In this note, we are going to discuss the anatomy and function of the eye, in particular the eyeball.

The human eye is a sensory organ essential for vision, composed of various anatomical structures (the eyeball and its accessory structures) that function together to capture and process light, providing our visual perception.

Eyeball

Let’s discuss the eyeball itself.

Each eyeball is located within the bony orbit of the cranium, surrounded by adipose tissue.

It is covered by a thin fascial layer called Tenon’s capsule.

The primary function of the eyeball is to detect visual stimuli and process this information to the central nervous system via the optic nerve (CN II).

The wall of the eyeball consists of three layers: the outer fibrous layer, the intermediate vascular layer, and the inner neural layer (or retina). Let’s discuss them in greater detail.

Fibrous Layer

The fibrous layer consists of the sclera and cornea. This layer provides mechanical support and physical protection for the eye, serves as an attachment site for the extraocular muscles, and contains structures involved in the focusing process.

Sclera

The sclera, often referred to as the “white of the eye”, covers most of the ocular surface.

It is composed of dense, fibrous connective tissue containing collagen and elastic fibers. The sclera is thickest at the posterior portion near the exit of the optic nerve and becomes progressively thinner towards the anterior surface.

The posterior pole of the sclera is perforated by the optic nerve, forming the posterior scleral foramen.

This structure also allows the passage of the central retinal artery and vein.

The sclera has three sets of apertures:

• Anterior apertures: transmit the anterior ciliary arteries.

• Middle apertures: transmit the vorticose veins.

• Posterior apertures: transmit long and short ciliary arteries, veins, and nerves.

The anterior margin of the sclera is continuous with the cornea at the corneoscleral junction, also known as the corneal limbus.

Posterior to this junction, within the inner surface of the sclera, is a circular canal called the internal scleral sulcus,

which contains the scleral venous sinus (canal of Schlemm).

The sclera is divided into three distinctive layers:

• Episclera

It contains an arterial episcleral plexus formed by branches of the anterior ciliary arteries.

• Scleral stroma. It gives the sclera its distinctive white color.

• Lamina Fusca or suprachoroid lamina. The innermost layer, named for its large number of melanocytes.

Cornea

The cornea is a transparent layer, that covers the anterior portion of the eyeball.

The stratified squamous epithelium covers its surface, and multiple layers of collagen fibers are located deeply. The cornea is avascular, with no blood vessels between it and the overlying conjunctiva.

The cornea serves as the primary and most significant refractory structure of the eye, playing a crucial role in vision. Additionally, due to numerous free nerve endings, it is the most sensitive part of the eye.

Vascular Layer

The vascular layer, or uvea, also known as uveal tract.

Contains numerous blood vessels and performs several essential functions:

• It provides a pathway for blood vessels and lymphatics to supply eye tissue. It regulates the amount of light entering the eye;
• It secretes and reabsorbs the aqueous humor that circulates within the eye. It is a transparent water-like fluid, similar to blood plasma, but containing lower concentrations of proteins.
• Controls the shape of the lens, which is crucial for the focusing process.

The uvea consists of sub-layers: the iris, the ciliary body, and the choroid.

Iris

The iris is the most anterior portion of the uvea.

It contains blood vessels, pigment cells, and two layers of smooth muscle cells.

The contraction of these muscles changes the diameter of the pupil, the central opening of the iris.

It varies in size from 1 to 8 millimeters, controlling the amount of light entering the eye. This ability to adjust pupil size ensures optimal light entry into the eye, essential for clear vision and protecting the retina from excessive light exposure.

• The sphincter pupillae, which is arranged in concentric circles around the pupil, contracts to decrease the pupil’s diameter.

• The dilator pupillae, which extends radially from the pupil’s edge, contracts to enlarge the pupil.

These antagonistic muscles are controlled by the autonomic nervous system, with parasympathetic activation constricting the pupil (known as miosis) and sympathetic activation dilating it (known as mydriasis).

