Overview and topographic anatomy of the orbit
General information
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Orbit— a pyramid-shaped bony recess in the anterior part of the skull, lined by periosteum called the periorbital fascia
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Contents include:
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Eye—organ associated with vision
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Extrinsic muscles
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Optic nerve
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Oculomotor nerve
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Ciliary ganglion
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Trochlear nerve
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Ophthalmic division of the trigeminal nerve
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Abducens nerve
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Ophthalmic artery and branches
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Superior and inferior ophthalmic veins
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Lacrimal apparatus
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Much fatty tissue
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Osteology of the orbit
Openings in the orbit
| Opening | Bony Boundaries | Structures Passing Through Opening |
|---|---|---|
| Optic canal | Lesser wing of the sphenoid |
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| Superior orbital fissure |
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| Inferior orbital fissure |
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| Supraorbital foramen (sometimes occurs as a notch) | Frontal |
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| Infraorbital groove and canal | Maxilla |
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| Zygomatic foramen (1 or 2 openings) | Zygomatic |
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| Nasolacrimal canal | Lacrimal | Nasolacrimal duct |
| Anterior ethmoidal foramen |
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| Posterior ethmoidal foramen |
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Bones and walls of the orbit
Bones creating the orbital margin
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Frontal
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Zygomatic
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Maxilla
| WALLS OF THE ORBIT | |
| Superior |
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| Inferior |
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| Medial |
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| Lateral |
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Contents of the orbit
Eye
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Eye— a spherical globe with a diameter of approximately 2.5 cm that lies in the orbit’s anterior portion
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Surrounded by a thin capsule called the fascia bulbi (Tenon’s capsule):
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Provides support
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Allows for movement
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Composed of 3 coats:
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Sclera
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Uveal tract
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Retina
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Divided into an anterior and a posterior segment
Anterior segment
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Filled with aqueous humor
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Separated into anterior and posterior chambers by the iris
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Contains aqueous humor secreted by the ciliary body and drained through a trabeculated network eventually into the superior ophthalmic vein
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Intraocular pressure is measured in the anterior segment, normally 10 to 20 mm Hg
Posterior segment
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Filled with vitreous fluid
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Called the vitreous cavity
Components
Sclera
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The outermost layer, very fibrous
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White along the periphery, except for the anterior portion—the cornea—which is transparent
Uveal tract
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Composed of choroid layer, ciliary body, and iris
Choroid
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The pigmented vascular layer between the sclera and the retina
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Extends posteriorly from the region of the optic nerve anteriorly, where it is continuous with the ciliary body near the ora serrata (anterior margin of the retina)
Ciliary body
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Located between the choroid and the iris
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Ring shaped; has a series of transparent fibers that form the suspensory ligament of the lens
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Within it is the ciliary muscle, which changes the shape of the lens
Iris
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A thin disclike structure with a central opening—the pupil
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Separates the aqueous humor into the anterior chamber (anterior to the iris) and the posterior chamber (between the iris and the lens)
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Contains the sphincter and dilator pupillae muscles, which change the pupil’s shape in response to light
Lens
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Located posterior to the iris
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A transparent biconcave structure responsible for focusing
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Connected to the ciliary body by the suspensory ligaments
Retina
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The innermost coat of the eye
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Thin and highly vascular
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3 areas located on the retina’s posterior portion:
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Optic disc
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Macula lutea
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Fovea centralis
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Optic disc
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Area where the optic nerve enters the retina is called the “blind spot”
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Retina’s central artery enters the eye through the optic disc and divides into superior and inferior branches
Macula lutea and fovea centralis
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Macula lutea—lateral to the optic disc
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A depressed, yellow-appearing area that contains the fovea centralis in its center
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Fovea centralis—area of highest visual acuity and contains high concentration of cone photoreceptors
Muscles
| ASSOCIATED EXTRINSIC MUSCLE OF THE ORBIT | |||||
| Muscle | Origin | Insertion | Action(s) | Nerve | Comment |
| Levator palpebrae superioris | Sphenoid (lesser wing) | Skin of the upper eyelid | Raises the upper eyelid |
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| EXTRINSIC MUSCLES OF THE EYE | |||||
| Muscle | Origin | Insertion | Action(s) on Eye | Nerve | Comment |
| Superior rectus | Common tendinous ring on sphenoid | Superior sclera | Elevation Adduction Intorsion |
Superior division of the oculomotor | A check ligament attaches it to the levator palpebrae superioris m. to help elevate the upper eyelid |
| Inferior rectus | Inferior sclera | Depression Adduction Extorsion |
Inferior division of the oculomotor | A check ligament attaches it to the inferior tarsal plate to help depress the lower eyelid | |
| Medial rectus | Medial sclera | Adduction | The most medial of the extraocular muscles | ||
| Lateral rectus | Lateral sclera | Abduction | Abducens | Impaired in abducens n. palsy | |
| Superior oblique | Body of the sphenoid | Superior portion of the posterolateral sclera | Depression Abduction Intorsion |
Trochlear | Tendon passes through the trochlea, a fibrocartilaginous pulley |
| Inferior oblique | Maxilla (lateral to the lacrimal groove) | Inferior portion of the posterolateral sclera | Elevation Abduction Extorsion |
Inferior division of the oculomotor | Only extraocular muscle that attaches to the maxilla |
Nerve supply
| ORBITAL INNERVATION | |
| Orbital Innervation | Description |
| Sensory |
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| Motor |
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| Cranial nn. |
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