Orbital and Ocular Anatomy Abdulrahman Al-Muammar, MD, FRCSC King Saud University

Periorbital Sinuses The eyes lie within two bony orbits, located on either side of the root of the nose. They border the nasal cavity anteriorly and the ethmoidal air cells and the sphenoid sinus posteriorly. The lateral walls border the middle cranial, temporal, and pterygopalatine fossae. Superior to the orbit are the anterior cranial fossa and the frontal and supraorbital sinus. The maxillary sinus and the palatine air cells are located inferiorly.

Orbital Volume The volume of each adult orbit is slightly less than 30 cc The orbital entrance averages about 35 mm in height and 45 mm in width. The maximum width is about 1 cm (behind the anterior orbital margin) In adults, the depth of the orbit varies from 40 to 45 mm from the orbital entrance to the orbital apex Both race and sex affect each of these measurements.

Bony Orbit Seven bones make up the bony orbit: Frontal Zygomatic Maxillary Ethmoidal Sphenoid Lacrimal Palatine

Orbital Roof The orbital roof formed from both the orbital plate of the frontal bone and the lesser wing of the sphenoid bone. Lacrimal gland Fovea trochlearis

Medial Orbital Wall Then medial wall of the orbit is formed from four bones: Frontal process of the maxillary Lacrimal Orbital plate of the ethmoidal Lesser wing of the sphenoid Lacrimal fossa Lamina papyracea

Orbital Floor The floor of the orbit is formed from three bones: Maxillary Palatine Orbital plate of the zygomatic Infraorbital groove Inferior oblique muscle

Lateral Orbital Wall Formed from two bones: Thickest and strongest Zygomatic Greater wing of the sphenoid Thickest and strongest Lateral orbital tubercle (Whitnall’s tubercle)

Orbital Foramina The optic foramen The supraorbital foramen, or notch The anterior ethmoidal foramen The posterior ethmoidal foramen The zygomatic foramen Nasolacrimal duct Infraorbital canal Superior orbital fissure Inferior orbital fissure

Extraocular Muscles The four recti and two oblique muscles All are supplied by oculomotor nerve III except superior oblique (Trochlear N) and lateral rectus (Abducens)

Optic nerve The optic nerve consists of more than 1 million axons that originate in the ganglion cell layer of the retina and extend toward the occipital cortex

Optic Nerve The optic nerve may be divided into the following topographic areas: Intraocular portion of the optic nerve: optic disc, or nerve head; prelaminar; and laminar portions Intraorbital portion (located within the muscle cone) Intracanalicular portion (located within the optic canal) Intracranial portion (ending in the optic chiasm)

Optic Chiasm The chiasm measures approximately 12 mm wide, 8 mm long in the anteroposterior direction, and 4 mm thick. The exact location of the chiasm with respect to the sella is variable. Most of the time it is directly superior Within the chiasm, the fibers coming from the nasal retina (approximately 53% of total fibers) cross to the opposite side to join the corresponding contralateral fibers

Optic Tract Each optic tract contains ipsilateral temporal and contralateral nasal fibers from the optic nerves . Fibers (both crossed and uncrossed) from the upper retinal projections travel medially in the optic tract; lower projections move laterally.

Lateral geniculate body The lateral geniculate body, or nucleus, is the synaptic zone for the higher visual projections It has six alternating layers of gray and white matter. Layers 1, 4, and 6 of the lateral geniculate body contain axons from the contralateral optic nerve. Layers 2, 3, and 5 arise from the ipsilateral optic nerve

Optic radiations The optic radiations connect the lateral geniculate body with the cortex of the occipital lobe The fibers of the optic radiations leave the lateral geniculate body and run around the temporal horn of the lateral ventricle, approaching the anterior tip of the temporal lobe (the so-called loop of Meyer)

Visual cortex The visual cortex, the thinnest area of the human cerebral cortex Macular function is extremely well represented in the visual cortex and occupies the most posterior position at the tip of the occipital lobe The posterior cerebral artery, a branch of the basilar artery, supplies the visual cortex almost exclusively. The blood supply to the occipital lobe does show anatomical variation, however, with the middle cerebral artery making a contribution in some persons.

Cranial Nerve III (Oculomotor) It supplies all the extraocular muscles except the superior oblique and the lateral rectus It also carries cholinergic innervation to the pupillary sphincter and the ciliary muscle

Cranial Nerve III The CN III nucleus consists of several distinct, large motor cell subnuclei, each of which subserves the extraocular muscle it innervates The Edinger-Westphal nucleus provides the parasympathetic preganglionic efferent innervation to the ciliary muscle and pupillary sphincter

Cranial Nerve III CN III usually divides into superior and inferior divisions after passing through the annulus of Zinn in the orbit The superior division of CN III innervates the superior rectus and levator palpebrae muscles. The larger inferior division splits into three branches to supply the medial and inferior rectus muscles and the inferior oblique.

