Lens (eye) – structure, function & diseases
Lens (eye)
The lens is a crystal-clear part of the human eye and is located in the eyeball ( bulbus oculi ) directly in front of the vitreous body. It is convexly curved on both sides (biconvex) and thus acts as a converging lens. It has the task of bundling incoming light in such a way that a sharp image is formed on the back of the vitreous body on the retina in the zone of sharpest vision ( fovea centralis ).
What is the lens
Since the ciliary muscle, which surrounds the lens capsule like a ring, works like a sphincter, the zonula fibers for near accommodation can only relax if the ciliary muscle contracts concentrically and vice versa.
When the ciliary muscle is tense, the diameter of the ciliary body decreases, so the zonule fibers become “loose” and vice versa. This process of accommodation takes place unconsciously. From the perspective of the ciliary muscle, near accommodation is an active state and far accommodation is a passive (relaxed) state.
Anatomy & Structure
The back of the lens rests against the front of the vitreous body and its front side, together with the iris , closes off the anterior chamber of the eye. Around the equator of the lens capsule, zonule fibers protrude in a star shape like spokes from a wheel hub. The other end of the fibers is connected to the ciliary or radiating body, which is part of the eye’s choroid as a ring-shaped bulge around the lens.Embedded within the ciliary body is the ciliary muscle which, when contracted, causes the inner diameter of the ciliary body to narrow. The lens itself consists of the lens nucleus, the lens cortex and the lens capsule. The lens consists of around 60% crystalline proteins, which are highly stable and largely UV-resistant.
A high proportion of vitamin C and enzymes that reduce oxidative stress largely prevent clouding caused by UV damage. The epithelium at the equator of the capsule produces lens fibers throughout life, which attach themselves to the old fibers with loss of organelles, so that the lens enlarges and becomes less elastic over the course of life. The veinless and nerveless lens is supplied by the aqueous humor, which is formed in the ciliary body.
Function & Tasks
The lens has the task of focusing incoming light in such a way that a sharp image is created on the retina at the point of sharpest vision, the fovea centralis. In order to achieve a sharp image at changing distances, either the distance from the lens to the retina would have to be variable (e.g. telescope) or the focal length of the lens itself would have to be variable.
Unlike in fish and reptiles, evolution in humans and in all vertebrates has opted for the latter variant and created a way of making the focal length variable within certain limits. In a mechanical secondary function, the lens, together with the iris, fulfills the task of separating the anterior from the posterior chamber of the eye, so that the aqueous humor cannot flow unhindered from the posterior to the anterior chamber and vice versa.
Diseases & Ailments
The most common lens dysfunction is clouding of the lens. Another malfunction can be caused by a mechanical displacement of the lens, a luxation. A clouding of the lens, known as a cataract or cataract, can have various causes.The most common manifestation is the senile cataract, which only occurs at an advanced age. An inherited genetic disposition plays a role in many cases. External factors that can favor the formation of a cataract are, for example, years of exposure of the unprotected eyes to UV-rich sunlight at sea, in the 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 become infected with rubella or mumps around the third month of pregnancy , there is a risk that the newborn will develop cataracts.
The disease initially manifests itself through difficulties with accommodation, later through increased sensitivity to glare and, in a more advanced stage, through clouding of vision (grey haze). From the outside, the disease can be recognized by the gray coloration of the pupil.
Another malfunction of the lens can occur when the lens capsule is damaged in such a way that aqueous humor penetrates the lens and causes the lens cortex to swell, which leads to accommodation problems and can cause further damage in the medium term. Dislocation of the lens can result from trauma or from zonular fiber lesions .
A tumor in the ciliary body can be the culprit, or inherited genetic defects can lead to dysfunction of the zonular fibers. A complete dislocation occurs when the lens either slides completely into the anterior chamber of the eye, i.e. in front of the iris, or completely dips into the vitreous body. Incomplete dislocations can sometimes remain symptom-free. More severe dislocations can result in monoocular double vision, which persists when the other eye is closed or covered.
Hello! I am Lisa Newlon, and I am a medical writer and researcher with over 10 years of experience in the healthcare industry. I have a Master’s degree in Medicine, and my deep understanding of medical terminology, practices, and procedures has made me a trusted source of information in the medical world.