The head and neck
5.14 The orbit and eye .1 The orbital margin
• Formed from:
■ Superior - Frontal bone
■ Lateral - Processes of frontal and zygomatic bones
■ Inferior - Zygomatic bone and maxilla
■ Medial - Process of maxilla and frontal bone
5.14.2 Eyelids
• Situated in front of the eye to protect it.
• Upper lid is larger and more mobile than lower lid.
• The orbital septum forms the framework of the eyelid and attaches to the orbital margin, where it becomes continuous with the periosteum.
• At the lid margins the orbital septum is thickened to form the tarsal plates, and these plates attach medially and laterally to a tubercle just inside the orbital margin.
• The meibomian glands (tarsal glands) are situated along the edges of the eyelids and secrete an oily substance that prevents tear film evaporation, prevents leakage of tears, and helps form an airtight seal when the eye is closed.
• Sensory innervation of the eyelids is via the ophthalmic division of the trigeminal nerve:
■ Upper lid:
■ Situated above the eyeball in the anterior and upper section of the lateral orbit, surrounding the lateral margin of levator palpebrae superioris
■ Serous gland, producing tears
■ Ducts open from the gland into the lateral part of the superior fornix of the conjunctiva and tears spread across the eye
• Lacus lacrimalis: Tears spread across the eye and accumulate here.
• Canaliculi: Tears enter the canaliculi and are transmitted medially to open into the lacrimal sac.
• Lacrimal sac:
■ Lies in the lacrimal groove, behind the medial palpebral ligament
■ Tears collect here before passing into the nasolacrimal duct
• Nasolacrimal duct: Travels through a bony canal from the lacrimal sac, downwards, backwards, and laterally, to open into the inferior meatus in the nose.
5.14.4 Orbital muscles
• Levator palpebrae superioris
■ Origin: Lesser wing of sphenoid
■ Insertion: Anterior surface and upper border of superior tarsal plate
■ Innervation:
• Oculomotor nerve (voluntary)
• Sympathetic nerves (involuntary)
■ Action: Raises upper eyelid
• Superior rectus
■ Origin: Common tendinous ring
■ Insertion: Sclera (6mm behind corneal margin)
■ Innervation: Oculomotor nerve
■ Action: Raises and medially rotates cornea
• Inferior rectus
■ Origin: Common tendinous ring
■ Insertion: Sclera (6mm behind corneal margin)
■ Innervation: Oculomotor nerve
■ Action: Depresses and medially rotates cornea
• Lateral rectus
■ Origin: Common tendinous ring
■ Insertion: Sclera (6mm behind corneal margin)
■ Innervation: Abducent nerve
■ Action: Moves cornea laterally
• Medial rectus
■ Origin: Common tendinous ring
■ Insertion: Sclera (6mm behind corneal margin)
■ Innervation: Oculomotor nerve
■ Action: Moves cornea medially
• Superior oblique
■ Origin: Body of sphenoid
■ Insertion: Pulley and attached to sclera
■ Innervation: Trochlear nerve
■ Action: Moves cornea down and laterally
• Inferior oblique
■ Origin: Anterior orbital floor
■ Insertion: Attached to sclera
■ Innervation: Oculomotor nerve
■ Action: Moves cornea up and laterally
5.14.5 Orbital stability
• The medial and lateral recti attach the eyeball to the medial and lateral orbital walls (by the medial and lateral check ligaments), providing stability and suspending the eyeball from the orbit, like a hammock.
5.14.6 Optic nerve
• Cranial nerve II
• Enters the orbit through the optic canal, accompanied by the ophthalmic artery.
• Ensheathed in all 3 dural layers (pia, arachnoid, dura mater).
• Meninges fuse with sclera, so the subarachnoid space extends into the orbit. When CSF pressure rises, this space fills, and results in papilloedema.
• The optic nerve receives its blood supply from the central artery of the retina, arising from the ophthalmic artery.
5.14.7 Orbital vessels
• The ophthalmic artery and its branches supply the eye, its muscles, and the lacrimal gland.
• Venous drainage of the orbit is via the superior ophthalmic vein (which communicates with the facial vein) and the inferior ophthalmic vein. Both of these veins pass through the superior orbital fissure and drain into the cavernous sinus.
• There are no lymph nodes in the orbit, but drainage of the region occurs via preauricular and parotid groups to the deep cervical nodes.
5.14.8 Structural anatomy of the eye
• The eyeball is coated in 3 layers:
■ Fibrous coat:
• Sclera - dense fibrous tissue
• Cornea - transparent area responsible for allowing light to enter
■ Pigmented coat:
• Choroid - highly vascular
• Ciliary body - attached to iris and lens and contains ciliary muscle, which alters refractive power of lens
• Iris - pigmented section of eye; surrounds the pupil. Involuntary muscle fibres control pupillary size
■ Nervous coat: Retina
• It has an anterior chamber, filled with aqueous humour, and a posterior chamber, filled with vitreous humour. The lens lies between the humours. See Figure A.5.12.
Figure A.5.12 The eye
5.14.9 Corneal reflex
• Consensual reflex in which blinking is stimulated by touching the cornea.
