Extraocular Muscles and Eye Structures Quiz

Anatomy of the Eye

• The retina consists of two layers: neural and pigmented layers
  • Neural layer is light receptive
  • Pigmented layer reinforces light-absorbing property of the choroid in reducing scattering of light in the eyeball
• The inner layer (inner coat) terminates at the ora serrata (serrated edge)

The Retina

• Optic part of the retina is sensitive to visual light rays
• Anterior continuation of the pigmented layer and a layer of supporting cells
• Extends over the ciliary body (ciliary part of retina) and the posterior surface of the iris (iridial part of the retina) to the pupillary margin

Refractive Media and Compartments of the Eyeball

• Refractive media of the eyeball include: cornea, aqueous humor, lens, and vitreous humor
• Anterior segment of the eyeball is occupied by aqueous humor
• Anterior chamber of the eye is the space between the cornea anteriorly and the iris/pupil posteriorly
• Posterior chamber of the eye is between the iris/pupil anteriorly and the lens and ciliary body posteriorly
• Lens is posterior to the iris, anterior to the vitreous body, and is a transparent, biconvex structure enclosed in a capsule
• Capsule of the lens is highly elastic and anchored by zonular fibers (collectively constitutes the suspensory ligament of the lens)

Ciliary Muscle and Accommodation

• Ciliary muscle changes the shape of the lens
• Accommodation is the process of changing the lens shape
• Ciliary muscle contraction reduces the tension on the zonular fibers, allowing the lens to become more convex

Supporting Apparatus of the Eyeball

• Fascial sheath of the eyeball envelops the eyeball and extends posteriorly from the conjunctival fornices to the optic nerve
• Medial and lateral check ligaments limit abduction and adduction
• Suspensory ligament of the eyeball consists of check ligaments and oblique muscles
• Inferior check ligament forms from the fascial sheath of the inferior rectus

Nerves of the Orbit

• Optic nerves (CN II) convey purely sensory nerve fibers transmitting impulses generated by optical stimuli
• Optic nerves begin at the lamina cribrosa of the sclera and are surrounded by extensions of cranial meninges and subarachnoid space
• Oculomotor (CN III), Trochlear (CN IV), and Abducens (CN VI) nerves supply ocular muscles
• Ophthalmic nerve (CN V1) supplies structures of the anterior orbit (lacrimal gland and eyelids), face, and scalp

Vasculature of the Orbit

• Ophthalmic artery is the main blood supply of the orbit
• Infra-orbital artery contributes blood to structures of the orbital floor
• Central retinal artery arises inferior to the optic nerve and supplies the retina
• Choriocapillaris supplies the external aspect of the retina
• 6 short posterior ciliary arteries directly supply the choroid and nourish the outer nonvascular layer of the retina
• 2 long posterior ciliary arteries anastomose with the anterior ciliary arteries to supply the ciliary plexus

Veins of the Orbit

• Superior and inferior ophthalmic veins pass through the superior orbital fissure and enter the cavernous sinus
• Central retinal vein usually enters the cavernous sinus directly
• Vorticose veins drain into the inferior ophthalmic vein
• Scleral venous sinus (canal of Schlemm) is a vascular structure encircling the anterior chamber of the eyeball

Lacrimal Apparatus

• Consists of: lacrimal gland, excretory ducts of lacrimal gland, lacrimal canaliculi, and nasolacrimal duct
• Lacrimal gland secretes lacrimal fluid – watery physiological saline containing lysozyme, a bactericidal enzyme
• Lacrimal fluid moistens and lubricates surfaces of conjunctiva and cornea, provides nutrients and dissolved oxygen to the cornea

Extraocular Muscles

• 4 recti muscles: Superior rectus, Inferior rectus, Medial rectus, and Lateral rectus
• 2 obliques: Superior oblique and Inferior oblique
• Levator palpebrae superioris elevates the superior eyelid