The lens is a vitreous part of the human eye and is located in the eyeball (bulbus oculi) just in front of the vitreous body. It is convexly curved on both sides (biconvex) and thus acts as a converging lens. Its function is to focus incident light so that a sharp image is formed at the back of the vitreous body on the retina in the zone of sharpest vision (fovea centralis).
What is the lens?
In the human eye, the lens, which is convexly curved on both sides, serves to focus incident light so that a sharp image is formed on the back of the vitreous body on the retina at the point of greatest resolving power (point of sharpest vision, fovea centralis). This is picked up by the color photosensors (mainly M and L cones for green and red) and transmitted to the visual center. The lens can be virtually “flattened” by pulling the zonula fibers at the edge of the capsule, thereby accommodating for distance vision. When the pull of the zonular fibers subsides again, the lens returns to its natural, almost spherical shape, which corresponds to near accommodation. Because the ciliary muscle, which annularly surrounds the lens capsule, works much like a sphincter, the zonular fibers can relax for near accommodation only when the ciliary muscle tenses concentrically and vice versa. When the ciliary muscle tenses, the diameter of the ciliary body decreases, causing the zonular fibers to “relax” and vice versa. This process of accommodation occurs unconsciously. From the perspective of the ciliary muscle, near accommodation is an active state and distance accommodation is a passive (relaxed) state.
Anatomy and structure
The lens rests with its posterior side against the anterior side of the vitreous body and with its anterior side, together with the iris, closes the anterior chamber of the eye. Around the equator of the lens capsule, zonula fibers project in a star shape like spokes from a wheel hub. The other end of the fibers is connected to the ciliary or ray body, which is an annular bead around the lens that is part of the choroid of the eye. Embedded in the ciliary body is the ciliary muscle, which when strained causes the inner diameter of the ciliary body to narrow. The lens itself is composed of the lens nucleus, the lens cortex, and the lens capsule. The lens consists of about 60% of proteins called crystallins, which are highly stable and largely insensitive to UV light. A high content of vitamin C and oxidative stress-degrading enzymes largely prevents clouding caused by UV damage. The epithelium at the equator of the capsule produces lifelong lens fibers, which attach to the old fibers with loss of organelles, so that the lens enlarges and becomes less elastic during life. The vein– and nerve-less lens is supplied by aqueous humor, which is formed in the ciliary body.
Function and tasks
The function of the lens is to focus incident light so that a sharp image is formed on the retina at the point of sharpest vision, the fovea centralis. To achieve a sharp image at varying distances, either the distance from the lens to the retina would have to be variable (example telescope) or the focal length of the lens itself would have to be variable. In humans and in all vertebrates, evolution has chosen the latter option – in contrast to fish and reptiles – and has created a possibility to vary the focal length within certain limits. In a secondary mechanical function, the lens, together with the iris, performs the task of separating the anterior from the posterior chamber of the eye, so that the chamber fluid cannot pass unimpeded from the posterior to the anterior chamber and vice versa.
Diseases and disorders
The most common lens dysfunction is opacification of the lens. Another functional disorder may be caused by mechanical displacement of the lens, or dislocation. Opacification of the lens, called cataract or cataract, can have various causes. The most common manifestation is senile cataract, which occurs at an older age. An inherited genetic disposition plays a role in many cases. External factors that can promote the development of cataracts include years of exposure of unprotected eyes to UV-rich sunlight at sea, in high mountains or in airplanes. Medications such as cortisone, drug use (including alcohol) and diabetes mellitus as well as neurodermatitis can cause the disease.If pregnant women are infected with rubella or mumps around the third month of pregnancy, there is a risk that the newborn will develop cataracts. The disease is initially manifested by difficulties in accommodation, later by an increased sensitivity to glare and, in more advanced stages, by a clouding of vision (cataract). From the outside, the disease can be recognized by the gray coloration of the pupil. Further dysfunction of the lens may occur if the lens capsule is damaged in such a way that aqueous humor enters the lens and causes the lens cortex to swell, which leads to accommodation problems and may cause further damage in the medium term. Dislocation of the lens can be caused by force or as a result of lesions of the zonular fibers. A tumor in the ciliary body may be the culprit, or inherited genetic defects may cause malfunction of the zonular fibers. Complete dislocation occurs when the lens either slips completely into the anterior chamber of the eye, i.e., in front of the iris, or completely submerges into the vitreous body. Incomplete luxations may remain symptom-free. More severe luxations may present with monocular double images that persist when the other eye is closed or occluded.