• The sensory innervation of the cornea is:
■ Long ciliary nerves - from the ophthalmic branch of trigeminal nerve
■ Short ciliary nerves - from the oculomotor nerve
• The reflex pathway is:
■ Cornea is touched with cotton wool
■ Afferent limb via long and short nasociliary nerves (trigeminal)
■ Impulse travels through trigeminal ganglion (does not synapse)
■ Impulse reaches trigeminal nucleus in pons
■ Impulse transmitted to facial nerve nucleus in pons
■ Efferent limb via facial nerve
■ Obicularis oculi stimulated - eyes closed
5.14.10 Control of pupillary size
• The muscle fibres of the iris are involuntary and have circulating and radiating fibres.
• The circular fibres form sphincter pupillae:
■ Supplied by parasympathetic fibres from oculomotor nerve
■ Constricts pupil
• The radiating fibres form dilator pupillae:
■ Supplied by sympathetic fibres that travel with long ciliary nerves
■ Dilates pupil
• The size of the pupil is therefore controlled by the balance of sympathetic and parasympathetic impulses, and this balance may be upset, resulting in pathology:
■ Argyll-Robertson pupil: Bilateral small pupils that don’t react to light but react to
accommodation. Seen in neurosyphillis. Light reactive fibres are damaged but fibres reacting to near vision remain intact.
■ Horner’s syndrome: Results in unilateral small pupil, due to damage to the sympathetic nervous system fibres that would normally be responsible for pupillary dilation.
5.14.11 Retina
• The retina is the light-sensitive portion of the eye, containing the photosensitive rods and cones.
• The optic nerve enters the retina via the optic disc.
• The optic disc is pierced by the central artery of the retina and also forms the ‘blind spot’ of the eye as it is devoid of photosensitive cells. See Figure A.5.13.
Figure A.5.13 Retina as seen on normal fundoscopy
• The macula lies temporal to the optic disc and has a depressed area within it, called the fovea, which has the most distinct area for vision and the highest density of photosensitive cells.
5.14.12 Oculomotor disorders
• Oculomotor innervation can be remembered using the mnemonic LR6(SO4)3:
■ Lateral Rectus is supplied by cranial nerve VI
■ Superior Oblique is supplied by cranial nerve IV
■ The other muscles are supplied by cranial nerve III
• Complete IIIrd nerve palsy:
■ Ptosis
■ Inability to move eye superiorly, inferiorly, or medially
■ Eye deviated down and outwards (lateral rectus and superior oblique are spared)
■ Pupil fixed and dilated
• Complete IVth nerve palsy:
■ Paralysis of superior oblique
■ Diplopia, especially on looking down or reading
• Complete VIth nerve palsy:
■ Paralysis of lateral rectus
■ Convergence of the eyes
■ Diplopia maximal on lateral gaze towards the side of the lesion
• Paralysis of individual muscles is summarized in Table A.5.2.
Table A.5.2 Orbital muscle paralysis
5.15 The ear
• The ear is divided into 3 sections:
■ External ear
■ Middle ear
■ Inner ear
5.15.1 External ear
• Pinna - Folded elastic cartilage, covered with skin
• External auditory meatus - Tube leading from the pinna to the tympanic membrane, approximately 3cm long
• Innervation
-■ Auriculotemporal branch of the mandibular division of trigeminal
■ Auricular branch of the vagus nerve
• The external auditory meatus ends at the tympanic membrane.
• The long handle of the malleus is attached to the inner surface of the tympanic membrane and can be seen on auriscope examination of the ear as the umbo, a small depression in the membrane. When illuminated this concavity produces a cone of light that radiates anteriorly and inferiorly.
• The tympanic membrane partitions the external and middle ear and transmits sound from the air to
the ossicles.
5.15.2 Middle ear
• The middle ear, or tympanic cavity, is an air-containing cavity in the petrous part of the temporal bone.
• It contains the auditory ossicles, which transmit the vibrations of the tympanic membrane to liquid-borne pulses in the perilymph.
• The malleus transmits vibrations to the incus, the incus to the stapes, and the stapes on to the oval window.
• As well as transmitting the impulses, the ossicles also amplify the vibrations by multiplying the effective pressure on the perilymph by 22 to 1.
5.15.3 Auditory tube (Eustachian tube)
• Extends from the anterior wall of the middle ear downwards, forwards, and medially to the nasopharynx.
• Function is to equalize air pressures in the middle ear and nasopharynx.
• The auditory tube provides a route for pathogenic organisms to gain entry to the middle ear from the pharynx.
5.15.4 Mastoid air cells
• The middle ear also connects with the mastoid air cells via the mastoid antrum.
• These are a series of communicating cavities within the mastoid process of the skull and are
clinically significant, as middle ear infections can pass through the mastoid antrum and into the air cells, resulting in mastoiditis.
• From the mastoid air cells infections can spread to the meninges, leading to meningitis and temporal lobe cerebral abscess, or to the inner ear or facial nerve, causing facial nerve palsy, labyrinthitis, and vertigo.
5.15.5 Inner ear
• The inner ear is concerned with 3 functions:
■ Hearing - via the cochlear
■ Static balance - via the utricle and saccule
■ Kinetic balance - via the semi-circular canals