The eye (latin: oculus) is a paired sensory organ, which is responsible for the sense of sight. Anatomically, each eye consists of an eyeball and accessory structures of the eye.
The wall of the eyeball consists of three layers surrounding the vitreous body:
- fibrous layer - the outer layer,
- vascular layer - the middle layer,
- retina - the inner layer of the eyeball.
The fibrous layer of the eyeball consists of the sclera - the largest posterior part, white in colour, and the cornea - the anterior part of the fibrous layer, a transparent plate.
The middle vascular layer is rich with blood vessels and pigment, and it includes the choroid - the largest part of the vascular layer, the ciliary body and the iris in the anterior part. One of the main functions of the ciliary body is the production of the aqueous fluid. The iris contains the pigment that determines the eye color. There is a hole in the middle of the iris called the pupil. The narrowing of the pupil is provided by the sphincter pupillae, while the widening is provided by the dilator pupillae. These are smooth muscles surrounding the pupil and innervated by the autonomic nervous system.
The retina is the third and inner layer of the wall of the eyeball, and is continuous with the optic nerve (CN II). The retina is the light-sensitive structure of the eye, where the neural visual pathway begins. It has two distinct layers: the external layer of pigmented epithelial cells and the internal layer called the neural retina. The neural retina contains three layers of neural cells: light-sensitive photoreceptor cells, bipolar cells, and ganglion cells of the retina. Light beams striking the retina initiate chemical and electrical processes ultimately leading to nerve impulses carried to the visual centers of the brain by the optic nerve.
The eyeball also includes such structures as the transparent vitreous body in the core and chambers filled with the aqueous humour, produced by the ciliary body. Another important structure is the lens - the most essential light-reflecting structure of the eyeball.
The accessory structures of the eye include the following:
- extra-ocular muscles,
- eyelids and conjunctiva,
- lacrimal apparatus,
- connective tissue of orbit.
There are seven extra-ocular muscles, also known as extrinsic muscles of the eyeball, in each eye. The extra-ocular muscles controlling eye movements include six striated muscles: the superior rectus, inferior rectus, medial rectus, lateral rectus, superior obliquus and inferior obliquus muscles. Additionally, there is a muscle that provides elevation of the upper eyelid, the levator palpebrae.
Each eye has two eyelids (upper and lower) that are covered by skin on the outside and by conjunctiva on the inside, which also partly covers the sclera.
The lacrimal apparatus is responsible for production of tears and is composed of the lacrimal gland, the lacrimal sac and excretory ducts.
The connective tissue structures of the orbit include the periosteum lining the bony orbit, the fascial sheath of the eyeball, and the orbital fat body.