Three pairs of extraocular muscles move each eye in three directions:
vertically (superior and inferior),
horizontally (medial and lateral), and
torsionally (intorsion when the eye rotates toward the patient's nose and extorsion when the eye rotates toward the patient's shoulder)
All of the muscles except the inferior oblique muscle arise from a common tendinous ring in the posterior part of the orbit.
To determine the action of each muscle, it should be noted that the apex of the orbit (where the superior and inferior rectus and oblique muscles originate) is not parallel with the optical axis (when looking directly forward). Therefore, when the superior and inferior rectus muscles and the oblique muscles contract, there are secondary actions on the eyeball movement, as follows:
- Superior rectus muscle (CN III). Elevation of the eye, with adduction and intorsion.
- Inferior rectus muscle (CN III). Depression of the eye, with adduction and extorsion.
- Lateral rectus muscle (CN VI). Only action is abduction of the eye.
- Medial rectus muscle (CN III). Only action is adduction of the eye.
- Superior oblique muscle (CN IV). Originates in common with the rectus muscles, courses along the medial wall of the orbit, and then turns and loops through a fibrous sling, called the trochlea, before inserting on the superolateral region of the eyeball. The superior oblique muscle moves the eyeball down and out (depression and abduction), with intorsion.
- Inferior oblique muscle (CN III). Originates on the medial wall of the bony orbit and courses laterally and obliquely to insert on the inferolateral surface of the eyeball. The inferior oblique moves the eyeball up and out (elevation and abduction), with extorsion.
Clinical Examination of the Extraocular Muscles
When performing a physical examination of the eye, a physician will test each of the extraocular muscles and their associated cranial nerves by drawing an “H” pattern in the air in front of the patient's face. The patient is instructed to follow the physician's finger with her eyes only.
- Horizontal line of the “H.” The horizontal line of the “H” will test the medial and lateral rectus muscles. The medial rectus muscle adducts the eye, whereas the lateral rectus muscle abducts the eye. The medial and lateral rectus muscles are the only muscles that move the eye in the horizontal plane and are therefore easy to test.
- Vertical lines of the “H.” The vertical motion of the eye is a little more complex than the horizontal motion. The superior and inferior rectus muscles and the superior and inferior oblique muscles control the vertical motion of the eyeball. When a patient's gaze is straight up, it occurs as a result of the combined action of the superior rectus and inferior oblique muscles. When the gaze is straight down, it is from the combined action of the inferior rectus and superior oblique muscles. Therefore, it is essential in testing the extraocular muscles to ensure that only one muscle is tested at a time, without influence from another extraocular muscle. To test any of these four muscles, each muscle must be isolated from the others.
- Lateral vertical line tests the rectus muscles. When the right eye is fully abducted, only the superior and inferior rectus muscles elevate and depress the eye. This is purely a mechanical property because of the axis of the eye lining up parallel to the line of contraction of the superior and inferior rectus muscles.
- Medial vertical line tests the oblique muscles. When the right eye is fully adducted, only the superior and inferior oblique muscles elevate and depress the eye. Again, this is due to the axis of the muscles paralleling the axis of the eye.
Six extraocular muscles control the movement of each eye: four rectus and two oblique muscles.
Rectus Muscles
The four rectus muscles originate at a common ring tendon (annulus of Zinn) surrounding the optic nerve at the posterior apex of the orbit. They are named according to their insertion into the sclera on the medial, lateral, inferior, and superior surfaces of the eye. The principal action of the respective muscles is thus to adduct, abduct, depress, and elevate the globe. The muscles are about 40 mm long, becoming tendinous 4–8 mm from the point of insertion, where they are about 10 mm wide. The approximate distances of the points of insertion from the corneal limbus are: medial rectus, 5.5 mm; inferior rectus, 6.5 mm; lateral rectus, 7 mm; superior rectus, 7.5 mm. With the eye in the primary position, the vertical rectus muscles make an angle of about 23° with the optic axis.
Oblique Muscles
The two oblique muscles primarily control torsional movement and, to a lesser extent, upward and downward movements of the globe.
The superior oblique is the longest and thinnest of the ocular muscles. It originates above and medial to the optic foramen and partially overlaps the origin of the levator palpebrae superioris muscle. The superior oblique has a thin, fusiform belly (30-mm long) and passes anteriorly in the form of a tendon (10-mm long) to its trochlea, or pulley. It is then reflected backwards and downwards as a further length of tendon to attach in a fan shape to the sclera beneath the superior rectus. The trochlea is a cartilaginous structure attached to the frontal bone 3 mm behind the orbital rim. The superior oblique tendon is enclosed in a synovial sheath as it passes through the trochlea.
The inferior oblique muscle originates from the nasal side of the orbital wall just behind the inferior orbital rim and lateral to the nasolacrimal duct. It passes beneath the inferior rectus and then under the lateral rectus muscle to insert onto the sclera with a short tendon. The insertion is into the posterotemporal segment of the globe and just over the macular area. The muscle is about 35-mm long.
In the primary position, the muscle plane of the superior and inferior oblique muscles forms an angle of 51–54° with the optic axis.
Fascia
All the extraocular muscles are ensheathed by fascia. Near the points of insertion of these muscles, the fascia is continuous with Tenon's capsule, and fascial condensations to adjacent orbital structures (check ligaments) act as the functional origins of the extraocular muscles (Figures 1–19 and 1–20).
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Blood Supply
The blood supply to the extraocular muscles is derived from the muscular branches of the ophthalmic artery. The lateral rectus and inferior oblique muscles are also supplied by branches from the lacrimal artery and the infraorbital artery, respectively.
Study Questions
All of the following intraocular muscles are innervated by cranial nerve III EXCEPT:
The correct answer is "C." All of the ocular muscles are innervated by cranial nerve III (CN3) except for the lateral rectus (CN6) and the superior oblique (CN4).
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