Anatomy of the Iris

Anatomy of the Iris

• The iris is the most anterior component of the uvea (iris + ciliary body + choroid) and divides the anterior chamber from the posterior chamber.
• It is a highly vascularized, muscular diaphragm with a central opening (pupil) and is approximately 12 mm in diameter.
• The iris is composed of two zones: the peripheral ciliary zone (3-4 mm) that contains the dilator muscle, and the central pupillary zone (1-2 mm) that contains the sphincter muscle.
• The iris collarette is the meeting point between the two zones and is the thickest portion of the iris (~500 µm).

Pupil

• The pupil is a circular opening between the irises that dilates (enlarges) in low lighting conditions and constricts (narrows) in bright conditions.
• Pupil size tends to decrease with age and is symmetrical between the two eyes.

Iris Function

• The iris controls pupil size, modulating retinal illuminance to optimize vision.
• A small pupil minimizes aberrations and increases depth of focus.
• The iris blocks stray light and plays a role in social signaling, with constricted pupils when asleep and dilated pupils when excited.

Iris Histology

• The iris is composed of three layers: the anterior border layer, stroma, and pigmented epithelial layer (anterior and posterior).
• The anterior border layer is a modified stroma with a relatively dense network of fibroblasts, melanocytes, and collagen.
• The stroma is a loose collagenous matrix with fibroblasts, melanocytes, and large 'clump cells' that are phagocytes containing melanosomes.
• The pigmented epithelial layer is a single layer of lightly pigmented cells divided into apical and basal processes.

Iris Muscles

• The iris has two muscles: the sphincter (miosis) and dilator (mydriasis).
• The sphincter muscle is a flat annulus of smooth muscle at the pupil margin, 0.75-1 mm wide, and constricts the pupil.
• The dilator muscle is composed of radially oriented basal processes of anterior epithelial cells (myoepithelium) and dilates the pupil.

Iris Surface

• The anterior iris surface has thin, radial trabeculae (collagen columns) seen in light-colored irises and depressions or openings called crypts.
• The posterior iris surface is covered in melanocytes that prevent light from entering the eye other than through the pupil.

Iris Vasculature

• The iris receives its arterial supply from the major arterial circle via the long posterior and anterior ciliary arteries.
• Radial vessels in the iris stroma supply the iris and converge in a spiral pattern towards the pupil, forming radial ridges on the anterior iris surface.
• The vessels are non-fenestrated with tight junctions between endothelial cells, making them impermeable to macromolecules and conferring immune privilege.

Acknowledgement of Country

• The University of Western Australia acknowledges that its campus is situated on Noongar land, and that Noongar people remain the spiritual and cultural custodians of their land.

Anatomy of the Eye

The Globe

• The globe has 3 coats (tunics): outer (fibrous tunic), middle (vascular tunic/uvea), and inner (retina).
• Fluid layers maintain the globe's structure and function: tear film, aqueous humour, and vitreous humour.
• Four major tissue types in the globe: epithelial, connective, muscular, and neural tissues.

Cornea

• The cornea is 12 mm horizontally and 11 mm vertically, with a radius of curvature of approximately 8 mm.
• The cornea has 5 layers: non-keratinised stratified epithelium, Bowman's layer, stroma, Descemet's membrane, and endothelium.

Limbus

• The limbus is the junction between the cornea and sclera, approximately 1.75 mm wide.
• There is a sudden flattening of curvature at the limbus, forming the scleral sulcus.

Conjunctiva

• The conjunctiva is the outermost layer of the globe, covering the posterior surface of the lids.
• It is a loose mucous membrane containing blood vessels, nerves, lymphatics, and secretory glands.
• The conjunctiva has four components: forniceal, orbital, tarsal, and bulbar.

Tenon's Capsule

• Tenon's capsule is a dense connective tissue covering the eyeball (excluding the cornea) and extraocular muscles.
• It supports and protects the globe, forming a socket for the globe to move.

Episclera

• The episclera is a loose connective tissue beneath Tenon's capsule and above the sclera.
• It contains two vascular networks: the superficial plexus and the deep plexus, which provide nutrients to the sclera.

Sclera

• The sclera has three layers: episclera, scleral stroma, and lamina fusca.
• It is mainly composed of type I collagen and has a radius of curvature of approximately 12 mm.
• The sclera is the main load-bearing connective tissue, providing stable mechanical support.

Anterior Chamber

• The anterior chamber contains the cornea, iris, limbus, and drainage structures.
• The anterior chamber depth is approximately 3 mm, with an angle of 45°.

Anterior Chamber Angle

• The anterior chamber angle contains the trabecular meshwork, which facilitates drainage of the aqueous humour.

Iris

• The iris is the aperture of the eye, with a diameter of approximately 12 mm.
• The pupil ranges from 1-9 mm in diameter, and the iris has three layers: anterior border, stroma, and posterior epithelium.

Posterior Chamber

• The posterior chamber is located behind the iris, next to the ciliary body, and anterior to the lens.
• It contains aqueous humour produced by the ciliary processes and secreted into the posterior chamber.

Ciliary Body

• The ciliary body consists of two zones: the pars plicata (anterior) and the pars plana (posterior).
• The ciliary muscle is located in the ciliary stroma.

Lens

• The lens has three main structures: capsule, cortex, and nucleus.
• The lens epithelium is only present on the anterior surface, and produces lens fibres.

Posterior/Vitreous Chamber

• The posterior/vitreous chamber is located posterior to the lens and anterior to the retina, occupying approximately 80% of the globe's volume.
• It contains vitreous humour, which is enclosed by the vitreous/hyaloid membrane.

Retina

• The retina is a light-sensing film, approximately 0.5 mm thick, with a diameter of 30-40 mm.
• It is subdivided into inner and outer retina based on vascular supply.

Acknowledgement of Country

• The University of Western Australia acknowledges that its campus is situated on Noongar land, and that Noongar people remain the spiritual and cultural custodians of their land.

Anatomy of the Eye

The Globe

• The globe has 3 coats (tunics): outer (fibrous tunic), middle (vascular tunic/uvea), and inner (retina).
• Fluid layers maintain the globe's structure and function: tear film, aqueous humour, and vitreous humour.
• Four major tissue types in the globe: epithelial, connective, muscular, and neural tissues.

Cornea

• The cornea is 12 mm horizontally and 11 mm vertically, with a radius of curvature of approximately 8 mm.
• The cornea has 5 layers: non-keratinised stratified epithelium, Bowman's layer, stroma, Descemet's membrane, and endothelium.

Limbus

• The limbus is the junction between the cornea and sclera, approximately 1.75 mm wide.
• There is a sudden flattening of curvature at the limbus, forming the scleral sulcus.

Conjunctiva

• The conjunctiva is the outermost layer of the globe, covering the posterior surface of the lids.
• It is a loose mucous membrane containing blood vessels, nerves, lymphatics, and secretory glands.
• The conjunctiva has four components: forniceal, orbital, tarsal, and bulbar.

