The eye is a specialised organ which collects and transmits visual information from the environment to the brain. A ~2.5 cm diameter sphere, it comprises of an anterior (cornea to lens) and a posterior segment (lens to optic nerve). Entering light is refracted by the cornea and lens onto the retina where it is detected by photoreceptors. Protected by the bony orbit, eyelids and lacrimal system, the eye has three coats:

• An outer fibrous (corneoscleral) coat, consisting of collagen bundles arranged anteriorly in highly ordered parallel lamellae to form the transparent cornea (responsible for 2/3 of the refractive power of the eye) and posteriorly in a more disordered fashion to form the opaque sclera.
• A vascular middle coat, comprising of the choroid, ciliary body and iris. The size of the pupillary aperture in the iris is controlled by the dilator pupillae (sympathetic) and sphincter pupillae (parasympathetic) regulating the amount of light that hits the retina.
• An inner neural layer known as the retina, from which retinal ganglion cells contribute axon fibres that form the optic nerve. At the posterior pole, the macula lutea, a pale yellowish area where 90% of optic nerve fibres arise, is responsible for sharp detailed central vision, with resolution highest specifically at the fovea. About 3 mm medially is the optic disc, the point where the optic nerve exits the eye, creating a blind spot in the visual field.

Lens

The lens is a highly organised system of special lens fibres contained in a capsule. Not only does it separate the aqueous humour anteriorly and the vitreous posteriorly, but it is also biconvex and accounts for 1/3 of the refractive power of the eye. Ciliary muscles in the ciliary body can change the shape of the lens to adjust the refractive power, allowing the eye to focus on objects at different distances.