Lateral rectus

The lateral rectus is a striated muscle of the orbit, part of the muscles of the eyeball and the most lateral of the four rectus muscles. It originates around the optic canal and enlarges as it projects rostrally towards the eyeball, where it terminates in an aponeurotic insertion lamina. It causes the abduction of the eye.

Configuration: Orbital straight striated muscle

Origin: Apex of the orbit, surrounding the optic canal

Insertion: Lateral part of the pre-equatorial region of the eyeball

Relationships: Posterior to eyeball, lateral to other eyeball muscles, overlying the retractor bulbi

Action: Mainly eye abduction

Antagonist: Medial rectus muscle

Vascularization: Musclar branches of external ophthalmic artery, ophthalmic plexus

Innervation: Abducent nerve

Description:

The lateral rectus muscle is one of the striated muscles of the orbit and one of the four rectus muscles of the eyeball.

It arises at the apex of the orbit near the optic canal, attaching to the periorbita.

The four rectus muscles are similar in shape, rectilinear and gradually widening towards their insertion on the eyeball, although there may be slight variations in size and length between them. The lateral rectus travels through the orbit in a lateral position in relation to the other orbital muscles. It inserts through an aponeurotic lamina, on the lateral part of the eyeball, in its pre-equatorial region, between the corneal limbus and the equator of the eyeball. With the other rectus muscles, it forms a cone that surrounds the optic nerve and the retractor bulbi muscle, thus covering it laterally.

Contraction of this muscle triggers abduction of the eye. The medial rectus muscle is antagonistic.

It is supplied by muscular branches of the external ophthalmic artery, and drained by veins forming part of the ophthalmic plexus.

Unlike the other recti muscles, it is not innervated by the oculomotor nerve, but by the abducens nerve, which enters the orbit through the orbital fissure and reaches the medial surface of the muscle.

Variations:

In equids, the recti muscles are inserted a few millimeters posterior to the corneal limbus, with thin aponeurosis.

In cattle, the recti muscles are particularly wide and thick. They are inserted close to the corneal limbus by tendinous blades.

In sheep, the recti muscles are attached to the sclera by tendinous blades, but their distance from the corneal limbus varies between muscles. Varying from 1 mm for the lateral rectus to 1 cm for the medial rectus.

In pigs, the recti muscles have strong origins in deep pits in the orbital wall. Their insertions are thin, close to the corneal limbus, except for the medial rectus, which inserts more posteriorly, close to the equator of the eyeball.

In the dog, the recti muscles are narrow and insert via aponeurotic endings between 3 and 7 mm posterior to the corneal limbus.

In cats, the recti muscles terminate in thin but broad aponeurotic blades. These insertions lie relatively far from the corneal limbus, between 6 and 10 mm, particularly the insertion of the lateral rectus muscle.

In rabbits, the recti muscles insert via wide aponeuroses into the eyeball, close to the corneal limbus, except for the medial rectus, whose insertion is more posterior.

In men, the recti muscles originate from a common tendinous ring around the superior orbital fissure and the optic canal. Their insertions are made by terminal aponeuroses between 5 and 8 mm posterior to the corneal limbus.