Eye is an organ that detects light and contributes to vision. It lies in the orbit, the bony socket at the front of the skull, where fat, connective tissue and muscles surround and support the eyeball. Its transparent front admits light, and its light-sensitive inner layer converts that light into nerve signals.
Where is the eye located?
The eyeball occupies the anterior part of the orbit, behind the eyelids. Orbital fat cushions it, while extraocular muscles, the muscles outside the eyeball, move it within the socket. Behind the eyeball, the optic nerve carries visual information towards the brain.
The orbital roof separates the eye from the cranial cavity, which contains the brain. The medial wall, nearest the nose, borders the ethmoidal air cells, air-filled spaces within the ethmoid bone. The floor separates the orbit from the maxillary sinus, an air-filled cavity in the upper jaw.
The accessory visual structures include the eyelids, the tear-producing apparatus and the muscles that move the eyeball. The conjunctiva, a thin mucous membrane, lines the inner eyelids and covers the front of the eyeball except the cornea, its transparent front surface.
What are the layers of the eyeball?
The wall of the eyeball has an outer fibrous coat, a middle vascular coat and an inner retina. Fibrous means rich in supporting connective tissue; vascular means containing blood vessels. The retina is the inner layer whose light-sensitive region contains the cells that detect light.
| Layer | Main structures | Main anatomical role |
|---|---|---|
| Outer fibrous coat | Sclera and cornea | Protects the eyeball; the cornea admits and bends light |
| Middle vascular coat, or uvea | Choroid, ciliary body and iris | Supplies tissues, supports lens adjustment and regulates light entry |
| Inner retina | Pigment layer and neural layer | Absorbs stray light and converts light into nerve signals |
The sclera is the tough, opaque outer coat, continuous with the cornea at the front. The choroid lies between the sclera and retina and contains blood vessels and pigment. The ciliary body forms a ring behind the iris, the coloured diaphragm surrounding the pupil; the pupil is an opening rather than a separate tissue.
What are the eye's chambers and internal contents?
The anterior segment includes the cornea, iris, ciliary body and lens, together with the aqueous-filled chambers. The lens is a transparent structure that helps focus light. It contains the anterior chamber between the cornea and iris, and the posterior chamber between the iris and lens. Both chambers contain aqueous humour, a clear fluid that nourishes the cornea and lens.
The ciliary processes, folds projecting from the ciliary body, produce aqueous humour. This fluid passes through the pupil into the anterior chamber. Much of it drains through the eye trabecular meshwork, a network of tissue at the junction of the cornea and iris, into the scleral venous sinus, a drainage channel within the sclera.
The posterior segment lies behind the lens and contains the vitreous body, a transparent gel occupying the large space in front of the retina. Fine fibres suspend the lens from the ciliary body. Contraction of the ciliary muscle reduces tension on these fibres, allowing the lens to become more rounded for near focus.
What is the blood supply and lymph drainage of the eye?
The ophthalmic artery, a branch of the internal carotid artery, provides the main arterial supply. Its central retinal artery supplies the inner retina. Posterior ciliary arteries supply the choroid, whose capillaries nourish the outer retina by diffusion, while anterior ciliary arteries contribute to the supply of the front of the eyeball.
Venous blood leaves through the central retinal vein and vortex veins, which drain the choroid. These routes communicate with the ophthalmic veins and the cavernous sinus, a venous channel within the cranial cavity.
The eyeball has no conventional lymphatic drainage network comparable to that of the eyelids and conjunctiva. These surrounding tissues drain towards preauricular nodes, in front of the ear, and submandibular nodes, beneath the lower jaw. Lymph nodes filter lymph, fluid collected from tissues.
Which nerves supply the eye?
The optic nerve carries signals from the retina to the brain. Sensation from the cornea and other parts of the eyeball travels mainly through branches of the ophthalmic division of the trigeminal nerve, the principal sensory nerve of the face.
Preganglionic parasympathetic fibres travel in the oculomotor nerve and synapse in the ciliary ganglion. Postganglionic fibres then reach the sphincter pupillae of the iris and the ciliary muscle through the short ciliary nerves, enabling pupil constriction and lens adjustment. Sympathetic fibres, another division of the autonomic nervous system, supply the muscle that widens the pupil.
Most extraocular muscles receive motor fibres from the oculomotor nerve. The lateral rectus muscle, which turns the eye outwards, receives the abducens nerve; the superior oblique muscle, which contributes to rotation and downward movement, receives the trochlear nerve.
How does the eye produce vision?
Within the sensory system, the eye receives light and turns it into information that the brain can process. The cornea provides most of the eye's light-bending action, while the lens adjusts focus. The iris changes pupil size to regulate light entry.
Rods and cones are the retina's light-sensitive cells. Rods support vision in dim light, while cones support colour vision and fine detail. Signals pass through retinal nerve cells before leaving in the optic nerve.
How can you orient the eye in the 3D model?
In the 3D model beside this article, rotate and zoom to establish the eye's position, then select and isolate it or fade neighbouring structures. You can also switch layers or cut a section along the sagittal, coronal or transverse plane to explore spatial relationships.
The right zonular fibers are the fine fibres that suspend the lens in the right eye. Distinguish this lens-supporting apparatus from the tissues that support the whole eyeball within the orbit.