Makindo Medical Notes

Related Subjects: |Anatomy of the Ear |Anatomy of the Oesophagus |Anatomy of the Diaphragm |Anatomy of Large Bowel |Anatomy of Small Bowel |Anatomy of the Biliary system |Anatomy of the Eye

Anatomy of the Eye

• Eyeball (Globe): The eye is a spherical structure approximately 24 mm in diameter in adults, composed of three main layers:
  • Fibrous Tunic: The outermost layer, consisting of the sclera (the white part of the eye) and the cornea (the transparent front part).
  • Vascular Tunic (Uvea): The middle layer, consisting of:
    • Iris: The coloured part of the eye, responsible for controlling the diameter of the pupil and regulating light entry.
    • Ciliary Body: Produces aqueous humor and contains muscles that control the shape of the lens (accommodation).
    • Choroid: A vascular layer that supplies oxygen and nutrients to the outer retina.
  • Retina: The innermost layer, consisting of light-sensitive cells (photoreceptors), including rods and cones, which convert light into electrical signals sent to the brain.
• Cornea: A transparent, avascular structure covering the anterior part of the eye, responsible for most of the eye's refractive power.
• Aqueous Humor: A clear fluid produced by the ciliary body, filling the anterior chamber (between the cornea and iris) and posterior chamber (between the iris and lens).
• Lens: A transparent, biconvex structure located behind the iris that focuses light onto the retina.
• Pupil: The central opening of the iris, through which light enters the eye.
• Vitreous Humor: A gel-like substance that fills the vitreous chamber (the space between the lens and retina), helping to maintain the shape of the eyeball.
• Macula: A specialized area of the retina responsible for sharp, central vision. The fovea at its center contains a high concentration of cones for colour vision.
• Optic Disc: The point where the optic nerve exits the eye, often called the "blind spot" because it lacks photoreceptors.
• Optic Nerve (Cranial Nerve II): Transmits visual information from the retina to the brain.

Anatomy of the Orbit

• The orbit is a bony cavity that houses the eye and its associated structures, including muscles, nerves, blood vessels, and fat. Each orbit is formed by seven bones:
  • Frontal bone: Forms the superior (roof) part of the orbit.
  • Maxilla: Forms the inferior (floor) part of the orbit.
  • Zygomatic bone: Forms the lateral wall.
  • Sphenoid bone: Forms part of the posterior wall and contains the optic canal through which the optic nerve passes.
  • Lacrimal bone: Contributes to the medial wall and houses the lacrimal sac (involved in tear drainage).
  • Ethmoid bone: Forms part of the medial wall.
  • Palatine bone: A small part of the orbital floor near the back.
• Extraocular Muscles: Six muscles control the movements of the eye:
  • Medial Rectus: Moves the eye medially (adduction).
  • Lateral Rectus: Moves the eye laterally (abduction), innervated by the abducens nerve (Cranial Nerve VI).
  • Superior Rectus: Moves the eye upward (elevation), innervated by the oculomotor nerve (Cranial Nerve III).
  • Inferior Rectus: Moves the eye downward (depression), innervated by the oculomotor nerve.
  • Superior Oblique: Moves the eye downward and medially (intorsion), innervated by the trochlear nerve (Cranial Nerve IV).
  • Inferior Oblique: Moves the eye upward and laterally (extorsion), innervated by the oculomotor nerve.
• Lacrimal Apparatus: Responsible for tear production and drainage:
  • Lacrimal Gland: Located in the superolateral orbit, it produces tears.
  • Lacrimal Canaliculi: Small ducts that drain tears from the eye's surface to the lacrimal sac.
  • Lacrimal Sac: Collects tears and drains them into the nasolacrimal duct.
  • Nasolacrimal Duct: Drains tears into the nasal cavity.
• Optic Nerve (Cranial Nerve II): Transmits visual information from the retina to the brain. It passes through the optic canal in the sphenoid bone.

Arterial Blood Supply to the Eye and Orbit

• The primary arterial blood supply to the eye and orbit is provided by the Ophthalmic Artery , a branch of the internal carotid artery.
  • Ophthalmic Artery: Enters the orbit through the optic canal alongside the optic nerve. It supplies the globe, extraocular muscles, and orbital structures through several branches.
• Branches of the Ophthalmic Artery:
  • Central Retinal Artery: Enters the optic nerve and provides blood to the inner layers of the retina. Its occlusion can cause sudden vision loss.
  • Lacrimal Artery: Supplies the lacrimal gland, lateral rectus muscle, and eyelids.
  • Posterior Ciliary Arteries: These arteries are further divided into short and long posterior ciliary arteries:
    • Short Posterior Ciliary Arteries: Supply the choroid and the outer layers of the retina, contributing to the nourishment of photoreceptors.
    • Long Posterior Ciliary Arteries: Extend forward to supply the ciliary body and iris.
  • Ethmoidal Arteries (Anterior and Posterior): Supply the ethmoid sinus and nasal cavity.
  • Supraorbital Artery: Travels through the supraorbital foramen to supply the forehead and scalp.
  • Supratrochlear Artery: Exits near the medial aspect of the orbit and supplies the forehead.
  • Dorsal Nasal Artery: Supplies the skin of the nose and the lacrimal sac.
• Venous Drainage: Blood from the orbit drains into the ophthalmic veins, which empty into the cavernous sinus.

Posterior Ciliary Arteries

• The posterior ciliary arteries are critical in supplying blood to the posterior part of the eye:
  • Short Posterior Ciliary Arteries: These multiple small branches supply the choroid, which nourishes the outer retina, including the photoreceptors. Their occlusion can lead to visual field defects.
  • Long Posterior Ciliary Arteries: These arteries run forward alongside the optic nerve and supply the ciliary body and the anterior segment of the eye, including the iris. They play an essential role in maintaining intraocular pressure by regulating blood flow to the ciliary body, which produces aqueous humor.

Ophthalmic Artery

• The ophthalmic artery is the main arterial supply to the orbit and eye. It branches off the internal carotid artery and provides oxygenated blood to several critical structures:
  • Globe: The ophthalmic artery supplies the globe through its various branches, ensuring adequate blood flow to the retina, choroid, and optic nerve head.
  • Extraocular Muscles: The ophthalmic artery’s muscular branches provide the necessary blood supply to the six extraocular muscles responsible for eye movement.
  • Optic Nerve: Blood is supplied to the optic nerve via small branches from the ophthalmic artery, particularly the central retinal artery, which nourishes the inner retina.
  • Orbital Structures: Additional branches such as the lacrimal, ethmoidal, and supraorbital arteries supply the lacrimal gland, nasal cavity, and surrounding bony and soft tissues of the orbit.
• Clinical Significance of the Ophthalmic Artery:
  • Occlusion of the central retinal artery (CRAO): A medical emergency that leads to sudden, painless vision loss due to a lack of blood flow to the retina. The classic fundoscopic finding is a "cherry-red spot" at the fovea.
  • Posterior ciliary artery occlusion: Can cause visual field loss and ischaemia in the choroid and optic nerve, potentially leading to conditions such as anterior ischaemic optic neuropathy (AION).

Venous Drainage

• The venous drainage of the eye and orbit is mainly through the superior and inferior ophthalmic veins , which drain into the cavernous sinus:
  • Superior Ophthalmic Vein: Drains blood from the upper part of the orbit, including the eyeball, upper eyelid, and extraocular muscles, and empties into the cavernous sinus.
  • Inferior Ophthalmic Vein: Drains blood from the lower orbit and can communicate with the pterygoid venous plexus.