The Orbit and Extraocular Muscles

The orbit is the cavity of the skull Skull The skull (cranium) is the skeletal structure of the head supporting the face and forming a protective cavity for the brain. The skull consists of 22 bones divided into the viscerocranium (facial skeleton) and the neurocranium. Skull in which the eye and its appendages are situated. The orbit is composed of 7 bones and has a pyramidal shape, with its apex pointed posteromedially. The orbital contents comprise the eye; orbital and retrobulbar fascia; extraocular muscles; cranial nerves Cranial nerves There are 12 pairs of cranial nerves (CNs), which run from the brain to various parts of the head, neck, and trunk. The CNs can be sensory or motor or both. The CNs are named and numbered in Roman numerals according to their location, from the front to the back of the brain. Overview of the Cranial Nerves II, III, IV, V, and VI; blood vessels; fat; lacrimal gland with its sac and nasolacrimal duct; eyelids; palpebral and suspensory ligaments; ciliary ganglion; and short ciliary nerves.

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Bones and Structure of the Orbit

  • Shaped like a quadrangular pyramidal cavern in the upper face, with a superficial base (orbital margin) and a deep, posteromedial apex
    • Orbital margin: 
      • Superior margin: frontal bone Bone Bone is a compact type of hardened connective tissue composed of bone cells, membranes, an extracellular mineralized matrix, and central bone marrow. The 2 primary types of bone are compact and spongy. Structure of Bones 
      • Medial margin: frontal process of the maxilla
      • Inferior margin: zygomatic process of the maxilla and zygomatic bone Bone Bone is a compact type of hardened connective tissue composed of bone cells, membranes, an extracellular mineralized matrix, and central bone marrow. The 2 primary types of bone are compact and spongy. Structure of Bones
      • Lateral margin: zygomatic process of the frontal bone Bone Bone is a compact type of hardened connective tissue composed of bone cells, membranes, an extracellular mineralized matrix, and central bone marrow. The 2 primary types of bone are compact and spongy. Structure of Bones and frontal process of the zygomatic bone Bone Bone is a compact type of hardened connective tissue composed of bone cells, membranes, an extracellular mineralized matrix, and central bone marrow. The 2 primary types of bone are compact and spongy. Structure of Bones
    • Apex: optic foramen
  • Walls: covered with periosteum (periorbita)
    • Superior (roof): orbital part of the frontal and lesser wing of the sphenoid
    • Medial: orbital plate of ethmoid bone Bone Bone is a compact type of hardened connective tissue composed of bone cells, membranes, an extracellular mineralized matrix, and central bone marrow. The 2 primary types of bone are compact and spongy. Structure of Bones, body of sphenoid bone Bone Bone is a compact type of hardened connective tissue composed of bone cells, membranes, an extracellular mineralized matrix, and central bone marrow. The 2 primary types of bone are compact and spongy. Structure of Bones, frontal bone Bone Bone is a compact type of hardened connective tissue composed of bone cells, membranes, an extracellular mineralized matrix, and central bone marrow. The 2 primary types of bone are compact and spongy. Structure of Bones, lacrimal bone Bone Bone is a compact type of hardened connective tissue composed of bone cells, membranes, an extracellular mineralized matrix, and central bone marrow. The 2 primary types of bone are compact and spongy. Structure of Bones, and maxilla
    • Inferior (floor): orbital surface of the maxilla, zygomatic bone Bone Bone is a compact type of hardened connective tissue composed of bone cells, membranes, an extracellular mineralized matrix, and central bone marrow. The 2 primary types of bone are compact and spongy. Structure of Bones, and palatine bone Bone Bone is a compact type of hardened connective tissue composed of bone cells, membranes, an extracellular mineralized matrix, and central bone marrow. The 2 primary types of bone are compact and spongy. Structure of Bones
    • Lateral: greater wing of sphenoid, orbital plate of frontal bone Bone Bone is a compact type of hardened connective tissue composed of bone cells, membranes, an extracellular mineralized matrix, and central bone marrow. The 2 primary types of bone are compact and spongy. Structure of Bones, and frontal process of zygomatic bone Bone Bone is a compact type of hardened connective tissue composed of bone cells, membranes, an extracellular mineralized matrix, and central bone marrow. The 2 primary types of bone are compact and spongy. Structure of Bones
  • Important openings: optic foramen or canal, anterior and posterior ethmoidal foramina, superior and inferior orbital fissures, infraorbital groove, and supraorbital notch
Location Contents
Optic foramen or canal Apex, bordered by the body and lesser wing of the sphenoid
  • Optic nerve (CN I)
  • Ophthalmic artery
Ethmoidal foramina
  • Junction between the superior and medial orbital walls
  • In the ethmoid bone Bone Bone is a compact type of hardened connective tissue composed of bone cells, membranes, an extracellular mineralized matrix, and central bone marrow. The 2 primary types of bone are compact and spongy. Structure of Bones, lateral to olfactory groove
Anterior and posterior ethmoidal veins Veins Veins are tubular collections of cells, which transport deoxygenated blood and waste from the capillary beds back to the heart. Veins are classified into 3 types: small veins/venules, medium veins, and large veins. Each type contains 3 primary layers: tunica intima, tunica media, and tunica adventitia. Veins, arteries Arteries Arteries are tubular collections of cells that transport oxygenated blood and nutrients from the heart to the tissues of the body. The blood passes through the arteries in order of decreasing luminal diameter, starting in the largest artery (the aorta) and ending in the small arterioles. Arteries are classified into 3 types: large elastic arteries, medium muscular arteries, and small arteries and arterioles. Arteries, and nerves
Superior orbital fissure Between the greater and lesser wings of the sphenoid bone Bone Bone is a compact type of hardened connective tissue composed of bone cells, membranes, an extracellular mineralized matrix, and central bone marrow. The 2 primary types of bone are compact and spongy. Structure of Bones Inside the common tendinous ring:
  • Nasociliary nerve
  • Superior and inferior branches of the oculomotor nerve (CN III)
  • Abducens (CN VI) nerve
Outside the common tendinous ring:
  • Frontal nerve
  • Lacrimal nerve
  • Trochlear (CN IV) nerve
  • Superior ophthalmic vein
Inferior orbital fissure
  • Lateral border of orbital floor
  • Formed by greater wing of sphenoid superiorly and palatine and maxillary bones inferiorly
  • Inferior ophthalmic vein
  • Infraorbital artery and vein
  • Branches of the maxillary division of the trigeminal nerve (CN V2)—zygomatic and infraorbital nerves
  • Orbital branches of the pterygopalatine ganglion
Infraorbital foramen Middle of the orbital floor (maxilla) Exit of the infraorbital vein, artery, and nerve
Supraorbital notch or foramen Superior margin of the orbit (frontal bone Bone Bone is a compact type of hardened connective tissue composed of bone cells, membranes, an extracellular mineralized matrix, and central bone marrow. The 2 primary types of bone are compact and spongy. Structure of Bones) Exit of the supraorbital vein, artery, and nerve