Cranial Nerve III The parasympathetic fibers wind around the periphery of the nerve Enter the inferior division, and course through the branch that supplies the inferior oblique muscle. They join the ciliary ganglion They synapse with the postganglionic fibers, which emerge as many short ciliary nerves

Cranial Nerve IV (Trochlear) Cranial nerve IV has the longest intracranial course The CN IV the only cranial nerve that is completely decussated and the only motor nerve to exit dorsally from the nervous system. CN IV enters the orbit through the superior orbital fissure outside the annulus of Zinn and runs superiorly to innervate the superior oblique muscle

Cranial Nerve V (Trigeminal) The largest cranial nerve Possesses both sensory and motor divisions The sensory portion subserves the greater part of the scalp, forehead, face, eyelids, eye, lacrimal gland, extraocular muscles, ear, dura mater, and tongue The motor portion innervates the muscles of mastication through branches of the mandibular division

Divisions of Cranial Nerve V Ophthalmic Frontal Lacrimal Nasociliary Maxillary Mandibular

Cranial Nerve VI (Abducens) The nucleus of cranial nerve VI is situated in the floor of the fourth ventricle, beneath the facial colliculus, in the caudal pons CN VI runs below and lateral to the carotid artery and may transiently carry sympathetic fibers from the carotid plexus It passes through the superior orbital fissure within the annulus of Zinn and innervates the lateral rectus muscle on its ocular surface

Arterial system Internal carotid artery Ophthalmic artery Lacrimal A Superior muscular A Inferior muscular A Posterior ethmoidal A Anterior ethmoidal A Supraorbital A Supratrochlear A Central retinal A Short post ciliary A Long post ciliary A Anterior ciliary A

Venous system Central retinal V Superior vortex V Superior episcleral plexus Superior ophthalmic V Cavernous sinus Inferior vortex V Inferior episcleral plexus Pterygoid plexus Jugular v

The Eye The adult human eye averages 24 mm in diameter The eye reaches maximum size before puberty. The transverse vertical diameter is less variable.

Cornea The cornea occupies the center of the anterior pole of the globe. In the adult, it measures about 12 mm in the horizontal meridian and about 11 mm in the vertical The cornea is transparent and form a positive lens of about 40 D and constitute the main refractive element of the eye

Cornea Epithelium Bowman’s layer Stroma Descemet’s membrane Endothelium

Sclera Sclera is opaque and white The sclera covers the posterior four fifths of the surface of the globe, with an anterior opening for the cornea and a posterior opening for the optic nerve

Limbus Transition zone between the peripheral cornea and the anterior sclera

Conjunctiva Mucous membrane with non keratinized squamous epithelium and goblet cells Thin, richy vascularized substantia propria Can be divided into three geographic zones: Palpebral Forniceal Bulbar

Anterior chamber The anterior chamber is bordered anteriorly by the cornea and posteriorly by the iris diaphragm and the pupil The depth of the anterior chamber varies The anterior chamber is filled with aqueous humor, which is produced by the ciliary epithelium

Anterior chamber The fluid passes through the pupil aperture and drains chiefly by the conventional pathway through the trabecular meshwork The anterior chamber angle, which lies at the junction of the cornea and the iris, consists of the following structures : Schwalbe's line Schlemm's canal and the trabecular meshwork Scleral spur Anterior border of the ciliary body

Lens The lens is a biconvex structure located directly behind the posterior chamber and pupil It is the lesser of the two refractive elements in the dioptric system The equatorial diameter in adult is about 9-10 mm The anteroposterior width of the lens is about 6 mm The lens has certain unusual features. It lacks innervation and is avascular.

Uveal Tract The uveal tract is the main vascular compartment of the eye. It consists of three parts: Iris Ciliary body (located in the anterior uvea) Choroid (located in the posterior uvea)

Iris

Ciliary Body The ciliary body, which is triangular in cross section. The apex of the ciliary body is directed posteriorly toward the ora serrata. Its base gives rise to the iris. The only attachment of the ciliary body to the sclera is at its base, via its longitudinal muscle fibers, where they insert into the scleral spur.

Ciliary body The ciliary body has three principal functions: aqueous humor formation and lens accommodation It also plays a role in the trabecular and uveoscleral outflow of aqueous humor Accommodation

Choroid The choroid, the posterior portion of the uveal tract, nourishes the outer portion of the retina. It averages 0.25 mm in thickness and consists of three layers of vessels: The choriocapillaris, the innermost layer A middle layer of small vessels An outer layer of large vessels

Retina The fundus is the part of the eye that is visible on ophthalmoscopy, including the retina and its vessels and the optic nerve head (or optic disc) The macula, 5–6 mm in diameter, lies between the temporal vascular arcades. At the macula's center lies the fovea, rich in cones and responsible for color vision and the highest visual acuity

Vitreous The vitreous cavity occupies four fifths of the volume of the globe The transparent vitreous humor is important to the metabolism of the intraocular tissues because it provides a route for metabolites used by the lens, ciliary body, and retina Its volume is close to 4.0 ml Although it has a gel-like structure, the vitreous is 99% water Its viscosity is approximately twice that of water, mainly because of the presence of the mucopolysaccharide hyaluronic acid

Ocular Embryology

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