Tenon's Capsule

• Tenon's capsule is a dense connective tissue covering the eyeball (excluding the cornea) and extraocular muscles.
• It supports and protects the globe, forming a socket for the globe to move.

Episclera

• The episclera is a loose connective tissue beneath Tenon's capsule and above the sclera.
• It contains two vascular networks: the superficial plexus and the deep plexus, which provide nutrients to the sclera.

Sclera

• The sclera has three layers: episclera, scleral stroma, and lamina fusca.
• It is mainly composed of type I collagen and has a radius of curvature of approximately 12 mm.
• The sclera is the main load-bearing connective tissue, providing stable mechanical support.

Anterior Chamber

• The anterior chamber contains the cornea, iris, limbus, and drainage structures.
• The anterior chamber depth is approximately 3 mm, with an angle of 45°.

Anterior Chamber Angle

• The anterior chamber angle contains the trabecular meshwork, which facilitates drainage of the aqueous humour.

Iris

• The iris is the aperture of the eye, with a diameter of approximately 12 mm.
• The pupil ranges from 1-9 mm in diameter, and the iris has three layers: anterior border, stroma, and posterior epithelium.

Posterior Chamber

• The posterior chamber is located behind the iris, next to the ciliary body, and anterior to the lens.
• It contains aqueous humour produced by the ciliary processes and secreted into the posterior chamber.

Ciliary Body

• The ciliary body consists of two zones: the pars plicata (anterior) and the pars plana (posterior).
• The ciliary muscle is located in the ciliary stroma.

Lens

• The lens has three main structures: capsule, cortex, and nucleus.
• The lens epithelium is only present on the anterior surface, and produces lens fibres.

Posterior/Vitreous Chamber

• The posterior/vitreous chamber is located posterior to the lens and anterior to the retina, occupying approximately 80% of the globe's volume.
• It contains vitreous humour, which is enclosed by the vitreous/hyaloid membrane.

Retina

• The retina is a light-sensing film, approximately 0.5 mm thick, with a diameter of 30-40 mm.
• It is subdivided into inner and outer retina based on vascular supply.

Eyelid Anatomy and Function

• The eyelid is divided into three parts: orbital, preseptal, and pretarsal orbicularis muscles, which control eyelid movement and tear drainage.
• The orbicularis oculi muscle has three components: orbital orbicularis, preseptal orbicularis, and pretarsal orbicularis.

Muscles of the Eyelid

• The orbicularis oculi muscle is responsible for blinking, winking, and moving tears towards the drainage system.
• The muscle of Müller (superior tarsal muscle) is a smooth muscle that maintains upper eyelid elevation.
• The frontalis muscle is vertically orientated and draws the scalp forward, raising eyebrows.
• The corrugator muscle moves the brow medially, creating vertical furrows.
• The procerus muscle pulls the medial portion of the eyebrow inferiorly, producing horizontal furrows over the bridge of the nose.

Fascia and Fat

• Periorbita is a dense connective tissue and periosteum of the bony orbit.
• The capsule of Tenon is a fascial sheath surrounding the globe and optic nerve.
• Fat in the eyelids is composed of skin, muscle, fibrous tissue, fat, mucous membrane, meibomian glands, and cilia (eyelashes).

Tear Film

• The tear film is 7-10 µm thick and has three layers: mucin, water, and lipid.
• The functions of the tear film include ocular surface lubrication, debris removal, oxygen supply to the cornea, smooth refractive surface, antibacterial substances, and growth factors for repair.
• The lipid layer slows evaporation of the tear film, the aqueous layer provides protection with antibacterial molecules and growth factors, and the mucin layer binds the tear film to the ocular surface.

Tear Drainage

• Tear drainage occurs through the puncta, canaliculi, nasolacrimal sac, and nasolacrimal duct.

Muscle Histology

• Striated muscle:
  • Named for the arrangement of contractile proteins giving a histological appearance of prominent cross-striations
  • Specialized for forceful contractions of short duration
  • Found in skeletal muscles and extraocular muscles
• Smooth muscle:
  • Histological appearance does not show cross-striations
  • Specialized for continuous contraction of relatively low force
  • Found in blood vessels, ciliary muscle, and iris sphincter/dilator
• Cardiac muscle:
  • Structural and functional characteristics intermediate between striated and smooth muscle
  • Provides continuous, rhythmic contraction of the heart

Striated Muscle: Gross Anatomy

• Entire muscle enveloped in a connective tissue sheath, the epimysium
• Groups of muscle fibers arranged into fascicles separated by a connective tissue sheath, the perimysium
• Individual muscle fibers surrounded by endomysium
• Muscle fibers made up of longitudinally aligned myofibrils, which are composed of repeating sarcomeres
• Fibre: >1000 myofibril
• Sarcomere: alternating myosin + actin
• Tendon (dense, regular CT) attaches muscle to bone

Smooth Muscle

• Forms sheets/bundles/layers
• Contractile proteins not organized into regular sarcomeres, resulting in a smooth, unstriated appearance
• Smooth muscle cell, equivalent to striated muscle fibers
• Length: 12-200 µm
• Gap junction between cells allows electrical excitation to pass, producing coordinated waves of contraction
• Slow to contract and relax
• No tendon equivalent, dense plaques attach cell to the surrounding extracellular matrix/reticular layer
• Cell-cell attachment via desmosomes, formed by two adjacent dense plaques

Extraocular Muscles (EOM)

• 7 muscles: levator palpebrae, 2 obliques, and 4 recti
• Striated muscle, differing from skeletal muscle in fewer muscle fibers, denser blood supply, and more delicate connective tissue sheaths with more elastic fibers
• Two distinct regions: outer, orbital layer (40-60% myofibers) and inner, global layer
• Fiber types:
  • Orbital layer: high number of mitochondria, small myofibrils, fast twitch, and fatigue-resistant
  • Global layer: slow twitch, fatigue-resistant, and large myofibrils
• Insertion: differing distances from the limbus, with the medial rectus inserting closest and the superior rectus inserting farthest
• Innervation:
  • CN III (Oculomotor Nerve): superior rectus, inferior rectus, medial rectus, and inferior oblique
  • CN IV (Trochlear Nerve): superior oblique
  • CN VI (Abducens Nerve): lateral rectus
• Blood supply:
  • Superior (lateral) branch of ophthalmic artery: superior rectus, lateral rectus, and superior oblique
  • Inferior (medial) branch of the ophthalmic artery: inferior rectus, medial rectus, and inferior oblique
• Actions:
  • Medial rectus: adduction
  • Lateral rectus: abduction
  • Superior rectus: elevation, incyclotorsion, and adduction
  • Inferior rectus: depression, excyclotorsion, and adduction
  • Superior oblique: incyclotorsion, depression, and abduction
  • Inferior oblique: excyclotorsion, elevation, and abduction

