The<unk> , or<unk> s oculi<unk> , is a fusiform<unk>  in the upper, medial side of the orbit whose primary action is intorsion and whose secondary actions are to abduct (laterally rotate) and depress the eyeball (i.e. it makes the eye move outward and downward). One of the extraocular<unk> s, the<unk><unk>  is the only<unk>  innervated by the trochlear nerve.
The primary action of the<unk>  is intorsion; the secondary action is depression (primarily in the adducted position); the tertiary action is abduction.  A brief survey of neurology and physiology texts and websites reveals much confusion about the role of the<unk> , with many sources claiming that its role is to move the eye towards the nose.  In fact, because of its positioning, it is able to rotate the eye away from the nose so that when the eye is already adducted (looking directly "inwards") its rotational action turns the pupil downwards to look towards the mouth, which many texts misinterpret as its primary action.
Usually the eye<unk> s do not work alone, which leads to the confusion of the primary actions.  The<unk><unk>  intorts, depresses, abducts the eye if it were to work by itself.  Normally, the<unk><unk> , in conjunction with the medial rectus, causes the eye to ADduct.
The origin for the<unk><unk>  is the annulus of Zinn at the orbital apex. It loops through a pulley like structure (the trochlea of<unk><unk>  and inserts on the posterotemporal surface of the eyeball.
It arises immediately above the margin of the optic foramen, above and medial to the origin of the<unk>  rectus, and, passing forward, ends in a rounded tendon, which plays in a fibrocartilaginous ring or pulley attached to the trochlear fovea of the frontal bone.
The contiguous surfaces of the tendon and ring are lined by a delicate mucous sheath, and enclosed in a thin fibrous investment.
The tendon is reflected backward, lateralward, and downward beneath the<unk>  rectus to the lateral part of the bulb of the eye, and is inserted onto the scleral surface, behind the equator of the eyeball, the insertion of the<unk>  lying between the<unk>  rectus and Rectus lateralis.
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