The six extraocular muscles of the orbit involved in eye movements are the superior, inferior, medial and lateral rectus muscles, and the superior and inferior oblique muscles.
The four rectus muscles arise from a thickening of the periosteum at the orbital apex known as the common tendinous ring (annulus of Zinn). The muscles pass forward as a muscle cone to be inserted into the anterior sclera of the eyeball.
Superior rectus muscle
Travels superiorly over the globe and has connective tissue links with the levator muscle, which ensures that these muscles work synergistically. Supplied by the superior division of the oculomotor (3rd) nerve the superior rectus primarily elevates the eye, although it also has secondary actions of medial movement (adduction) and nasal rotation (intorsion).
Inferior rectus muscle
Passing underneath the eyeball the inferior rectus shares a connective tissue sheath with the inferior oblique muscle. Supplied by the inferior division of the oculomotor (3rd) nerve the inferior rectus primarily depresses the globe. Additional secondary actions are medial movement (adduction) and temporal rotation (extorsion).
Lateral rectus muscle
The lateral rectus is the only extraocular muscle supplied by the abducen (6th) nerve and is responsible for moving the eye laterally (abduction).
Medial rectus muscle
The medial rectus is the largest of the extraocular muscles, probably due to its importance with allowing convergence for near vision. Supplied by the inferior division of the oculomotor (3rd) nerve the medial rectus is responsible for moving the eye medially (adduction).
Superior oblique muscle
Arising from the sphenoid bone, just above and medial to the optic canal, this long slender muscle travels between the medial wall and roof of the orbit. Anteriorly it forms a tendon which, having passed through the trochlear pulley, turns sharply backwards to pass obliquely over the superior surface of the globe.
The superior oblique is the only extraocular muscle supplied by the trochlear (4th) nerve and the primary action is nasal rotation (intorsion) of the eye. Secondary actions depression and moving the eye laterally (abduction).
Inferior oblique muscle
The inferior oblique is the only extraocular muscle which originates from the anterior orbit, arising from the medial orbital floor. It runs laterally under the inferior rectus muscle to insert into the posterolateral sclera of the globe. Supplied by the inferior division of the oculomotor (3rd) nerve the inferior oblique chiefly acts to rotate the laterally (extorsion). Secondary actions are elevation and lateral movement (abduction) of the globe.
Although discussed separately the position of the eyeball, at any given time, is determined by the tone in all six extraocular muscles. For the globe to move in any given direction no single muscle acts alone, but groups of muscles act as agonists, antagonists or synergists in a highly co-ordinated fashion. Binocular vision requires even finer detailed control so that the eyes move in a conjugate (fused) fashion, allowing the image to fall on identical positions on both retinae and therefore avoiding diplopia (double vision).