Other Muscles in the Orbit

• Levator muscle: responsible for raising the upper lid, originates at the orbital apex, and inserts into the upper lid posterior to the orbital septum and attaches to the anterior face of the superior tarsal plate
• Müller's muscle: smooth muscle that arises from the inferior surface of the levator muscle, inserts into the upper edge of the superior tarsal plate, and is responsible for increasing the width of the palpebral aperture during fear or excitement

Muscle Histology

• Striated muscle:
  • Named for the arrangement of contractile proteins giving a histological appearance of prominent cross-striations
  • Specialized for forceful contractions of short duration
  • Found in skeletal muscles and extraocular muscles
• Smooth muscle:
  • Histological appearance does not show cross-striations
  • Specialized for continuous contraction of relatively low force
  • Found in blood vessels, ciliary muscle, and iris sphincter/dilator
• Cardiac muscle:
  • Structural and functional characteristics intermediate between striated and smooth muscle
  • Provides continuous, rhythmic contraction of the heart

Striated Muscle: Gross Anatomy

• Entire muscle enveloped in a connective tissue sheath, the epimysium
• Groups of muscle fibers arranged into fascicles separated by a connective tissue sheath, the perimysium
• Individual muscle fibers surrounded by endomysium
• Muscle fibers made up of longitudinally aligned myofibrils, which are composed of repeating sarcomeres
• Fibre: >1000 myofibril
• Sarcomere: alternating myosin + actin
• Tendon (dense, regular CT) attaches muscle to bone

Smooth Muscle

• Forms sheets/bundles/layers
• Contractile proteins not organized into regular sarcomeres, resulting in a smooth, unstriated appearance
• Smooth muscle cell, equivalent to striated muscle fibers
• Length: 12-200 µm
• Gap junction between cells allows electrical excitation to pass, producing coordinated waves of contraction
• Slow to contract and relax
• No tendon equivalent, dense plaques attach cell to the surrounding extracellular matrix/reticular layer
• Cell-cell attachment via desmosomes, formed by two adjacent dense plaques

Extraocular Muscles (EOM)

• 7 muscles: levator palpebrae, 2 obliques, and 4 recti
• Striated muscle, differing from skeletal muscle in fewer muscle fibers, denser blood supply, and more delicate connective tissue sheaths with more elastic fibers
• Two distinct regions: outer, orbital layer (40-60% myofibers) and inner, global layer
• Fiber types:
  • Orbital layer: high number of mitochondria, small myofibrils, fast twitch, and fatigue-resistant
  • Global layer: slow twitch, fatigue-resistant, and large myofibrils
• Insertion: differing distances from the limbus, with the medial rectus inserting closest and the superior rectus inserting farthest
• Innervation:
  • CN III (Oculomotor Nerve): superior rectus, inferior rectus, medial rectus, and inferior oblique
  • CN IV (Trochlear Nerve): superior oblique
  • CN VI (Abducens Nerve): lateral rectus
• Blood supply:
  • Superior (lateral) branch of ophthalmic artery: superior rectus, lateral rectus, and superior oblique
  • Inferior (medial) branch of the ophthalmic artery: inferior rectus, medial rectus, and inferior oblique
• Actions:
  • Medial rectus: adduction
  • Lateral rectus: abduction
  • Superior rectus: elevation, incyclotorsion, and adduction
  • Inferior rectus: depression, excyclotorsion, and adduction
  • Superior oblique: incyclotorsion, depression, and abduction
  • Inferior oblique: excyclotorsion, elevation, and abduction

Other Muscles in the Orbit

• Levator muscle: responsible for raising the upper lid, originates at the orbital apex, and inserts into the upper lid posterior to the orbital septum and attaches to the anterior face of the superior tarsal plate
• Müller's muscle: smooth muscle that arises from the inferior surface of the levator muscle, inserts into the upper edge of the superior tarsal plate, and is responsible for increasing the width of the palpebral aperture during fear or excitement

The Nervous System

• Divided into two sub-systems:
  • Central nervous system (CNS): brain and spinal cord
  • Peripheral nervous system (PNS): spinal and cranial nerves
• Responsible for sending, receiving, and interpreting messages from the body
• CNS: processing center, regulates complex functions throughout the body, assists the whole body to adapt quickly

Peripheral Nervous System (PNS)

• Sends and receives info to/from CNS
• Divided into two components:
  • Autonomic nervous system (involuntary)
  • Somatic nervous system (voluntary)
• Autonomic nervous system:
  • Controls involuntary actions (e.g., heart rate, digestion)
  • Divided into three parts:
    • Sympathetic nervous system: mobilization of energy stores for increased activity (fight-or-flight)
    • Parasympathetic nervous system: conservation of energy stores
    • Enteric nervous system: neurons intrinsic to the gastrointestinal tract
• Somatic nervous system:
  • Controls voluntary motor fibers to skeletal muscle
  • Afferent/sensory nerves and efferent/motor nerves

Cranial Nerves

• 12 pairs of cranial nerves
• Transmit sensory information from the head and neck to the brain
• Control the actions of the head and neck
• Organization:
  • Motor and/or sensory nerves
  • Comprise a central nucleus in the brain and a peripheral nerve fiber

Cranial Nerve Functions

• CN I – Olfactory nerve: olfaction (sensory)
• CN II – Optic nerve: vision (sensory)
• CN III – Oculomotor nerve: movement of eye, elevation of upper lid (motor), pupillary constriction and accommodation (parasympathetic)
• CN IV – Trochlear nerve: movement of eye (motor)
• CN V – Trigeminal nerve:
  • Ophthalmic branch: sensation from forehead, scalp, eyelids, nose, and eye (cornea and conjunctiva)
  • Maxillary branch: sensation from skin of anterior temporal and upper cheek, palate, teeth, maxillary sinus
  • Mandibular branch: motor to masticatory muscles, sensory to anterior 2/3 of tongue, skin posterior temporal, cheek, chin, teeth, lower jaw
• ... (functions of remaining cranial nerves)

Additional Important Concepts

• Oculocardiac reflex: decrease in heart rate following globe pressure or traction of extraocular muscles, causing dizziness and fainting, notable in strabismus surgery

The Nervous System

• Divided into two sub-systems:
  • Central nervous system (CNS): brain and spinal cord
  • Peripheral nervous system (PNS): spinal and cranial nerves
• Responsible for sending, receiving, and interpreting messages from the body
• CNS: processing center, regulates complex functions throughout the body, assists the whole body to adapt quickly

Peripheral Nervous System (PNS)

