Inferior oblique muscle

At a glance

English

Inferior oblique muscle

Latin (TA2)

Musculus obliquus inferior bulbi oculi

System

Muscle

Region

Head

Side

Paired, left and right

About the inferior oblique muscle

Of the muscles that move the eyeball, the inferior oblique is the odd one out. It is a slim, narrow strap lying near the front edge of the orbital floor, and it stands apart from the others in where it starts. The four recti and the superior oblique arise at the back of the orbit, around the common tendinous ring, whereas the inferior oblique arises at the front, from bone just inside the orbital rim.

Its origin is the maxilla's orbital surface, just lateral to the groove for the nasolacrimal duct. From there it runs backwards, outwards and upwards, passing below the inferior rectus between that muscle and the orbital floor, and then curving up under the lateral rectus. It ends on the sclera on the lower, outer and back part of the globe, behind the equator, between the insertions of the inferior and lateral recti. Because it pulls on the back of the eye from in front, its line of action resembles that of the superior oblique tendon after it has turned at the trochlea.

Its chief action is extorsion, turning the top of the eye outwards. It also elevates the eye and, to a lesser degree, abducts it. Its lifting power is greatest when the eye is turned in towards the nose, and in that position it is the only muscle able to raise the gaze; this is why it is tested by asking the patient to look up and in. Its motor nerve comes from the lower division of the oculomotor nerve; this branch enters the muscle from behind and also carries the parasympathetic fibres bound for the ciliary ganglion.

An isolated palsy of this muscle is rare; it is usually weakened together with other muscles in an oculomotor palsy. Excess upward drift of an adducted eye, often called overaction of the inferior oblique, is a frequent feature of childhood squint, and it may be treated by recessing, cutting or repositioning the muscle. Surgeons operating on the lower eyelid also look out for it, because it lies close beneath the orbital fat there.

Attachments, action and supply

Origin

The maxilla's orbital surface, just lateral to the nasolacrimal groove, near the front of the orbital floor.

Insertion

Sclera on the posterolateral, lower part of the eyeball, behind the equator, between the inferior and lateral recti.

Action

Extorts the eye, elevates it (most strongly when adducted) and assists abduction.

Nerve supply

Oculomotor nerve (cranial nerve III), lower division.

Blood supply

Inferior muscular branch of the ophthalmic artery and the infraorbital artery.