Mnemonic 1

To help memorize the bones that make up the orbit, remember: Many Friendly Zebras Enjoy Lazy Summer Picnics

  • Many: M axilla Axilla The axilla is a pyramid-shaped space located between the upper thorax and the arm. The axilla has a base, an apex, and 4 walls (anterior, medial, lateral, posterior). The base of the pyramid is made up of the axillary skin. The apex is the axillary inlet, located between the 1st rib, superior border of the scapula, and clavicle. Axilla and Brachial Plexus
  • Friendly: Frontal bone Bone Bone is a compact type of hardened connective tissue composed of bone cells, membranes, an extracellular mineralized matrix, and central bone marrow. The 2 primary types of bone are compact and spongy. Structure of Bones
  • Zebras: Zygomatic bone Bone Bone is a compact type of hardened connective tissue composed of bone cells, membranes, an extracellular mineralized matrix, and central bone marrow. The 2 primary types of bone are compact and spongy. Structure of Bones
  • Enjoy: Ethmoid bone Bone Bone is a compact type of hardened connective tissue composed of bone cells, membranes, an extracellular mineralized matrix, and central bone marrow. The 2 primary types of bone are compact and spongy. Structure of Bones
  • Lazy: Lacrimal bone Bone Bone is a compact type of hardened connective tissue composed of bone cells, membranes, an extracellular mineralized matrix, and central bone marrow. The 2 primary types of bone are compact and spongy. Structure of Bones
  • Summer: Sphenoid bone Bone Bone is a compact type of hardened connective tissue composed of bone cells, membranes, an extracellular mineralized matrix, and central bone marrow. The 2 primary types of bone are compact and spongy. Structure of Bones
  • Picnics: Palatine bone Bone Bone is a compact type of hardened connective tissue composed of bone cells, membranes, an extracellular mineralized matrix, and central bone marrow. The 2 primary types of bone are compact and spongy. Structure of Bones