• Sends and receives info to/from CNS
• Divided into two components:
  • Autonomic nervous system (involuntary)
  • Somatic nervous system (voluntary)
• Autonomic nervous system:
  • Controls involuntary actions (e.g., heart rate, digestion)
  • Divided into three parts:
    • Sympathetic nervous system: mobilization of energy stores for increased activity (fight-or-flight)
    • Parasympathetic nervous system: conservation of energy stores
    • Enteric nervous system: neurons intrinsic to the gastrointestinal tract
• Somatic nervous system:
  • Controls voluntary motor fibers to skeletal muscle
  • Afferent/sensory nerves and efferent/motor nerves

Cranial Nerves

• 12 pairs of cranial nerves
• Transmit sensory information from the head and neck to the brain
• Control the actions of the head and neck
• Organization:
  • Motor and/or sensory nerves
  • Comprise a central nucleus in the brain and a peripheral nerve fiber

Cranial Nerve Functions

• CN I – Olfactory nerve: olfaction (sensory)
• CN II – Optic nerve: vision (sensory)
• CN III – Oculomotor nerve: movement of eye, elevation of upper lid (motor), pupillary constriction and accommodation (parasympathetic)
• CN IV – Trochlear nerve: movement of eye (motor)
• CN V – Trigeminal nerve:
  • Ophthalmic branch: sensation from forehead, scalp, eyelids, nose, and eye (cornea and conjunctiva)
  • Maxillary branch: sensation from skin of anterior temporal and upper cheek, palate, teeth, maxillary sinus
  • Mandibular branch: motor to masticatory muscles, sensory to anterior 2/3 of tongue, skin posterior temporal, cheek, chin, teeth, lower jaw
• ... (functions of remaining cranial nerves)

Additional Important Concepts

• Oculocardiac reflex: decrease in heart rate following globe pressure or traction of extraocular muscles, causing dizziness and fainting, notable in strabismus surgery

Blood Supply to the Eye

• Blood to the eye comes from the right atrium, through the pulmonary artery, lung, pulmonary vein, left atrium, left ventricle, and aorta.
• The right and left internal carotid arteries supply the right and left eyes, respectively.
• The internal carotid artery runs through the neck, enters the cranial vault via the carotid canal, and then into the cavernous sinus.
• It bifurcates into the ophthalmic artery as it exits the cavernous sinus.

Ophthalmic Artery

• The ophthalmic artery enters the orbit via the optic canal (along with the optic nerve) and bifurcates into multiple arteries.
• The central retinal artery is the first branch of the ophthalmic artery.
• It pierces the dura ~ 1 cm from the globe and enters the optic nerve.
• The central retinal artery has many collateral branches that serve the optic nerve fibers prior to the optic nerve head.
• It exits the optic nerve and is visible when viewing the fundus.

Inner Retinal Supply

• The inner retina has a dual blood supply: short posterior choroid and anterior optic nerve, and long posterior choroid and ciliary body.
• The cilioretinal artery arises from the short posterior ciliary artery and makes a characteristic bend as it leaves the disk margin.
• It is present in ~30% of the population and supplies the fovea.

Superficial Vascular Complex

• The superficial vascular complex consists of the radial peripapillary capillary plexus, superficial vascular plexus, intermediate capillary plexus, and deep capillary plexus.

Inner Retinal Venous Return

• The post-capillary venules drain into the branched retinal veins, which then drain into the central retinal vein.
• The central retinal vein empties into the superior ophthalmic vein or cavernous sinus (different between people).

Short Posterior Ciliary Arteries

• The short posterior ciliary arteries pierce the sclera in an annulus around the optic nerve head and supply the choroid.
• Other branches form the circle of Zinn Haller to supply the optic nerve.

Long Posterior Ciliary Arteries

• The long posterior ciliary arteries pierce the posterior sclera to supply the choroid.
• They then join the major circle of the iris to supply the ciliary body and iris.

Muscular Arteries

• The muscular arteries arise from the ophthalmic artery and have two branches:
  • Superior branch, which runs laterally and supplies the levator, superior rectus, superior oblique, and partially lateral rectus.
  • Inferior branch, which runs medially and supplies the inferior rectus, inferior oblique, and anterior ciliary arteries.

Anterior Ciliary Arteries

• The anterior ciliary arteries branch from the muscular arteries.
• One branch joins the major circle of the iris, while the other branch supplies the episclera and conjunctiva.

Lateral Branches

• The lacrimal artery is the 2nd branch off the ophthalmic artery and runs along the orbital wall with the lateral rectus.
• It has three branches:
  • Muscular branch, which supplies the lateral and superior rectus muscle.
  • Glandular branch, which supplies the lacrimal gland.
  • Recurrent branch, which anastomoses with the meninges.

Zygomatic Artery

• The zygomatic artery is a branch off the lacrimal artery before the lacrimal gland.
• It supplies the orbicularis and other facial muscles.

Lateral Palpebral Artery

• The lateral palpebral artery is the terminal branch of the lacrimal artery.
• It supplies the lower and upper lids.
• It anastomoses with the medial palpebral artery.

Medial Branches

• The supraorbital artery arises from the ophthalmic artery and supplies the periosteum of the frontal bone, skin of the forehead, superior rectus, and palpebrae superioris.
• The supratrochlear artery arises from the ophthalmic artery and supplies the forehead and scalp.
• The dorsal nasal artery is the terminal branch of the lacrimal artery and supplies the lacrimal sac and nose.

Conjunctival/Episcleral Return

• The conjunctival/episcleral return drains to the anterior ciliary veins, which then drain into the ophthalmic veins.
• Aqueous humor also drains via this route.

Vortex/Vorticose Veins

• The vortex veins drain the choroid.
• Some vortex veins drain into the superior ophthalmic vein, while others drain into the inferior ophthalmic vein.
• The superior ophthalmic vein exits via the superior orbital fissure and drains into the cavernous sinus.
• The inferior ophthalmic vein exits via the inferior orbital fissure and drains into the pterygoid plexus.
• A normal eye has 4 to 8 vortex veins, at least 1 vein in each quadrant.

Summary

• Retinal drainage: central retinal vein → ophthalmic vein → cavernous sinus.
• Conjunctival/episcleral drainage: anterior ciliary vein → cavernous sinus.
• Uveal drainage: vortex vein → superior ophthalmic vein → cavernous sinus or vortex vein → inferior ophthalmic vein → pterygoid plexus.

Blood Supply to the Eye

• Blood to the eye comes from the right atrium, through the pulmonary artery, lung, pulmonary vein, left atrium, left ventricle, and aorta.
• The right and left internal carotid arteries supply the right and left eyes, respectively.
• The internal carotid artery runs through the neck, enters the cranial vault via the carotid canal, and then into the cavernous sinus.
• It bifurcates into the ophthalmic artery as it exits the cavernous sinus.

Ophthalmic Artery

• The ophthalmic artery enters the orbit via the optic canal (along with the optic nerve) and bifurcates into multiple arteries.
• The central retinal artery is the first branch of the ophthalmic artery.
• It pierces the dura ~ 1 cm from the globe and enters the optic nerve.
• The central retinal artery has many collateral branches that serve the optic nerve fibers prior to the optic nerve head.
• It exits the optic nerve and is visible when viewing the fundus.

