Eye anatomy: the parts of the eyeball, its three layers and a labelled 3D eye model

The eyeball is the largest structure in the orbit. It is round, with a forward bulge at the front where the cornea sits. (Kenhub: Eye and orbit anatomy.)

The bony orbits surround the eyeballs, protecting them and anchoring the soft tissues of the eye. Six extraocular muscles move each eyeball within its orbit, and the eyelids, with lashes along their edges, protect the front of the eye. (OpenStax Anatomy and Physiology 2e, 14.1: Sensory Perception.)

On CT, a normal adult eye measures about 24 mm from side to side and about 23.5 mm from front to back. The front-to-back (axial) length varies with focusing errors: it can be as short as about 20 mm in long-sightedness (hypermetropia) and as long as about 26 mm in short-sightedness (myopia). (Radiopaedia: Globe.)

The wall of the eyeball is built from three layers, from outside in: the fibrous layer, the vascular layer and the inner (neural) layer. (TeachMeAnatomy: The Eyeball.)

OpenStax calls them tunics. The outer fibrous tunic includes the white sclera and the clear cornea. The middle vascular tunic is made mostly of the choroid, ciliary body and iris. The innermost neural tunic is the retina. (OpenStax Anatomy and Physiology 2e, 14.1: Sensory Perception.)

The outer layer gives the eyeball its shape and resists damage. The sclera covers the posterior five-sixths of the eyeball and is the attachment site for the eye muscles. (Kenhub: Eye and orbit anatomy.)

The sclera is the opaque white coat of the eye. Its main layer is dense fibrous tissue mixed with fine elastic fibres; its type I and III collagen fibres are arranged irregularly, which is why it is white rather than clear. It has a loose outer layer (episclera) and a pigmented inner layer (lamina fusca). (Radiopaedia: Sclera.)

At the front the sclera is continuous with the cornea. At the back it is perforated by the optic nerve at the lamina cribrosa, and it is continuous with the dura, arachnoid and pia sheaths around the optic nerve. (Radiopaedia: Sclera.)

The sclera accounts for five-sixths of the surface of the eye, most of it hidden in the orbit. Humans are unusual among species in having so much of the white of the eye visible. (OpenStax Anatomy and Physiology 2e, 14.1: Sensory Perception.)

The cornea is the clear tissue at the front of the eye. Light entering the eye is refracted (bent) by the cornea before it reaches the lens. (TeachMeAnatomy: The Eyeball.)

The choroid of the eye is a pigmented layer of connective tissue and blood vessels between the sclera and the retina. Its main job is to supply oxygen and nutrients to the other layers of the eyeball. (Kenhub: Eye and orbit anatomy.)

In particular, the choroid nourishes the outer layers of the retina. The ciliary body, in front of the choroid, contains the ciliary muscle and the ciliary processes: it controls the shape of the lens and helps produce aqueous humour. (TeachMeAnatomy: The Eyeball.)

The ciliary body is attached to the lens by suspensory ligaments (zonular fibres). By pulling on these ligaments it changes the shape of the lens so that light is focused on the back of the eye. (OpenStax Anatomy and Physiology 2e, 14.1: Sensory Perception.)

The iris is the coloured part of the eye, a ring of smooth muscle around a central hole, the pupil, through which light enters. The iris constricts the pupil in bright light and dilates it in dim light. (OpenStax Anatomy and Physiology 2e, 14.1: Sensory Perception.)

The iris lies between the lens and the cornea, and its smooth muscle is under autonomic (involuntary) control. (TeachMeAnatomy: The Eyeball.)

The retina has an outer pigmented layer, which absorbs stray light, and an inner neural layer that contains the photoreceptors. The macula, near the centre of the retina, contains the fovea centralis, the area of sharpest vision. The optic disc, where the optic nerve leaves the eye, has no photoreceptors. (TeachMeAnatomy: The Eyeball.)

Kenhub divides the retina into a light-sensitive optic part, which has the neural and pigmented layers, and a non-visual part. The 3D model on this page shows the optic part of the retina. (Kenhub: Eye and orbit anatomy.)

