Lens (eye)

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Average human eyeball:
1. sclera (sclera)
2. choroid ( choroid )
3. Schlemm's canal ( sinus venosus sclerae / venous plexus sclera )
4. orris root ( Radix iris )
5. cornea ( cornea )
6. iris ( Iris )
7. Pupil ( Pupilla )
8. Anterior chamber of the eye ( Camera anterior bulbi )
9. Posterior chamber of the eye ( Camera posterior bulbi )
10. Ciliary body ( Corpus ciliare )
11. Lens ( Lens )
12. Vitreous body ( Corpus vitreum )
13. Retina ( Retina )
Optic nerve ( Nervus opticus )
15. Zonula fibers (
Fibrae zonulares )
Outer eye skin ( Tunica externa bulbi ): 1. + 5.
Middle eye skin ( Tunica media bulbi / uvea ): 2. + 6. + 10.
Inner eye skin ( Tunica interna bulbi ): 13.

The lens ( gr. Phakos ) of the eye is a clear, transparent and elastic body, which is convexly curved both on the front and on the back - here more strongly . It is known in technical terms as lens crystallina and, as a converging lens, focuses the light entering through the pupil on the back of the eye so that a sharp image can be created on the retina . In ophthalmology , the presence of the natural lens is called phakia .


The lens is of ectodermal origin. It develops from a lens vesicle, which consists of a capsule and underlying immature cells , but is otherwise hollow. As the process progresses, the cells of the posterior wall take on an elongated shape and develop into lens fibers that eventually fill the lens vesicle. This process is called primary lens fiber differentiation and leads to the formation of the embryonic lens nucleus.


In addition to the capsule, the mature lens itself consists of the lens cortex and a lens nucleus. An epithelium is just under the anterior capsule of the lens. In the equatorial region under the lens capsule, new cells are formed throughout life through cell division in a growth zone ( zona germinativa ) . These transform themselves into elongated fibers in the further course. In doing so, they produce and enrich transparent proteins , the crystallines , and then lose their cell organelles. The newly formed fibers finally lie on the outside of the older fibers, similar to the skins of an onion. The bark shrinks with age in favor of the core. This happens through water loss. Since the nucleus, unlike the cortex, is hardly elastic, the originally very elastic lens becomes more and more rigid. To a correspondingly stronger lens curvature for near vision is finally often no longer capable ( presbyopia ), which wearing a pair of reading glasses required.

Although the lens is a very metabolically active organ, it has neither nerves nor blood vessels , but is only supplied by the nutrients , electrolytes and other substances contained in the aqueous humor . This guarantees the crystal clear transparency of the eye lens.

The lens is suspended from the ciliary body by the zonular fibers ( Fibrae zonulares ) radiating laterally into the equator of the lens capsule .

Monofocal and multifocal lenses

The cat's pupils narrow to slits when it is bright

In the course of evolution , two different types of lenses emerged in vertebrates : monofocal lenses and multifocal lenses. At the same time, round or slit-shaped pupils developed, depending on the respective lens type , with the aim of optimally complementing the specific optical properties of the respective lens type. Slit-shaped pupils only occur in animals with multifocal lenses, e.g. B. in cats , geckos or some snakes . Species with round pupils (e.g. humans , dogs ) in turn have monofocal lenses.

Multifocal lenses focus light of different wavelengths through different concentric (ring-shaped) zones. This creates a sharper image than eyes with round pupils, whose lenses focus incident light on a single point in the center. In the case of a multifocal lens, as a round pupil contracted, it would gradually cover the outer annular regions of the lens, each of which is optimized for focusing a specific wavelength of light. A slit-shaped pupil, on the other hand, always leaves sections of the outer rings of the lens free, so that the different wavelengths are bundled even with a narrow pupil.


For accommodation (near and far setting ) the refractive power of the lens - it is normally around 19 D in the setting for distance vision in the healthy human eye - can be increased to around 33 D, depending on age, by reducing the radius of curvature. This is done by the ciliary body , the muscle ( musculus ciliaris , ciliary muscle) regulates the degree of curvature of the lens. The contraction of this smooth muscle leads to a relaxation of the zonular fibers, whereby the lens takes on a more curved, spherical shape due to the inherent elasticity of the lens fibers and thus focuses on close focus. When the muscle relaxes, the pulling of the zonular fibers leads to a flattening of the lens and thus an adjustment to distant vision. The elasticity of the lens decreases with age and ultimately leads to presbyopia .

Transparency of the lens of the eye

The lens of the eye consists of cells that, due to various factors, ensure transparency without color distortion (in the healthy eye):

  • no organelles and no nucleus
  • regular and close alignment of the fibers, which are hexagonal in cross-section
  • Production of transparent proteins (crystalline)
  • low water content
  • practically the same refractive index of the cell membranes as the cytoplasm inside the cells of the eye lens

The transparency of the lens of the eye is an example of extreme specialization through biological evolution , since on the one hand the cells of the eye lens can live despite the lack of organelles and without a nucleus , and on the other hand this lack makes transparency possible. Mechanisms of programmed cell death ( apoptosis ) seem to play a role in the formation of these special cells .

Diseases and changes in the lens

Classification according to ICD-10
Z96.1 Pseudophakia
ICD-10 online (WHO version 2019)
Lens opacity

A clouding of the lens due to various causes is known as cataract or cataract . Although this disease is successfully treated in large numbers in industrialized countries by surgery and the implantation of intraocular artificial lenses , it is considered to be the most common cause of blindness worldwide. Lens cancer does not occur.

The lack of a natural or an artificial lens is called aphakia in medicine . The condition after removal or loss of the natural lens and its replacement by an implant is called pseudophakia .

If the lens moves from its original position into the anterior chamber or the vitreous humor of the eye, it is called lens dislocation .

The deviation of the lens surface from a purely spherical shape can lead to lens astigmatism .

See also


Individual evidence

  1. Tim Malmström, Ronald HH Kröger: Pupil shapes and lens optics in the eyes of terrestrial vertebrates. In: The Journal of Experimental Biology. Vol. 209, 2005, ISSN  0022-0949 , pp. 18-25, doi : 10.1242 / jeb.01959 .