Inferior rectus muscle

At a glance

English

Inferior rectus muscle

Latin (TA2)

Musculus rectus inferior

System

Muscle

Region

Head

Side

Paired, left and right

About the inferior rectus muscle

The inferior rectus is one of the four straight, or rectus, muscles that move the eye, and it lies along the floor of the orbit beneath the eyeball. Like the other recti it begins at the apex of the orbit and runs forward to the front half of the globe, but because the orbit points outwards while the eye looks forwards, its pull is not straight down. That angle explains the extra movements it produces.

It arises from the lower part of the common tendinous ring, the fibrous ring around the optic canal and the middle of the superior orbital fissure. The muscle runs forwards and slightly laterally, just above the orbital floor and the inferior oblique, which crosses beneath it near its front. It ends in a broad, thin tendon on the lower part of the sclera, in front of the equator and a little less than a centimetre from the edge of the cornea. Its sheath is linked to the lower eyelid, so when the eye looks down the lid follows it slightly.

Its main action is to depress the eye. Because it pulls backwards and medially as well, it also adducts and extorts the eye. Its downward pull is purest when the eye is turned outwards, and in that abducted position it is the only muscle able to lower the gaze; it is tested by asking the patient to look down and out. Its nerve is the lower division of the oculomotor nerve, and its blood comes from the ophthalmic artery's lower muscular branch, sometimes helped by the infraorbital artery, with matching veins.

Weakness or paralysis of the inferior rectus, or very rarely its absence from birth, leaves the eye drifting upwards because the superior rectus is unopposed, giving vertical squint and double vision. Mild cases can be helped with prism glasses; more marked ones may need surgery, such as weakening the superior rectus. The nerve to the inferior oblique runs along the lateral border of the inferior rectus carrying fibres for the ciliary ganglion, so operating here risks disturbing control of the pupil. In thyroid eye disease the inferior rectus is often the first muscle to swell and stiffen, limiting upward gaze.

Attachments, action and supply

Origin

Lower part of the common tendinous ring at the apex of the orbit.

Insertion

Lower surface of the sclera in front of the equator, a little under 1 cm behind the corneal limbus.

Action

Depresses the eye (most purely when abducted); also adducts and extorts it.

Nerve supply

Oculomotor nerve (cranial nerve III), lower division.

Blood supply

The ophthalmic artery's lower muscular branch, at times joined by twigs of the infraorbital artery; matching veins drain it.