There are two kinds of photoreceptor. Rods work in low light and do not sense colour; cones work in brighter light and give colour vision. Because the optic disc has no photoreceptors, it creates a blind spot in the retina and a matching blind spot in the visual field. (OpenStax Anatomy and Physiology 2e, 14.1: Sensory Perception.)

The lens sits behind the iris. Its shape is changed by the ciliary body to focus light. Behind it, the vitreous body is a clear gel that helps refract light, supports the lens and holds the retina in place; a fetal remnant called the hyaloid canal runs through it. (TeachMeAnatomy: The Eyeball.)

The front part of the eye (anterior segment) is divided into two chambers: the anterior chamber between the cornea and the iris, and the posterior chamber between the iris and the lens. Both are filled with aqueous humour. The posterior segment, behind the lens, contains the gel-like vitreous humour. (Radiopaedia: Globe.)

Aqueous humour is made all the time by the ciliary body and drains through the trabecular meshwork near the edge of the cornea. Glaucoma is a group of diseases that damage the optic nerve, and in closed-angle glaucoma the iris is pushed against the trabecular meshwork. (TeachMeAnatomy: The Eyeball.)

Light first meets the cornea, the most superficial layer of the bulge at the front of the eye, which refracts it. It then passes through the anterior chamber and the lens, which focuses it onto the retina. (Kenhub: Eye and orbit anatomy.)

On its way, light passes through the pupil, whose size is set by the iris, and then through the vitreous body to the back of the eye. (OpenStax Anatomy and Physiology 2e, 14.1: Sensory Perception.)

Inside the retina the light-sensitive cells face away from the incoming light. The photoreceptors lie behind the retinal ganglion cells, bipolar cells and blood vessels, so light must pass through these layers first, and a significant amount is absorbed before it reaches the rods and cones. (OpenStax Anatomy and Physiology 2e, 14.1: Sensory Perception.)

The photoreceptors pass signals to bipolar cells and then to retinal ganglion cells, whose axons gather at the optic disc and leave the eye as the optic nerve. (OpenStax Anatomy and Physiology 2e, 14.1: Sensory Perception.)

The sclera is part of the wall of the eyeball: an opaque white coat of dense fibrous tissue, continuous with the cornea in front and with the sheaths of the optic nerve behind. (Radiopaedia: Sclera.)

The conjunctiva is not part of the eyeball wall. It is a transparent membrane attached at the margin of the cornea. It is loosely attached to the sclera, then folds back to line the inner surface of the eyelids, where it is firmly attached to the tarsal plates and blends with the skin at the lid margins. (Radiopaedia: Conjunctiva.)

The part on the eyeball is the bulbar conjunctiva and the part lining the lids is the palpebral conjunctiva, which forms the deepest layer of the eyelid. TeachMeAnatomy describes the conjunctiva as a thin mucous membrane. (TeachMeAnatomy: The Eyelids.)

In short, the conjunctiva covers the white of the eye (the sclera) and connects the eyelids to the eyeball, whereas the sclera itself forms the eye's outer wall. (OpenStax Anatomy and Physiology 2e, 14.1: Sensory Perception.)

From superficial to deep, the eyelid has five main layers: skin and subcutaneous tissue, the orbicularis oculi muscle, the tarsal plate, the levator apparatus and the conjunctiva. The skin of the eyelid lacks subcutaneous fat, so it swells easily with fluid or blood. (TeachMeAnatomy: The Eyelids.)

The tarsal plates are made of dense connective tissue and act as the scaffolding of the upper and lower eyelids. They contain the Meibomian glands, specialised sebaceous glands that secrete an oily layer onto the eye to slow evaporation of the tear film. The levator palpebrae superioris, supplied by the oculomotor nerve (CN III), lifts the upper lid. (TeachMeAnatomy: The Eyelids.)