This way of functioning is similar to the functioning of a camera’s diaphragm.

The number of melanocytes (pigment cells) is associated with different eye colors among individuals, ranging from light blue to dark brown.

The periphery of the iris, known as the root or ciliary margin, forms the iridocorneal angle (filtration angle) with the cornea.

This angle contains the trabecular meshwork, essential for draining aqueous humor into the Schlemm canal.

Ciliary body

The ciliary body is the middle portion of the uvea.

It begins at the junction of the cornea and sclera and extends posteriorly to the so-called ora serrata, which is the site of junction with the retina.

The ciliary body consists of three components:

• Ciliary epithelium. This cuboidal epithelium covers the deep surface of the ciliary body. It has numerous folds called ciliary processes. They produce the already mentioned aqueous humor.

• Ciliary stroma. Composed of loose connective tissue and rich in blood vessels, that form the arterial circle at the base of the ciliary body.
• Ciliary muscle.

When it contracts, it pulls the ciliary body anteriorly, loosening the zonular fibers of the lens and allowing the lens to become more convex. This change increases the lens’s refractive power, which is essential for the process of accommodation.

Choroid

The choroid is the posterior portion of the uvea.

It is a vascular layer, containing an extensive capillary network that delivers oxygen and nutrients to the retina.

The choroid is divided into three distinct layers:

• Vessel layer. It is formed by branches of the short posterior ciliary arteries and tributaries of the vorticose veins. It also contains numerous melanocytes, which help to absorb light and prevent internal reflection within the eye.
• Capillary layer. It consists of smaller branches from the vessel layer and numerous melanocytes.
• Bruch’s membrane. This layer is attached to the outer retinal layer.

Inner Layer – Retina

The innermost layer of the eyeball is called the retina.

It converts light into neural impulses, which are then conveyed to the brain via the optic nerve for interpretation and analysis.

The retina consists of two sub-layers: the outer retinal pigmented layer and the inner neural layer.

Retinal pigment epithelium

It is located on the previously mentioned Bruch’s Membrane.

The cells of this epithelium contain a high amount of dark pigment, which absorbs light passing through the retina, preventing its reflection back to the neurosensory layer.

Furthermore, the retinal pigment epithelium plays a crucial role in nourishing the retina by forming the blood-retinal barrier, which regulates the exchange of substances between the bloodstream and the retina.

Neural retina

The retina contains several cell layers.

The outermost is the layer of rods and cones, known as photoreceptor cells, which are responsible for detecting light.

They are the first-order neurons of the visual pathway. There are three types of cones that detect different colors and provide us with color vision. Rod cells don’t detect colors, but they are highly sensitive to light and function in night vision.

The photoreceptors are not evenly distributed across the retina. The macula is the region with the greatest density of cones and is located in the center of the posterior retina.

It doesn’t have rods at all. Its central part is called the fovea centralis and is considered as the site of sharpest color vision.

When we look directly at an object, its image falls on this portion of the retina.

The optic disc is located a bit medially to the macula, and it is a site where the optic nerve exits the eye.

The optic disc doesn’t contain any of the photoreceptors, that’s why it’s also known as the “blind spot” of the eye.

The second layer is the outer limiting membrane.

Then goes the outer nuclear layer, made up of cell bodies of the cones and rods.

The outer plexiform layer. It consists of the synapses between the terminal processes of the rods and cones, bipolar and horizontal cells (we’re going to cover them a bit later).

The inner nuclear layer. It consists of the cell bodies of the amacrine cells, bipolar cells, and horizontal cells.

The bipolar cells process information from the photoreceptors and pass it further. They are the second-order neurons of the visual pathway.

The horizontal and amacrine cells modulate the photoreceptive process by stimulating and inhibiting other cells.

Then goes the inner plexiform layer. It consists of the synapses between the bipolar, amacrine, and ganglion cells.