Inner Retinal Supply

• The inner retina has a dual blood supply: short posterior choroid and anterior optic nerve, and long posterior choroid and ciliary body.
• The cilioretinal artery arises from the short posterior ciliary artery and makes a characteristic bend as it leaves the disk margin.
• It is present in ~30% of the population and supplies the fovea.

Superficial Vascular Complex

• The superficial vascular complex consists of the radial peripapillary capillary plexus, superficial vascular plexus, intermediate capillary plexus, and deep capillary plexus.

Inner Retinal Venous Return

• The post-capillary venules drain into the branched retinal veins, which then drain into the central retinal vein.
• The central retinal vein empties into the superior ophthalmic vein or cavernous sinus (different between people).

Short Posterior Ciliary Arteries

• The short posterior ciliary arteries pierce the sclera in an annulus around the optic nerve head and supply the choroid.
• Other branches form the circle of Zinn Haller to supply the optic nerve.

Long Posterior Ciliary Arteries

• The long posterior ciliary arteries pierce the posterior sclera to supply the choroid.
• They then join the major circle of the iris to supply the ciliary body and iris.

Muscular Arteries

• The muscular arteries arise from the ophthalmic artery and have two branches:
  • Superior branch, which runs laterally and supplies the levator, superior rectus, superior oblique, and partially lateral rectus.
  • Inferior branch, which runs medially and supplies the inferior rectus, inferior oblique, and anterior ciliary arteries.

Anterior Ciliary Arteries

• The anterior ciliary arteries branch from the muscular arteries.
• One branch joins the major circle of the iris, while the other branch supplies the episclera and conjunctiva.

Lateral Branches

• The lacrimal artery is the 2nd branch off the ophthalmic artery and runs along the orbital wall with the lateral rectus.
• It has three branches:
  • Muscular branch, which supplies the lateral and superior rectus muscle.
  • Glandular branch, which supplies the lacrimal gland.
  • Recurrent branch, which anastomoses with the meninges.

Zygomatic Artery

• The zygomatic artery is a branch off the lacrimal artery before the lacrimal gland.
• It supplies the orbicularis and other facial muscles.

Lateral Palpebral Artery

• The lateral palpebral artery is the terminal branch of the lacrimal artery.
• It supplies the lower and upper lids.
• It anastomoses with the medial palpebral artery.

Medial Branches

• The supraorbital artery arises from the ophthalmic artery and supplies the periosteum of the frontal bone, skin of the forehead, superior rectus, and palpebrae superioris.
• The supratrochlear artery arises from the ophthalmic artery and supplies the forehead and scalp.
• The dorsal nasal artery is the terminal branch of the lacrimal artery and supplies the lacrimal sac and nose.

Conjunctival/Episcleral Return

• The conjunctival/episcleral return drains to the anterior ciliary veins, which then drain into the ophthalmic veins.
• Aqueous humor also drains via this route.

Vortex/Vorticose Veins

• The vortex veins drain the choroid.
• Some vortex veins drain into the superior ophthalmic vein, while others drain into the inferior ophthalmic vein.
• The superior ophthalmic vein exits via the superior orbital fissure and drains into the cavernous sinus.
• The inferior ophthalmic vein exits via the inferior orbital fissure and drains into the pterygoid plexus.
• A normal eye has 4 to 8 vortex veins, at least 1 vein in each quadrant.

Summary

• Retinal drainage: central retinal vein → ophthalmic vein → cavernous sinus.
• Conjunctival/episcleral drainage: anterior ciliary vein → cavernous sinus.
• Uveal drainage: vortex vein → superior ophthalmic vein → cavernous sinus or vortex vein → inferior ophthalmic vein → pterygoid plexus.

Blood Supply to the Eye

• Blood to the eye comes from the right atrium, through the pulmonary artery, lung, pulmonary vein, left atrium, left ventricle, and aorta.
• The right and left internal carotid arteries supply the right and left eyes, respectively.
• The internal carotid artery runs through the neck, enters the cranial vault via the carotid canal, and then into the cavernous sinus.
• It bifurcates into the ophthalmic artery as it exits the cavernous sinus.

Ophthalmic Artery

• The ophthalmic artery enters the orbit via the optic canal (along with the optic nerve) and bifurcates into multiple arteries.
• The central retinal artery is the first branch of the ophthalmic artery.
• It pierces the dura ~ 1 cm from the globe and enters the optic nerve.
• The central retinal artery has many collateral branches that serve the optic nerve fibers prior to the optic nerve head.
• It exits the optic nerve and is visible when viewing the fundus.

Inner Retinal Supply

• The inner retina has a dual blood supply: short posterior choroid and anterior optic nerve, and long posterior choroid and ciliary body.
• The cilioretinal artery arises from the short posterior ciliary artery and makes a characteristic bend as it leaves the disk margin.
• It is present in ~30% of the population and supplies the fovea.

Superficial Vascular Complex

• The superficial vascular complex consists of the radial peripapillary capillary plexus, superficial vascular plexus, intermediate capillary plexus, and deep capillary plexus.

Inner Retinal Venous Return

• The post-capillary venules drain into the branched retinal veins, which then drain into the central retinal vein.
• The central retinal vein empties into the superior ophthalmic vein or cavernous sinus (different between people).

Short Posterior Ciliary Arteries

• The short posterior ciliary arteries pierce the sclera in an annulus around the optic nerve head and supply the choroid.
• Other branches form the circle of Zinn Haller to supply the optic nerve.

Long Posterior Ciliary Arteries

• The long posterior ciliary arteries pierce the posterior sclera to supply the choroid.
• They then join the major circle of the iris to supply the ciliary body and iris.

Muscular Arteries

• The muscular arteries arise from the ophthalmic artery and have two branches:
  • Superior branch, which runs laterally and supplies the levator, superior rectus, superior oblique, and partially lateral rectus.
  • Inferior branch, which runs medially and supplies the inferior rectus, inferior oblique, and anterior ciliary arteries.

Anterior Ciliary Arteries

• The anterior ciliary arteries branch from the muscular arteries.
• One branch joins the major circle of the iris, while the other branch supplies the episclera and conjunctiva.

Lateral Branches

• The lacrimal artery is the 2nd branch off the ophthalmic artery and runs along the orbital wall with the lateral rectus.
• It has three branches:
  • Muscular branch, which supplies the lateral and superior rectus muscle.
  • Glandular branch, which supplies the lacrimal gland.
  • Recurrent branch, which anastomoses with the meninges.

Zygomatic Artery

• The zygomatic artery is a branch off the lacrimal artery before the lacrimal gland.
• It supplies the orbicularis and other facial muscles.

Lateral Palpebral Artery

• The lateral palpebral artery is the terminal branch of the lacrimal artery.
• It supplies the lower and upper lids.
• It anastomoses with the medial palpebral artery.