Is the orbital septum a flat sheet of tissue inside the eyelid? Yes: it is a thin sheet of fibrous tissue that starts from the periosteum of the orbital rim. In the upper lid it blends with the tendon of the levator palpebrae superioris, and in the lower lid it inserts into the tarsal plate. (Radiopaedia: Orbital septum.)

The orbital septum separates the fat inside the orbit from the eyelid and the orbicularis oculi muscle. It also acts as a barrier against the spread of infection between the preseptal space (in front of it) and the postseptal space (the orbit proper), which is why doctors distinguish preseptal from orbital infections. (Radiopaedia: Orbital septum.)

Kenhub describes the orbital septum as extensions of the periosteum from the orbital margin that extend through both eyelids and support them. (Kenhub: Eye and orbit anatomy.)

Tears are made by the lacrimal gland, which lies just inside the orbit, above and to the outer side of the eyeball. (OpenStax Anatomy and Physiology 2e, 14.1: Sensory Perception.)

The gland sits in the lacrimal fossa of the frontal bone and has a larger orbital part and a smaller palpebral part. Its secretion is driven by parasympathetic fibres that come from the facial nerve through the pterygopalatine ganglion. (TeachMeAnatomy: The Lacrimal Gland.)

Tears are released into the upper conjunctival fornix and spread over the cornea by blinking. They collect in the lacrimal lake at the inner (medial) corner of the eye, drain through small canals into the lacrimal sac, and then pass down the nasolacrimal duct, which opens into the inferior meatus of the nasal cavity. (TeachMeAnatomy: The Lacrimal Gland.)

Kenhub lists the lacrimal canaliculi, the lacrimal sac and the nasolacrimal duct as the drainage parts of the lacrimal apparatus, alongside the lacrimal gland, which is supplied by the lacrimal nerve from the ophthalmic division of the trigeminal nerve (CN V1). (Kenhub: Eye and orbit anatomy.)

The eyeball gets its arterial blood mainly from the ophthalmic artery, a branch of the internal carotid artery. Its central retinal artery supplies the inner retina; blocking it quickly causes blindness in that eye. Veins drain through the superior and inferior ophthalmic veins to the cavernous sinus. (TeachMeAnatomy: The Eyeball.)

Swelling of the optic disc, called papilloedema, is caused by raised pressure inside the skull, most commonly from a mass, bleeding, meningitis or hydrocephalus. (TeachMeAnatomy: The Eyeball.)

Anterior chamber of eyeball

The anterior chamber is the fluid-filled space between the cornea and iris.

Choroid

The choroid is the vascular, pigmented layer of the eye between the retina and the sclera.

Cornea

The cornea is the transparent front part of the outer coat of the eye and is the eye's major refractive surface.

Iris

The iris is the pigmented, contractile diaphragm in front of the lens that forms the pupil.

Lacrimal canaliculus

The lacrimal canaliculi are small ducts that drain tears from the eyelid margins to the lacrimal sac.

Lacrimal gland

The lacrimal gland is the main tear-secreting gland of the eye.

Lacrimal lake

The lacrimal lake is the small pool of tears at the medial angle of the eye.

Lacrimal sac

The lacrimal sac is the upper dilated end of the nasolacrimal duct.

Lens

The lens is a transparent, biconvex structure behind the iris that focuses light on the retina.

Nasolacrimal duct

The nasolacrimal duct carries tears from the lacrimal sac into the nasal cavity.

Optic nerve

The optic nerve (CN II) carries visual information from the retina to the brain.

Optic part of retina

The optic part of the retina is the light-sensitive neural layer lining the back of the eye.

Sclera

The sclera is the tough, white outer coat of the eyeball.

Suspensory ligament of lens

Zonular fibres that hold the lens of the eye.

Tarsal plate of lower eyelid

The tarsal plate of the lower eyelid is a thin plate of dense connective tissue that stiffens the lower lid.

Tarsal plate of upper eyelid

The tarsal plate of the upper eyelid is a firm plate of dense connective tissue that gives the upper lid its shape.

Vitreous body

The vitreous body is the clear gel filling the large space behind the lens.