The ganglion cell layer consists of the nuclei of the ganglion cells. They are the third-order neurons in the optic pathway.

The axons of the ganglion cells converge towards the optic disc and form the next layer, the nerve fiber layer. The optic disc is the origin of the optic nerve.

The central retinal artery

and central retinal vein join it at this point.

And the last layer of the retina is the inner limiting membrane.

Chambers of the Eye

So, we have already discussed the layers of the eyeball’s wall. However, it is a hollow organ, and deep inside, it is divided into two cavities.

The posterior cavity contains the gelatinous vitreous body.

The anterior cavity has two chambers: the anterior chamber (between the cornea and the iris)

and the posterior chamber (between the iris and the lens).

The shape of the eye is stabilized by the vitreous body and the aqueous humor – the water-like fluid filling the anterior cavity.

Refractive media of the eyeball

The already mentioned lens, vitreous body, and aqueous humor, as well as the cornea, belong to the refractive media of the eyeball.

The role of refractive structures is to bend the direction of the incoming light, focusing it onto the retina for clear visual perception.

But each of them has some other functions, so let’s discuss them in greater detail.

Aqueous Humor

The aqueous humor is a water-like fluid, produced by the epithelial cells of the ciliary processes and delivered into the posterior chamber of the eye.

Then it passes through the iris, to reach the anterior chamber, and eventually flows into the scleral venous sinus (canal of Schlemm), which is connected with the veins of the eye.

It is similar in composition to cerebrospinal fluid and circulates within the eyeball to transport nutrients and remove waste products.

Vitreous Body

The vitreous body, or vitreous humor, is a gelatinous mass that occupies the posterior cavity, also known as the vitreous chamber.

It maintains the shape of the eye, supports the posterior surface of the lens, and holds the retina in place.

The hyaloid fossa is a depression in the anterior part of the vitreous body, which is shaped to fit with the convexity of the lens.

Lens

The lens is a circular, biconvex structure, located behind the iris.

It is held in place by the small ligamentous bands known as the zonular fibers, which extend from the ciliary processes.

Collectively, these fibers form the suspensory ligament of the lens.

The lens changes shape through accommodation, controlled by the ciliary muscle.

In a resting state, when focusing on distant objects, the ciliary body maintains tension on the zonular fibers, keeping the lens in a flattened state.

When focusing on close objects, the ciliary muscle contracts, relaxing the suspensory ligament of the lens. This relaxation allows the lens to increase its anterior curvature, thereby increasing its refractive power. Simultaneously, the pupil undergoes miosis, directing light rays through the thickest central part of the lens towards the retina.

Extraocular muscles

Eye movement is controlled by six extraocular muscles:

1. Superior rectus muscle – provides elevation, adduction, internal rotation of the eyeball

2. Inferior rectus muscle – provides depression, adduction, external rotation of the eyeball

3. Medial rectus muscle – provides adduction of the eyeball

4. Lateral rectus muscle – provides abduction of the eyeball

5. Superior oblique muscle – provides depression, abduction, internal rotation of the eyeball

6. Inferior oblique muscle – provides elevation, abduction, external rotation of the eyeball.

And one other muscle, the levator palpebrae superioris, elevates the upper eyelid.

The lateral rectus muscle is innervated by the abducens nerve (CN VI),

the superior oblique muscle – by the trochlear nerve (CN IV)

and all other muscles – by the oculomotor nerve (CN III).

Visual Pathway

The anatomy of the visual pathway is discussed in detail in our video about the optic nerve. But for now, let’s just quickly recap the structures involved in visual processing.

