The transparent biconvex structure in the eye is known as the lens. The lens which is along with the cornea helps to refract light that is focused on the retina. As the lens changes in shape, it helps in adjusting the focal distance so as to focus on the objects at different distances. It therefore allows the image to form on the retina.
The other name for the lens is aquula or crystalline lens. The refractive power of the lens in humans is about 18 dioptres which is approximately one-third of the eye’s power.
|Gray's||subject #226 1019|
The lens is divided into three parts- the lens capsule, the lens epithelium and the lens fibers. The lens capsule is the outer layer of the lens and the lens fibers form the bulk of the anterior of the lens. The cells of the epithelium that are between the outer layer of the lens fibers and the lens capsule are only on the anterior side of the lens.
It is a transparent, smooth membrane that surrounds the lens completely. The capsule is stretchable and contains collagen. The lens capsule is synthesized by the lens epithelium and Type IV collagen and sulfated glycosaminoglycans are its main components. Since the capsule is stretchable therefore the lens is more globular shaped when it’s not under the zonular fibers.
The lens epithelium is in the anterior portion of the lens between the lens fibers and lens capsule. The cells in the lens of the epithelium regulate the homeostatic functions of the lens. As nutrients, ions and liquid enter the lens from the aqueous humor the Na+/K+ ATPase pumps out the ions out of the lens and pumps in the epithelial cells to balance the volume and osmolarity. The Na+/K+ ATPases keep current and water flowing through the lens.
The lens fibers form the lens. These fibers are transparent cells, thin, long and firmly packed with a diameter between 4-7 and length up to 12 mm. The lens fibers stretch in length from posterior to the anterior poles. These firmly packed lens are known as laminae. Mature lens fibers do not contain nuclei or organelles and new lens fibers grow from the lens epithelium.
The curvature of the lens is controlled by ciliary muscles and the lens is flexible. When the curvature of the lens changes, a person can therefore focus objects at different distances. This is known as accommodation.
The refractive index of the lens is approximately 1.406 at the central layers to 1.386 in less dense cortex of the lens. The optical power of the lens is therefore enhanced by the index gradient.
When it comes to marine animals, they have to depend completely on their lens. It would be for providing refractive power as well as focusing. The lens in marine animals is therefore hard and round.
When objects are at a great distance, the lens has to focus and relax the ciliary muscle which increases the tension on the zonules by making the lens flat and increasing the focal distance.