Extraocular Muscles

Muscle Origin Insertion Irrigation Innervation Function
Medial rectus Annulus of Zinn (common tendinous ring) Anterior, medial surface of the eye Inferior muscular branch of ophthalmic artery Inferior branch of oculomotor nerve (CN III) Adduction
Lateral rectus Anterior, lateral surface of the eye Lacrimal artery Abducens nerve (CN VI) Abduction
Inferior rectus Anterior, inferior surface of the eye Inferior muscular branch of ophthalmic artery and the infraorbital branch of the maxillary artery Inferior branch of oculomotor nerve (CN III) Depression, extorsion, and adduction. In abduction: only depresses
Superior rectus Anterior, superior surface of the eye Superior muscular branch of ophthalmic artery Superior branch of the oculomotor nerve (CN III) Elevation, intorsion, and adduction. In abduction: only elevates
Superior oblique Lesser wing of sphenoid, medial to optic canal Posterior, superior, lateral surface of the eye Superior muscular branch of the ophthalmic artery Trochlear nerve (CN IV) Intorsion, depression, and abduction
Inferior oblique Lateral to the lacrimal groove (maxilla) Posterior, inferior, lateral surface of the eye Inferior branch of the ophthalmic artery and infraorbital artery Inferior branch of oculomotor nerve (CN III) Extorsion, elevation, and abduction
Levator palpebrae superioris Lesser wing of the sphenoid, above the optic canal Tarsal plate of upper eyelid Supraorbital branch of the ophthalmic artery Superior branch of oculomotor nerve (CN III).
Sympathetic fibers innervate the smooth muscle fibers on the inferior surface of this muscle.
Retracting and elevating the eyelid

Mnemonic 2

To help memorize the innervation of the extraocular muscles, remember: LR6, SO4, 3

  • Lateral rectus innervated by the abducens nerve (CN VI)
  • Superior oblique innervated by the trochlear nerve (CN IV)
  • The remaining extraocular muscles are innervated by the oculomotor nerve (CN III)

Mnemonic 3

To help memorize the actions of the muscles, remember: RAD

Recti are Adductors, except the lateral rectus.

Lacrimal Apparatus

  • Lacrimal gland:
    • Serous; releases aqueous layer of tear fluid directly onto the eyeball
    • Minimizes friction, protects and cleans the eye (“tear film”)
    • Located on upper lateral aspect of orbit, drains via lacrimal ducts to the superior conjunctival fornix
    • Innervated by parasympathetic fibers of the facial nerve (VII) via the pterygopalatine ganglion
  • Tears:
    • Isotonic solution
    • Contains bactericidal enzymes Enzymes Enzymes are complex protein biocatalysts that accelerate chemical reactions without being consumed by them. Due to the body's constant metabolic needs, the absence of enzymes would make life unsustainable, as reactions would occur too slowly without these molecules. Basics of Enzymes (lysozyme and lactoferrin), immunoglobulin A, and lipocalin
  • Tarsal or Meibomian glands:
    • Sebaceous glands; produce meibum, which decreases evaporation of the tear film
    • Located within tarsal plate of eyelids (within the superior eyelid) with orifices at the rim of the marginal zone of the conjunctiva
  • Tear drainage system—consists of:
    • Lacrimal canaliculi
    • Lacrimal sac: dilated portion of the nasolacrimal duct
    • Nasolacrimal duct: drains into the inferior nasal meatus
Orbit lacrimal apparatus