Medial Branches

• The supraorbital artery arises from the ophthalmic artery and supplies the periosteum of the frontal bone, skin of the forehead, superior rectus, and palpebrae superioris.
• The supratrochlear artery arises from the ophthalmic artery and supplies the forehead and scalp.
• The dorsal nasal artery is the terminal branch of the lacrimal artery and supplies the lacrimal sac and nose.

Conjunctival/Episcleral Return

• The conjunctival/episcleral return drains to the anterior ciliary veins, which then drain into the ophthalmic veins.
• Aqueous humor also drains via this route.

Vortex/Vorticose Veins

• The vortex veins drain the choroid.
• Some vortex veins drain into the superior ophthalmic vein, while others drain into the inferior ophthalmic vein.
• The superior ophthalmic vein exits via the superior orbital fissure and drains into the cavernous sinus.
• The inferior ophthalmic vein exits via the inferior orbital fissure and drains into the pterygoid plexus.
• A normal eye has 4 to 8 vortex veins, at least 1 vein in each quadrant.

Summary

• Retinal drainage: central retinal vein → ophthalmic vein → cavernous sinus.
• Conjunctival/episcleral drainage: anterior ciliary vein → cavernous sinus.
• Uveal drainage: vortex vein → superior ophthalmic vein → cavernous sinus or vortex vein → inferior ophthalmic vein → pterygoid plexus.

Lid Anatomy

• Comprised of skin, muscle, fibrous tissue, mucous membrane, meibomian glands, and cilia (eyelashes)
• Skin: stratified squamous keratinized epithelium, transitions to cuboidal epithelial cells of the palpebral conjunctiva at the mucocutaneous junction
• Lid wiper epitheliopathy: increased friction between lid wiper and ocular/contact lens surface

Muscles of the Lid

• Orbicularis oculi: most anterior muscle group, consists of palpebral (preseptal, pretarsal) and orbital portions
• Riolan's muscle: prevents lipid expression unless eyelids touch during blink
• Levator: elevates upper eyelid
• Müller's muscle (aka superior tarsal muscle): smooth muscle, sympathetic innervation, maintains upper lid elevation
• Upper lid responsible for most of the palpebral opening

Ligaments and Tendons

• Lids muscles attached laterally and medially to tendons
• Transverse ligaments within the lids (affects eyelid apposition if becomes tight/loose)

Innervation

• Sensory: trigeminal (ophthalmic [V1] and maxillary [V2] division)
• Motor:
  • Levator: CNIII, superior division
  • Orbicularis: CNVII
  • Müller's: sympathetic nervous system, from superior cervical ganglion

Tarsal Plate

• Dense connective tissue
• Gives lid rigidity and shape
• Studded with meibomian glands

Vessels in the Tarsal Conj

• Highly vascularized, supply from lateral and medial palpebral arteries
• Clinically, inflamed tarsal conj presents as papillae/follicles

Palpebral Conjunctiva

• Consists of epithelium + stroma
• Epithelium: stratified columnar mucoepithelium, thickest at lid wiper
• Stroma: loose vascular tissue with lymphoid structures

Glands of the Lid

• Lacrimal gland:
  • Produces basal + reflex + emotional tears
• Glands of Moll & Zeis:
  • Zeis: sebaceous gland, exact function unknown
  • Moll: apocrine gland, immune defense
• Meibomian glands:
  • Sebaceous glands producing lipid layer (oil) of tear film
  • Embedded in the tarsal plate
  • Long & multilobed
  • Openings on eyelid margin
  • Upper lid: 30-40
  • Lower lid: 20-30
• Accessory lacrimal glands:
  • Contribute to 5% of water component
  • Similar functions, differ in location
  • Krause: conjunctival fornix
  • Wolfring: border of tarsal plate
  • Produce basal tears

Tear Film

• Layers:
  • Lipid layer:
    • Thinnest layer
    • Slows evaporation of the tear film
  • Aqueous layer:
    • Main bulk of tear film
    • Protection: antibacterial molecules & growth factor
  • Mucin layer:
    • Binds tears film to ocular surface
• The distinct, three-layered structure is superseded by multiple blended-phase (i.e. lipid layer, aqueous layer with graded concentration of mucin)

Lipid Layer/Meibum

• 50 – 100 nm thick
• Complex mixture of lipids and proteins
• Most stored in the meibomian gland ducts
• Released onto ocular surface with each blink
• Prevents evaporation of aqueous layer/prevents sebum from lashes entering tear film

Aqueous Layer

• Bulk of the tears
• Complex mixture of water, electrolytes, proteins, and antimicrobials
• Provides O2 and nutrients to the cornea
• Note that there is a GRADED concentration of mucin within the aqueous layer, not separate aqueous and mucin layers

Mucin Layer

• Produced by goblet cells in the conjunctiva
• Distribution varies amongst species
• In humans, highest density in nasal region
• Goblet cells secrete mucin via apocrine secretion

Lid Anatomy

• Comprised of skin, muscle, fibrous tissue, mucous membrane, meibomian glands, and cilia (eyelashes)
• Skin: stratified squamous keratinized epithelium, transitions to cuboidal epithelial cells of the palpebral conjunctiva at the mucocutaneous junction
• Lid wiper epitheliopathy: increased friction between lid wiper and ocular/contact lens surface

Muscles of the Lid

• Orbicularis oculi: most anterior muscle group, consists of palpebral (preseptal, pretarsal) and orbital portions
• Riolan's muscle: prevents lipid expression unless eyelids touch during blink
• Levator: elevates upper eyelid
• Müller's muscle (aka superior tarsal muscle): smooth muscle, sympathetic innervation, maintains upper lid elevation
• Upper lid responsible for most of the palpebral opening

Ligaments and Tendons

• Lids muscles attached laterally and medially to tendons
• Transverse ligaments within the lids (affects eyelid apposition if becomes tight/loose)

Innervation

• Sensory: trigeminal (ophthalmic [V1] and maxillary [V2] division)
• Motor:
  • Levator: CNIII, superior division
  • Orbicularis: CNVII
  • Müller's: sympathetic nervous system, from superior cervical ganglion

Tarsal Plate

• Dense connective tissue
• Gives lid rigidity and shape
• Studded with meibomian glands

Vessels in the Tarsal Conj

• Highly vascularized, supply from lateral and medial palpebral arteries
• Clinically, inflamed tarsal conj presents as papillae/follicles

Palpebral Conjunctiva

• Consists of epithelium + stroma
• Epithelium: stratified columnar mucoepithelium, thickest at lid wiper
• Stroma: loose vascular tissue with lymphoid structures

