As these muscles pass forward in the orbit to attach to the anterior half of the eyeball, they are also directed laterally (Fig. 8.95). Because of these orientations:
Contraction of the superior rectus elevates, adducts, and internally rotates the eyeball (Fig. 8.97A).
Contraction of the inferior rectus depresses, adducts, and externally rotates the eyeball (Fig. 8.97A).
The superior branch of the oculomotor nerve [III] innervates the superior rectus, and the inferior branch of the oculomotor nerve [III] innervates the inferior rectus.
To isolate the function of and to test the superior and inferior rectus muscles, a patient is asked to track a physician’s finger laterally and then either upward or downward (Fig. 8.97B). The first movement brings the axis of the eyeball into alignment with the long axis of the superior and inferior rectus muscles. Moving the finger upward tests the superior rectus muscle and moving it downward tests the inferior rectus muscle (Fig. 8.97B).
The orientation and actions of the medial and lateral rectus muscles are more straightforward than those of the superior and inferior rectus muscles.
The medial rectus originates from the medial part of the common tendinous ring medial to and below the optic canal, whereas the lateral rectus originates from the lateral part of the common tendinous ring as the common tendinous ring bridges the superior orbital fissure (Fig. 8.96).
The medial and lateral rectus muscles pass forward and attach to the anterior half of the eyeball (Fig. 8.95). Contraction of medial rectus adducts the eyeball, whereas contraction of lateral rectus abducts the eyeball (Fig. 8.97A).
The inferior branch of the oculomotor nerve [III] innervates the medial rectus, and the abducent nerve [VI] innervates the lateral rectus.
To isolate the function of and test the medial and lateral rectus muscles, a patient is asked to track a physician’s finger medially and laterally, respectively, in the horizontal plane (Fig. 8.97B).
The oblique muscles are in the superior and inferior parts of the orbit, do not originate from the common tendinous ring, are angular in their approaches to the eyeball, and, unlike the rectus muscles, attach to the posterior half of the eyeball (Table 8.8).
The superior oblique arises from the body of the sphenoid, superior and medial to the optic canal and medial to the origin of the levator palpebrae superioris (Figs. 8.95 and 8.96). It passes forward, along the medial border of the roof of the orbit, until it reaches a fibrocartilaginous pulley (the trochlea), which is attached to the trochlear fovea of the frontal bone.
The tendon of the superior oblique passes through the trochlea and turns laterally to cross the eyeball in a posterolateral direction. It continues deep to the superior rectus muscle and inserts into the outer posterior quadrant of the eyeball.
Contraction of the superior oblique therefore directs the pupil down and out (Fig. 8.97A).
The trochlear nerve [IV] innervates the superior oblique along its superior surface.
To isolate the function of and to test the superior oblique muscle, a patient is asked to track a physician’s finger medially to bring the axis of the tendon of the muscle into alignment with the axis of the eyeball, and then to look down, which tests the muscle (Fig. 8.97B).
The inferior oblique is the only extrinsic muscle that does not take origin from the posterior part of the orbit. It arises from the medial side of the floor of the orbit, just posterior to the orbital rim, and is attached to the orbital surface of the maxilla just lateral to the nasolacrimal groove (Fig. 8.95).