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TSM47: THE ORBIT
25/10/08
BONES AND VESSELS OF THE ORBIT
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The bony orbit is made up of seven different bones:
o The roof is formed by the frontal bone
o The lateral and inferior walls are formed by the zygomatic bone and maxilla
o From lateral the medial, the posterior wall is formed by sphenoid, palatine, ethmoid, lacrimal
There are a number of openings in the orbital walls that allow passage of various vessels:
o Optic canal – rounded hole transmitting the optic nerve and ophthalmic artery
o Supra-orbital fissure – infero-medial slope leading to the middle cranial fossa transmitting:
 All oculomotor cranial nerves – oculomotor, trochlear and abducens (CNIII, IV, VI)
 Branches of the ophthalmic division of the trigeminal (CNV1 – see below)
 Superior ophthalmic vein
o Infra-orbital fissure – infero-lateral slope leading to pterygopalatine and temporal fossae:
 Maxillary nerve (CNV2)
 Inferior ophthalmic vein
LEARNING OUTCOMES
Describe the basic anatomy of lacrimation
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Lacrimation is the production of tears
The lacrimal gland is situated in the lacrimal fossa in the antero-lateral roof of orbit
o Secretory ducts project onto the superior temporal conjunctiva
o Tears flow into the medial angle of the eye and pass through the lacrimal puncta
o Medial to the lacrimal caruncle the two lacrimal ducts join at the lacrimal sac
o The orbicularis oculi muscle compresses the lacrimal sac to encourage drainage
o The nasolacrimal duct conducts the tears into the nasal cavity below the inferior concha
Sensory innervation to the lacrimal gland is via the lacrimal nerve (branch of V1 – see below)
Secretomotor innervation to the lacrimal gland is via parasympathetic branches
o Fibres originate from the facial nerve and follow complicated pathway
o Greater petrosal nerve to pterygopalatine ganglion; follows zygomatic branch of V2
Describe the types of eye movements
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Eye movements are described relative to the visual axis:
o Elevation and depression – movement in the vertical plane
o Abduction and adduction – directing the pupil laterally and medially, respectively
o Intorsion and extorsion – internal and external rotation
The pupil can also dilate and constrict:
o Miosis – constriction via parasympathetic innervation of circumferential sphincter pupillae
o Mydriasis – dilation via sympathetic innervation of radial dilator pupillae
Describe the basic anatomy of extra-ocular muscles
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The orbital axis is oriented slightly lateral to the visual axis
o The muscles of the eye act along the orbital axis and as such often have multiple effects
o Most movements of the eye involve simultaneous contributions from multiple muscles
The seven extra-ocular muscles control movements of the eye and upper eyelid
Elevation of the upper eyelid is achieved by a combination of striated (main) and smooth muscles:
o Levator palpebrae superioris – most superior muscle of orbit, superior CNIII innervation
o Superior tarsal (smooth) – assists, superior cervical ganglion sympathetic innervation
Simple abduction and adduction of the eye are achieved by two dedicated extra-ocular muscles:
o Medial rectus – adducts the eye, inferior CNIII innervation
o Lateral rectus – abducts the eye, CNVI innervation
Elevation and depression with adduction are achieved by the remaining rectus muscles:
o Superior rectus – elevates and adducts the eye, superior CNIII innervation
o Inferior rectus – depresses and adducts the eye, inferior CNIII innervation
Elevation and depression with abduction are achieved by the two oblique muscles:
o Superior oblique – depresses and abducts the eye, CNIV innervation
o Inferior oblique – elevates and abducts the eye, inferior CNIII innervation
Describe the nerve supply to the eye
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The ophthalmic division of the trigeminal nerve (CNV1) transmits three branches into the orbit:
o Nasociliary nerve – supplies ciliary ganglion, mucous membrane of the nose, conjunctiva
o Frontal nerve – sensory supply to supra-orbital regions of the face
o Lacrimal nerve – sensory to lacrimal gland, superficial temporal conjunctiva
The oculomotor nerve (CNIII) gives pre-ganglionic parasympathetic branches to the ciliary ganglion
o Short post-ganglionic fibres to the sphincter pupillae (of pupil) and ciliary muscles (of lens)
Thoracic spinal nerves (T1) give pre-ganglionic sympathetic branches to the upper cervical ganglion
o Post-ganglionic fibres travel with the ophthalmic artery to the dilator pupillae muscle
To summarise the motor nerve supply to the extra-ocular muscles as detailed above:
o Superior oblique is supplied by the trochlear nerve (CNIV)
o Lateral rectus is supplied by the abducens (CNVI)
o All others are supplied by the oculomotor nerve (CNIII)
Describe the pupillary light reflex
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The pupillary light reflex causes the pupil to constrict when the retina receives a bright light stimulus
This is primarily a subcortical reflex and so its absence clinically implies brainstem damage
o Afferent fibres decussate at the optic chiasm and proceed bilaterally
o Impulses bypass the lateral geniculate nuclei and synapse in brainstem pretectal nuclei
o Relays to the Edinger-Westphal nucleus stimulate sympathetic innervation of dilator pupillae
Bright light stimulation of one eye therefore results in bilateral pupillary dilation
o Dilation of the stimulated eye is the direct response; dilation of the other eye is consensual