Glands of the Lid

• Lacrimal gland:
  • Produces basal + reflex + emotional tears
• Glands of Moll & Zeis:
  • Zeis: sebaceous gland, exact function unknown
  • Moll: apocrine gland, immune defense
• Meibomian glands:
  • Sebaceous glands producing lipid layer (oil) of tear film
  • Embedded in the tarsal plate
  • Long & multilobed
  • Openings on eyelid margin
  • Upper lid: 30-40
  • Lower lid: 20-30
• Accessory lacrimal glands:
  • Contribute to 5% of water component
  • Similar functions, differ in location
  • Krause: conjunctival fornix
  • Wolfring: border of tarsal plate
  • Produce basal tears

Tear Film

• Layers:
  • Lipid layer:
    • Thinnest layer
    • Slows evaporation of the tear film
  • Aqueous layer:
    • Main bulk of tear film
    • Protection: antibacterial molecules & growth factor
  • Mucin layer:
    • Binds tears film to ocular surface
• The distinct, three-layered structure is superseded by multiple blended-phase (i.e. lipid layer, aqueous layer with graded concentration of mucin)

Lipid Layer/Meibum

• 50 – 100 nm thick
• Complex mixture of lipids and proteins
• Most stored in the meibomian gland ducts
• Released onto ocular surface with each blink
• Prevents evaporation of aqueous layer/prevents sebum from lashes entering tear film

Aqueous Layer

• Bulk of the tears
• Complex mixture of water, electrolytes, proteins, and antimicrobials
• Provides O2 and nutrients to the cornea
• Note that there is a GRADED concentration of mucin within the aqueous layer, not separate aqueous and mucin layers

Mucin Layer

• Produced by goblet cells in the conjunctiva
• Distribution varies amongst species
• In humans, highest density in nasal region
• Goblet cells secrete mucin via apocrine secretion

Lid Anatomy

• Comprised of skin, muscle, fibrous tissue, mucous membrane, meibomian glands, and cilia (eyelashes)
• Skin: stratified squamous keratinized epithelium, transitions to cuboidal epithelial cells of the palpebral conjunctiva at the mucocutaneous junction
• Lid wiper epitheliopathy: increased friction between lid wiper and ocular/contact lens surface

Muscles of the Lid

• Orbicularis oculi: most anterior muscle group, consists of palpebral (preseptal, pretarsal) and orbital portions
• Riolan's muscle: prevents lipid expression unless eyelids touch during blink
• Levator: elevates upper eyelid
• Müller's muscle (aka superior tarsal muscle): smooth muscle, sympathetic innervation, maintains upper lid elevation
• Upper lid responsible for most of the palpebral opening

Ligaments and Tendons

• Lids muscles attached laterally and medially to tendons
• Transverse ligaments within the lids (affects eyelid apposition if becomes tight/loose)

Innervation

• Sensory: trigeminal (ophthalmic [V1] and maxillary [V2] division)
• Motor:
  • Levator: CNIII, superior division
  • Orbicularis: CNVII
  • Müller's: sympathetic nervous system, from superior cervical ganglion

Tarsal Plate

• Dense connective tissue
• Gives lid rigidity and shape
• Studded with meibomian glands

Vessels in the Tarsal Conj

• Highly vascularized, supply from lateral and medial palpebral arteries
• Clinically, inflamed tarsal conj presents as papillae/follicles

Palpebral Conjunctiva

• Consists of epithelium + stroma
• Epithelium: stratified columnar mucoepithelium, thickest at lid wiper
• Stroma: loose vascular tissue with lymphoid structures

Glands of the Lid

• Lacrimal gland:
  • Produces basal + reflex + emotional tears
• Glands of Moll & Zeis:
  • Zeis: sebaceous gland, exact function unknown
  • Moll: apocrine gland, immune defense
• Meibomian glands:
  • Sebaceous glands producing lipid layer (oil) of tear film
  • Embedded in the tarsal plate
  • Long & multilobed
  • Openings on eyelid margin
  • Upper lid: 30-40
  • Lower lid: 20-30
• Accessory lacrimal glands:
  • Contribute to 5% of water component
  • Similar functions, differ in location
  • Krause: conjunctival fornix
  • Wolfring: border of tarsal plate
  • Produce basal tears

Tear Film

• Layers:
  • Lipid layer:
    • Thinnest layer
    • Slows evaporation of the tear film
  • Aqueous layer:
    • Main bulk of tear film
    • Protection: antibacterial molecules & growth factor
  • Mucin layer:
    • Binds tears film to ocular surface
• The distinct, three-layered structure is superseded by multiple blended-phase (i.e. lipid layer, aqueous layer with graded concentration of mucin)

Lipid Layer/Meibum

• 50 – 100 nm thick
• Complex mixture of lipids and proteins
• Most stored in the meibomian gland ducts
• Released onto ocular surface with each blink
• Prevents evaporation of aqueous layer/prevents sebum from lashes entering tear film

Aqueous Layer

• Bulk of the tears
• Complex mixture of water, electrolytes, proteins, and antimicrobials
• Provides O2 and nutrients to the cornea
• Note that there is a GRADED concentration of mucin within the aqueous layer, not separate aqueous and mucin layers

Mucin Layer

• Produced by goblet cells in the conjunctiva
• Distribution varies amongst species
• In humans, highest density in nasal region
• Goblet cells secrete mucin via apocrine secretion

Structure of the Conjunctiva

• The conjunctiva consists of 2 layers: epithelium and stroma.
• Epithelium: 2-7 cell layers thick, depending on location, and contains goblet cells.
• Stroma: a connective tissue layer beneath the epithelium, housing blood vessels, nerves, and specialized cells.

Types of Conjunctiva

• Marginal conjunctiva: a transition zone from skin to conjunctiva, consisting of 5-7 layers of non-keratinized stratified squamous epithelium.
• Palpebral conjunctiva: firmly attached to the inner surface of eyelids, consisting of 2 layers: superficial columnar and deep cuboidal.
• Palpebral conjunctiva (orbital component): a transition zone between tarsal conjunctiva and forniceal conjunctiva, with an increase in loose submucosal tissue and containing a large number of mucosa-associated lymphoid tissue.
• Forniceal conjunctiva: a very loose tissue forming a pocket, with 3 layers of stratified squamous epithelium and containing a high number of goblet cells.
• Bulbar conjunctiva: the thinnest stroma of all conjunctiva, almost transparent, with 3 layers of stratified epithelium and loosely attached to the sclera.
• Limbus: the junction between conjunctiva and cornea, fusing with corneal epithelium, with ~10 layers of squamous epithelium and containing a capillary network that feeds the avascular cornea.

Specialized Cells in the Conjunctiva

• Goblet cells: secrete mucin.
• Langerhans cells: immune cells, with high density in tarsal, forniceal, and bulbar conjunctiva.

Blood Supply to the Conjunctiva

• The conjunctiva has 2 different sources of blood supply: palpebral arcades and conjunctival arteries.
• Palpebral arcades: supply most of the conjunctiva, with 2 arcades: marginal tarsal arcades and peripheral tarsal arcades.
• Conjunctival arteries: branch of the anterior ciliary artery, supplying the bulbar conjunctiva in the limbal area.

