Eye Anatomy: The Lens

The eye’s crystalline lens is a biconvex structure (outwardly curved on both sides), and is suspended behind the colored iris. Both are behind the protective clear covering known as the cornea. The cornea is also a lens, meaning that it is clear and refracts (bends) incoming light to focus it on the retina, the eye’s “camera film”.

The lens has two functions in vision:

To allow light an unimpeded path to the retina
To refract light and help focus it clearly on the retina

While the corneal curvature is fixed, the lens can change its curvature. This ability is called accommodation. The lens curvature becomes steeper for near vision and flatter for distance vision, which makes it an adaptable partner for the cornea. In a 20/20 eye, they work together to provide clear vision at all distances.

Three Lens Elements

There are three structural elements that make up the lens:

Lens Fibers -- long protein fibers that are closely packed and parallel. This arrangement keeps the lens transparent. They make up most of the lens volume and are laid out horizontally, front to back, and concentrically like layers of an onion. There is also water in and around these fibers. They contain no elements that would scatter light, such as a cell nucleus or mitochondria.
Lens Epithelium – a layer of cells on the anterior (front) side of the lens which act like a pump. The lens has no blood vessels but receives nourishment from the aqueous fluid [link to Internal Eye Fluids] that flows all around it and carries away its waste products. The lens does not need much oxygen, but receives glucose as its main energy source. The epithelial cells distribute fluid, ions and glucose through the entire lens. They also create new fibers throughout a person’s life.
Lens Capsule – a transparent membrane surrounding the lens like a pocket. It is elastic to allow lens curvature to steepen for near vision. When an artificial lens replaces the natural lens in cataract surgery, it is placed in this same capsule. Please see Intraocular Lenses for more information.

The Ciliary Body

Around the lens is a multi-function structure called the ciliary body. It contains:

Ligaments called zonules that attach the lens to the ciliary body and support it behind the iris
The two ciliary muscles that flatten and steepen the lens curvature for clear vision at varying distances
The ciliary processes, arranged like a black circular frill around the lens, which produce aqueous humor

The ciliary body connects to the choroid, the middle layer of the eyeball wall. The choroid is filled with blood vessels that nourish the retina and other parts of the eye, and it contains a lot of melanin (pigment) to absorb excess light.

For more information, please see Eye Anatomy: Overview.

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