The eyeball consists of three distinct layers, each composed of specialized structures with unique functions.

Fibrous layer (Read more!) - the outer protective layer that supports the inner structures, gives the eyeball its shape, and provides resistance; formed by the transparent cornea (Read more!) anteriorly and the opaque sclera (Read more!) posteriorly;
Vascular layer (Read more!) - the middle layer that contains blood vessels and provides oxygen and nutrients to the eyeball; it also regulates light entry into the eyeball; it contains the choroid (Read more!), ciliary body (Read more!), and iris (Read more!); this layer is also known as the pigmented layer, as it contains melanocytes that produce melanin;
Nervous layer (Read more!) - the inner layer that is formed mainly by nerve cells and contains photoreceptors that detect visual light and convert it into nerve impulses; it is formed by the retina; this is the most important layer, as it receives external visual stimuli.

All these layers surround the eyeball from the outside, but the structures within the eyeball include the vitreous body (Read more!) and the lens (Read more!). Both of these structures, together with the aqueous humor and the cornea (Read more!), are collectively known as the refractive media. They cooperate to refract incoming light rays and focus them onto the retina.

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Layers and structures of eyeball