21st February 2024

Layers of the Eyeball – Inner Layer (Retina)

The innermost layer of the eye is the retina, responsible for detecting light. The retina has two distinct layers: The pigmented (outer) layer, consisting of a single layer of cells, is closely connected to the choroid. Its primary function is to support the choroid in absorbing light, preventing light from scattering within the eyeball. This layer extends around the entire inner surface of the eye. The neural (inner) layer is positioned at the back and sides of the eye. This layer contains photoreceptors, specialised cells responsible for detecting light.

At the centre of the retina lies the macula, distinguished by its yellowish colour and abundant pigmentation. Within the macula is the fovea centralis, a specialised depression that houses a high density of light-detecting cells. This region is responsible for achieving precise and detailed vision. Conversely, where the optic nerve enters the retina is termed the optic disc; it notably lacks any light-sensitive cells.

Lens

The lens is a transparent, biconvex structure positioned directly behind the iris, suspended by the ciliary body. The lens serves a vital role in the accommodation reflex and refraction. 

The lens is primarily composed of specialised lens fibres. These fibres lack cell nuclei and organelles, contributing to the lens’s transparency and optical clarity. It has an outer capsule that encapsulates the lens fibres, and its outermost portion is known as the lens capsule. The lens fibres are arranged concentrically like the layers of an onion.

The ability of the lens to change its shape is called accommodation. This process is controlled by the ciliary muscle and the zonular fibres. When focusing on near objects, the ciliary muscle contracts, causing the lens to thicken and increase its refractive power. Conversely, for distant objects, the ciliary muscle relaxes, causing the lens to flatten.

Vitreous Body

The vitreous body is a gel-like, transparent substance that occupies the rear part of the eyeball, located posterior to the lens. It serves to support the lens anteriorly and the retina posteriorly. It also contributes to the magnifying power of the eye.

Vitreous humour consists mainly of water (approximately 99%) along with a network of fine collagen fibres, hyaluronic acid, and other macromolecules. It has a gel-like consistency, allowing it to maintain the shape of the eye and support delicate structures such as the retina. Structurally, the vitreous can be divided into two regions: the vitreous cortex, which is adjacent to the retina, and the vitreous core, located deeper within the eye. The core is more liquid in nature than the cortex.