Schematic diagram of the location of the lacrimal gland and apparatus

Image: “Microbiology” by CNX OpenStax. License: CC BY 4.0

Clinicopathologic Correlations

Nerve palsy Causes Symptoms involving affected eye
Oculomotor (CN III)
  • Infarction of midbrain
  • Berry aneurysm Aneurysm An aneurysm is a bulging, weakened area of a blood vessel that causes an abnormal widening of its diameter > 1.5 times the size of the native vessel. Aneurysms occur more often in arteries than in veins and are at risk of dissection and rupture, which can be life-threatening. Extremity and Visceral Aneurysms at junction of posterior communicating and internal carotid arteries Arteries Arteries are tubular collections of cells that transport oxygenated blood and nutrients from the heart to the tissues of the body. The blood passes through the arteries in order of decreasing luminal diameter, starting in the largest artery (the aorta) and ending in the small arterioles. Arteries are classified into 3 types: large elastic arteries, medium muscular arteries, and small arteries and arterioles. Arteries
  • Lesions of cavernous sinuses (neoplasm, vascular, or inflammatory)
  • Horizontal temporal deviation
  • Down and out gaze
  • Ptosis
  • Pupillary dilation
Trochlear (CN IV)
  • Head trauma Head trauma Head trauma occurs when external forces are directed to the skull and brain structures, resulting in damage to the skull, brain, and intracranial structures. Head injuries can be classified as open (penetrating) or closed (blunt), and primary (from the initial trauma) or secondary (indirect brain injury), and range from mild to severe and life-threatening. Head Trauma
  • Tumor at base of skull Skull The skull (cranium) is the skeletal structure of the head supporting the face and forming a protective cavity for the brain. The skull consists of 22 bones divided into the viscerocranium (facial skeleton) and the neurocranium. Skull
  • Microvasculopathy
  • Idiopathic
Eye is up and in
Abducens (CN VI)
  • Postviral syndrome
  • Ischemic mononeuropathy Mononeuropathy Neuropathy is a nerve pathology presenting with sensory, motor, or autonomic impairment secondary to dysfunction of the affected nerve. The peripheral nerves are derived from several plexuses, with the brachial and lumbosacral plexuses supplying the major innervation to the extremities. Mononeuropathies affect a single nerve. Mononeuropathy and Plexopathy
Eye directed medially
  • Orbital and preseptal cellulitis Preseptal cellulitis Orbital and preseptal cellulitis are infections differentiated by the anatomic sites affected in the orbit. Infection anterior to the orbital septum is preseptal cellulitis. Inoculation with the pathogen can occur through trauma or surgery. Cellulitis also occurs via extension from a nearby structure (such as from sinus infection or sinusitis). Orbital and Preseptal Cellulitis: possible complications to paranasal sinusitis Sinusitis Sinusitis refers to inflammation of the mucosal lining of the paranasal sinuses. The condition usually occurs concurrently with inflammation of the nasal mucosa (rhinitis), a condition known as rhinosinusitis. Acute sinusitis is due to an upper respiratory infection caused by a viral, bacterial, or fungal agent. Sinusitis or trauma to the eye orbit. Patients may present with fever Fever Fever is defined as a measured body temperature of at least 38°C (100.4°F). Fever is caused by circulating endogenous and/or exogenous pyrogens that increase levels of prostaglandin E2 in the hypothalamus. Fever is commonly associated with chills, rigors, sweating, and flushing of the skin. Fever, malaise, proptosis ophthalmoplegia, toxic shock Shock Shock is a life-threatening condition associated with impaired circulation that results in tissue hypoxia. The different types of shock are based on the underlying cause: distributive (↑ cardiac output (CO), ↓ systemic vascular resistance (SVR)), cardiogenic (↓ CO, ↑ SVR), hypovolemic (↓ CO, ↑ SVR), obstructive (↓ CO), and mixed. Types of Shock, and impaired vision.
  • Orbital fractures Orbital fractures An orbital fracture is a break in the continuity of one or multiple bones of the eye socket, caused by direct or indirect trauma to the orbit. Patients frequently present with lacerations around the eye, orbital pain, edema, ecchymosis, diplopia on upward gaze, numbness around the eye, and signs of muscle entrapment. Orbital Fractures: fractures to the eye sockets are classified as orbital rim fractures, direct orbital floor fracture Fracture A fracture is a disruption of the cortex of any bone and periosteum and is commonly due to mechanical stress after an injury or accident. Open fractures due to trauma can be a medical emergency. Fractures are frequently associated with automobile accidents, workplace injuries, and trauma. Overview of Bone Fractures, and blowout fractures. These fractures can present with blurry, decreased, or double vision, and black-and-blue bruising around the eyes.
  • Dacryoadenitis: an inflammation Inflammation Inflammation is a complex set of responses to infection and injury involving leukocytes as the principal cellular mediators in the body's defense against pathogenic organisms. Inflammation is also seen as a response to tissue injury in the process of wound healing. The 5 cardinal signs of inflammation are pain, heat, redness, swelling, and loss of function. Inflammation of the lacrimal glands commonly due to a bacterial or viral infection. May present as swelling of the outer portion of the upper lid, with redness and tenderness, pain Pain Pain has accompanied humans since they first existed, first lamented as the curse of existence and later understood as an adaptive mechanism that ensures survival. Pain is the most common symptomatic complaint and the main reason why people seek medical care. Physiology of Pain in the area of swelling, excess tearing, and swelling of the preauricular lymph nodes.
  • Sjögren’s syndrome: an autoimmune disease that affects the moisture-producing glands of the body. Primary symptoms are dry mouth and dry eyes. 
  • Strabismus Strabismus Strabismus is the misalignment of the eyes while fixating the gaze on an object. Strabismus can be idiopathic, but it may also be caused by cerebral palsy, uncorrected refractive errors, and extraocular muscle or cranial nerve dysfunction. Strabismus: a condition characterized by a misalignment of the eyes. If left untreated throughout childhood, strabismus may result in amblyopia or loss of depth perception. 
  • Blepharitis Blepharitis Blepharitis is an ocular condition characterized by eyelid inflammation. Anterior blepharitis involves the eyelid skin and eyelashes, while the posterior type affects the meibomian glands. Often, these conditions overlap. Blepharitis: one of the most common ocular conditions; characterized by inflammation Inflammation Inflammation is a complex set of responses to infection and injury involving leukocytes as the principal cellular mediators in the body's defense against pathogenic organisms. Inflammation is also seen as a response to tissue injury in the process of wound healing. The 5 cardinal signs of inflammation are pain, heat, redness, swelling, and loss of function. Inflammation, scaling, reddening, and crusting of the eyelid. May also present with a burning, itching, or grainy sensation

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