Clinical Relevance

• Conjunctival inflammation: engorged superficial vessels, bulbar conjunctiva hyperaemic, and characteristic ciliary flush (perilimbal injection).
• Subconjunctival haemorrhage (ecchymosis): caused by multiple blood vessels within the conjunctiva, bleeding between the conjunctiva and sclera.

Venous Drainage

• Palpebral and forniceal conjunctiva: veins from the conjunctiva drain into the venous plexus of the eyelids, then into the superior/inferior ophthalmic veins, and finally into the cavernous sinus.
• Bulbar conjunctiva: veins from the bulbar conjunctiva drain into the circumcorneal zone of veins, then into the anterior ciliary veins, ophthalmic veins, and finally into the cavernous sinus.

Conjunctival Lymph Drainage

• Lymph drains laterally to the superficial parotid nodes and medially to the submandibular nodes.

Conjunctival Sensory Innervation

• Bulbar conjunctiva: innervated by long ciliary nerves.
• Superior palpebral and forniceal conjunctiva: innervated by the frontal and lacrimal branches of the ophthalmic division of the trigeminal nerve.
• Inferior palpebral and forniceal conjunctiva: innervated by the lacrimal branch of the ophthalmic division and the infraorbital branch of the maxillary division.

Structure of the Conjunctiva

• The conjunctiva consists of 2 layers: epithelium and stroma.
• Epithelium: 2-7 cell layers thick, depending on location, and contains goblet cells.
• Stroma: a connective tissue layer beneath the epithelium, housing blood vessels, nerves, and specialized cells.

Types of Conjunctiva

• Marginal conjunctiva: a transition zone from skin to conjunctiva, consisting of 5-7 layers of non-keratinized stratified squamous epithelium.
• Palpebral conjunctiva: firmly attached to the inner surface of eyelids, consisting of 2 layers: superficial columnar and deep cuboidal.
• Palpebral conjunctiva (orbital component): a transition zone between tarsal conjunctiva and forniceal conjunctiva, with an increase in loose submucosal tissue and containing a large number of mucosa-associated lymphoid tissue.
• Forniceal conjunctiva: a very loose tissue forming a pocket, with 3 layers of stratified squamous epithelium and containing a high number of goblet cells.
• Bulbar conjunctiva: the thinnest stroma of all conjunctiva, almost transparent, with 3 layers of stratified epithelium and loosely attached to the sclera.
• Limbus: the junction between conjunctiva and cornea, fusing with corneal epithelium, with ~10 layers of squamous epithelium and containing a capillary network that feeds the avascular cornea.

Specialized Cells in the Conjunctiva

• Goblet cells: secrete mucin.
• Langerhans cells: immune cells, with high density in tarsal, forniceal, and bulbar conjunctiva.

Blood Supply to the Conjunctiva

• The conjunctiva has 2 different sources of blood supply: palpebral arcades and conjunctival arteries.
• Palpebral arcades: supply most of the conjunctiva, with 2 arcades: marginal tarsal arcades and peripheral tarsal arcades.
• Conjunctival arteries: branch of the anterior ciliary artery, supplying the bulbar conjunctiva in the limbal area.

Clinical Relevance

• Conjunctival inflammation: engorged superficial vessels, bulbar conjunctiva hyperaemic, and characteristic ciliary flush (perilimbal injection).
• Subconjunctival haemorrhage (ecchymosis): caused by multiple blood vessels within the conjunctiva, bleeding between the conjunctiva and sclera.

Venous Drainage

• Palpebral and forniceal conjunctiva: veins from the conjunctiva drain into the venous plexus of the eyelids, then into the superior/inferior ophthalmic veins, and finally into the cavernous sinus.
• Bulbar conjunctiva: veins from the bulbar conjunctiva drain into the circumcorneal zone of veins, then into the anterior ciliary veins, ophthalmic veins, and finally into the cavernous sinus.

Conjunctival Lymph Drainage

• Lymph drains laterally to the superficial parotid nodes and medially to the submandibular nodes.

Conjunctival Sensory Innervation

• Bulbar conjunctiva: innervated by long ciliary nerves.
• Superior palpebral and forniceal conjunctiva: innervated by the frontal and lacrimal branches of the ophthalmic division of the trigeminal nerve.
• Inferior palpebral and forniceal conjunctiva: innervated by the lacrimal branch of the ophthalmic division and the infraorbital branch of the maxillary division.

Sclera

• Whish outer coat of the eye
• Derived from Greek "skleros" meaning hard
• Dense, irregular connective tissue (CT)

Functions of Sclera

• Protection: tough barrier separating intraocular structures from orbital walls and the outside world
• Refractive stability: keeps eye length stable during changes in intraocular pressure and blood pressure, and during eye movements and accommodation
• Conduit for nerves and vessels: allows nerve and vessel access to underlying ocular structures
• Anchor for extraocular muscles: provides stable anchorage for extraocular muscle tendons, contributing to stability of torsional movements
• Anchor for ciliary muscle: opposes contractile forces of ciliary muscle, allowing stable accommodation
• Facilitates ocular drainage: majority of aqueous outflow through intrascleral venous plexus to the episcleral plexus

Openings in the Sclera

• Minor foramina: allow blood vessels and nerves to pierce the eyeball
• Two major foramina: anterior scleral foramen (cornea) and posterior scleral foramen (scleral canal)

Layers of the Sclera

• Episclera: outermost layer, merges with the underlying scleral stroma
• Scleral stroma: dense, irregular CT
• Lamina fusca: innermost layer, regarded as part of choroid by some, but separated from choroid by perichoroidal space

Biochemical Composition of the Sclera

• Collagen (around 90% of scleral dry weight, predominantly type I)
• Proteoglycans and glycosaminoglycans (around 1% of dry weight)
• Glycoproteins (e.g. fibronectin)
• Fibroblasts, myofibroblasts, and some melanocytes
• Non-collagenous proteins (e.g. elastin, proteases, etc.)
• Water

Scleral Cells

• Fibroblasts: synthesise collagen, elastic, and reticular fibres, reside close to collagen matrices
• Myofibroblasts: modified fibroblasts, synthesise collagen, express contractile proteins (e.g. α-smooth muscle actin), present in mammals, including humans

Scleral Collagens

• Structural unit: tropocollagen/collagen (~3 intertwined polypeptide chains, rich in amino acids proline and hydroxyproline)
• Staggered collagen molecules assemble into a fibril

Scleral Proteoglycans

• ~0.7-0.9% of the total scleral dry weight
• Modulate collagen fibril assembly and arrangement
• Aggrecan, biglycan, and decorin: sequester water within the scleral ECM, highest concentration in the posterior sclera, possibly maintaining posterior pole flexibility