The eyeball (Latin: bulbus oculi) is a spheroidal structure that contains sense receptors for vision. The eyeball contains many structures, with each being responsible for different functions. The eyeball lies in orbit, surrounded by the adipose tissue. The eyeball is covered by a thin fascial sheath known as the Tenon's capsule. The eyeball consists of the layers of the eyeball and the refractive media of the eyeball.

The eyeball consists of the segments of two spheres of different sizes placed one in front of the other. The anterior segment is smaller and transparent and creates around 1/6 of the eyeball. The posterior segment is larger and opaque, forming 5/6 of the eyeball. The eyeball has an anterior and posterior pole. The anterior pole of the eye is in the center of the transparent segment, while the posterior pole - the center of the posterior curvature.

The optic axis connects both poles. The equator is midway between both poles. In its anteroposterior diameter, the eyeball is around 24 mm, the vertical diameter is 23 mm, but the horizontal diameter - 23.5 mm. The eyeball has three layers surrounding the vitreous body: the fibrous layer, the vascular pigmented layer, and the nervous layer.

The fibrous layer or the outer layer consists of the sclera and cornea. The sclera is a white outer layer that surrounds the posterior 5/6 of the eyeball. The sclera can also be divided into three layers: the episclera, the scleral stroma, and the lamina fusca. The cornea is a transparent layer surrounding the anterior 1/6 of the eyeball. The cornea is continuous with the sclera.

The vascular pigmented layer is the middle layer, also called the uvea. This layer is rich in blood vessels and pigment. The vascular pigmented layer has three parts that are connected with each other. These parts are the choroid, ciliary body, and iris. The ciliary body produces the aqueous fluid. The iris contains the pigment that determines the eye colour. In the middle of the iris is a hole called the pupil.

The nervous layer or the retina is the deepest layer. The retina can be further divided into two layers: pigmented or outer layer and neurosensory or inner layer. The retina is the light-sensitive structure of the eye, where the neural visual pathway begins. The neural retina contains three layers of neural cells: light-sensitive photoreceptor cells, bipolar cells, and ganglion cells of the retina. The retina is continuous with the optic nerve.

The eyeball also includes such structures that are part of the refractive media of the eyeball. One of the structures is already above mentioned cornea. Other structures are the transparent vitreous body in the core and chambers filled with the aqueous humour, produced by the ciliary body. The aqueous humor firstly drains into the posterior chamber of the eyeball and then through the pupil into the anterior chamber. The aqueous humor supplies the lens and cornea with nutrients. Also, the lens is part of the refractive media - the most essential light-reflecting structure of the eyeball. The eyeball has two chambers: the anterior and posterior chambers. The anterior chamber is between the cornea and iris, while the posterior chamber is between the iris and lens.

Fascial sheath

The fascial sheath is also called Tenon's capsule. The fascial sheath creates a socket around the eyeball. The sheath attaches to the sclera, while on the dorsal part, it fuses with the optic nerve covering meninges. The fascial sheath's inner part is separated from the sclera by the episcleral space. The outer part of the fascial sheath is an attachment site for the extraocular muscles.

The fascial sheath covers the muscle tendons and creates sleeve-like projections. The projections are responsible for limiting the actions of the muscles by attaching to other structures within the eyeball. Two distinctive sleeves are tendons from the medial and lateral rectus muscles. The medial is called the medial check ligament attaching to the lacrimal bone, but the lateral is known as the lateral check ligament attaching to the zygomatic bone.

The fascial sheath protects the eyeball, positions it in orbit, and permits the extraocular muscles to perform their actions. The fascial sheath and the eye move together due to the lack of movement between both structures.