Attachments
The four recti arise from the common tendinous ring (annulus of Zinn), a fibrous cuff that encircles the optic canal and the medial part of the superior orbital fissure. They run forward as a cone around the optic nerve and insert into the sclera just in front of the equator of the globe, medial rectus closest to the corneal margin and superior rectus furthest from it.
The obliques
The superior oblique arises from the body of the sphenoid, above and medial to the optic canal. Its thin tendon passes through the trochlea, a fibrocartilaginous pulley attached to the trochlear fossa of the frontal bone, then turns backwards and laterally beneath the superior rectus to insert into the posterosuperolateral quadrant of the sclera. The inferior oblique is the only extraocular muscle that arises from the front of the orbit: it starts on the orbital surface of the maxilla just lateral to the nasolacrimal canal, passes backwards and laterally beneath the inferior rectus, and inserts into the posteroinferolateral sclera under the lateral rectus.
Levator palpebrae superioris
Levator palpebrae superioris arises from the lesser wing of the sphenoid above the optic canal and fans out into an aponeurosis that inserts into the skin of the upper eyelid and the front of the superior tarsal plate. Its deep, smooth-muscle part, the superior tarsal muscle (of Müller), inserts into the upper margin of the tarsus and carries a sympathetic supply.
Actions
Each muscle's action depends on the direction of gaze, because the orbital axis points about 23 degrees lateral to the visual axis. Only the medial and lateral recti act in a single plane. The superior and inferior recti pull along the orbital axis, so in the primary position they also adduct and rotate the globe; the obliques approach from behind and in front, so they abduct.
| Muscle | Primary action | Secondary actions | Tested by asking the patient to look |
|---|---|---|---|
| Medial rectus | Adduction | None | Medially |
| Lateral rectus | Abduction | None | Laterally |
| Superior rectus | Elevation | Adduction, intorsion | Out and up |
| Inferior rectus | Depression | Adduction, extorsion | Out and down |
| Superior oblique | Intorsion | Depression, abduction | In and down |
| Inferior oblique | Extorsion | Elevation, abduction | In and up |
The clinical test positions isolate each vertical muscle. With the eye abducted, the superior and inferior recti line up with the visual axis and become pure elevator and depressor. With the eye adducted, the obliques take over vertical movement: the superior oblique is then the main depressor and the inferior oblique the main elevator.
Relations
The recti form a muscular cone whose apex is the common tendinous ring. Inside the cone lie the optic nerve, the ophthalmic artery, the ciliary ganglion (lateral to the optic nerve near the apex), the nasociliary nerve and orbital fat. Structures that pass through the ring enter the cone directly: the superior and inferior divisions of the oculomotor nerve, the nasociliary nerve and the abducens nerve.
The frontal, lacrimal and trochlear nerves pass through the superior orbital fissure above the ring and stay outside the cone. The trochlear nerve therefore reaches the upper surface of the superior oblique without entering the cone, and the lacrimal nerve runs along the upper border of the lateral rectus.
Blood supply and innervation
The muscles receive muscular branches of the ophthalmic artery, with the lacrimal artery contributing to the lateral rectus and the infraorbital artery to the inferior rectus and inferior oblique. Venous blood drains to the superior and inferior ophthalmic veins.
The oculomotor nerve divides near the superior orbital fissure. Its superior division supplies superior rectus and levator palpebrae superioris. Its inferior division supplies medial rectus, inferior rectus and inferior oblique, and carries the parasympathetic fibres that relay in the ciliary ganglion for the sphincter pupillae and ciliary muscle. The superior tarsal muscle receives postganglionic sympathetic fibres from the superior cervical ganglion.
Clinical relevance
A lesion of each nerve produces a recognisable pattern of squint and diplopia.
- Oculomotor (III) palsy: the eye rests down and out, with complete ptosis. A dilated pupil suggests compression, such as a posterior communicating artery aneurysm, because the pupillary fibres run superficially in the nerve; ischaemic palsies from diabetes often spare the pupil.
- Trochlear (IV) palsy: vertical diplopia worst when looking down and in, noticed on stairs or when reading. Patients tilt the head away from the affected side to reduce it.
- Abducens (VI) palsy: horizontal diplopia worst looking towards the affected side, with the eye adducted at rest. Its long intracranial course makes it a false localising sign of raised intracranial pressure.
- Partial ptosis with miosis: loss of the superior tarsal muscle in Horner syndrome.
An orbital floor blowout fracture can trap the inferior rectus and inferior oblique in the maxillary sinus, restricting upgaze. In thyroid eye disease the inferior and medial recti are the muscles most often enlarged.