:Eye_and_ray_path3.svg) Lens of eye changing shape to focus near and far. | |
&sq=Qlik&lang=&file=File:Schematic_diagram_of_the_human_eye_en.svg) Schematic diagram of the human eye. | |
| Details | |
| Part of | Eyeball |
| System | Visual system |
| Function | Refract light |
| Identifiers | |
| Latin | lens crystallin |
| MeSH | D007908 |
| TA98 | A15.2.05.001 |
| TA2 | 6798 |
| FMA | 58241 |
| Anatomical terminology | |
The lens, or crystalline lens, is a transparent biconvex structure in most land vertebrate eyes. Relatively long, thin fiber cells make up the majority of the lens. These cells vary in architecture and are arranged in concentric layers. New layers of cells are recruited from a thin epithelium at the front of the lens, just below the basement membrane surrounding the lens. As a result the vertebrate lens grows throughout life. The surrounding lens membrane referred to as the lens capsule also grows in a systematic way, ensuring the lens maintains an optically suitable shape in concert with the underlying fiber cells. Thousands of suspensory ligaments are embedded into the capsule at its largest diameter which suspend the lens within the eye. Most of these lens structures are derived from the epithelium of the embryo before birth.
Along with the cornea, aqueous, and vitreous humours, the lens refracts light, focusing it onto the retina. In many land animals the shape of the lens can be altered, effectively changing the focal length of the eye, enabling them to focus on objects at various distances. This adjustment of the lens is known as accommodation (see also below). In many fully aquatic vertebrates, such as fish, other methods of accommodation are used, such as changing the lens's position relative to the retina rather than changing the shape of the lens. Accommodation is analogous to the focusing of a photographic camera via changing its lenses. In land vertebrates the lens is flatter on its anterior side than on its posterior side, while in fish the lens is often close to spherical.
Accommodation in humans is well studied and allows artificial means of supplementing our focus, such as glasses, for correction of sight as we age. The refractive power of a younger human lens in its natural environment is approximately 18 dioptres, roughly one-third of the eye's total power of about 60 dioptres. By 25 years of age the ability of the lens to alter the light path has reduced to 10 dioptres and accommodation continues to decline with age.