• Rods and cones (photoreceptor cells)
• Bipolar cells
• Ganglion cells
• Optic nerves (CN II)

• Optic chiasm

• Optic tracts

• Lateral geniculate body

• Optic radiation

• Visual cortex and its cortical projections

Golosary

Anatomy of the Eye: Accessory structures

eye

oculus

eyeball

bulbus oculi

eyelids

palpebrae

palpebral fissure

rima palpebrarum

medial canthus

angulus oculi medialis

lateral canthus

angulus oculi lateralis

plate

lamella

anterior plate

lamella anterior

orbicularis oculi muscle

musculus orbicularis oculi

ciliary glands

glandulae ciliares

sebaceous glands

glandulae sebaceae Zeis

middle plate

lamella media

orbital septum

septum orbitale

posterior plate

lamella posterior

superior tarsal plate

tarsus superior

inferior tarsal plate

tarsus inferior

conjunctiva

conjunctiva

levator palpebrae superioris muscle

musculus levator palpebrae superioris

superior tarsal muscle

musculus tarsalis superior

inferior tarsal muscle

musculus tarsalis inferior

tarsal glands

glandulae tarsales

lacrimal caruncle

caruncula lacrimalis

fornix of conjunctiva

fornix conjunctivae

lacrimal apparatus

apparatus lacrimalis

lacrimal gland

glandula lacrimalis

lacrimal fossa

fossa lacrimalis

palpebral part of lacrimal gland

pars palpebralis

orbital part of lacrimal gland

pars orbitalis

lacrimal lake

lacus lacrimalis

lacrimal puncta

puncta lacrimalia

lacrimal canaliculi

canaliculi lacrimales

lacrimal sac

saccus lacrimalis

lacrimal groove

sulcus lacrimalis

nasolacrimal duct

ductus nasolacrimalis

nasolacrimal canal

canalis nasolacrimalis

nasal cavity

cavitas nasalis

Tenon’s capsule

fascia bulbi

fibrous layer of eyeball

tunica fibrosa bulbi

sclera

sclera

lamina cribrosa sclerae

lamina cribrosa sclerae

anterior ciliary arteries

arteriae ciliares anteriores

vortex veins

venae vorticosae

long ciliary arteries

arteriae ciliares longae

short ciliary arteries

arteriae ciliares breves

corneal limbus

limbus corneae

internal scleral sulcus

sulcus sclerae internus

scleral venous sinus (Schlemm’s canal)

sinus venosus sclerae

episclera

lamina episcleralis

episcleral space

spatium episclerale

episcleral arterial plexus

plexus episcleralis

scleral stroma

stroma sclerae

suprachoroid lamina

lamina fusca

cornea

cornea

vascular layer of eyeball

tunica vasculosa bulbi

iris

iris

pupil

pupilla

sphincter pupillae muscle

musculus sphincter pupillae

dilator pupillae muscle

musculus dilatator pupillae

ciliary margin

margo ciliaris

iridocorneal angle

angulus iridocornealis

ciliary body

corpus ciliare

ora serrata

ora serrata

ciliary epithelium

epithelium ciliare

ciliary processes

processus ciliares

ciliary stroma

stroma ciliare

ciliary muscle

musculus ciliaris

choroid

choroidea

vascular layer

stratum vasculosum

capillary layer

stratum capillare

Bruch’s membrane

lamina basalis

retina

retina

retinal pigment epithelium

epithelium pigmentosum retinae

optic disc

discus nervi optici

blind spot

punctum caecum

chambers of the eye

camerae bulbi

vitreous body

corpus vitreum

vitreous chamber

camera vitrea

anterior chamber

camera anterior

posterior chamber

camera posterior

aqueous humor

humor aquosus

lens

lens

zonular fibers

fibrae zonulares

refractive media of the eye

media refractiva bulbi

superior rectus muscle

musculus rectus superior

inferior rectus muscle

musculus rectus inferior

medial rectus muscle

musculus rectus medialis

lateral rectus muscle

musculus rectus lateralis

superior oblique muscle

musculus obliquus superior

inferior oblique muscle

musculus obliquus inferior

abducens nerve

nervus abducens

trochlear nerve

nervus trochlearis

oculomotor nerve

nervus oculomotorius