The final common pathways that influence the position of the eye within the orbit are the numerous soft-tissue elements connected to the globe. In addition to the EOMs, these tissues include the optic nerve, Tenon capsule, blood vessels, and the conjunctiva anteriorly. Orbital anatomy is discussed in BCSC Section 7, Oculofacial Plastic and Orbital Surgery.
Of the 6 EOMs, 4 are rectus muscles (lateral, medial, superior, and inferior), and 2 are oblique (superior and inferior). The rectus muscles originate—along with the levator palpebrae superioris muscle—at the annulus of Zinn, a condensation of tissue around the optic nerve at the orbital apex. They run forward within sheaths that are connected by intermuscular septa to pierce the posterior Tenon capsule and insert on the anterior sclera, at points variably posterior to the corneal limbus, increasing from the medial through the inferior and lateral to the superior (spiral of Tillaux). The rectus muscles are also maintained in position by septal attachments to the orbital periosteum that act as pulleys.
The 2 oblique muscles insert on the posterior lateral aspect of the globe. The origin of the inferior oblique muscle is in the anteromedial periorbita near the posterior margin of the lacrimal fossa. The effective origin of the superior oblique muscle is the trochlea, a pulleylike structure located at the notch in the superior medial orbit. The superior oblique muscle runs anteriorly in the superior medial orbit to the trochlea, where its tendon reverses its direction of action.
The EOMs are of variable mass and cross section: The inferior oblique is the thinnest, and the medial rectus is the largest. Thus, with normal tonic innervation, the somewhat stronger medial rectus reduces the divergent phoria.
Each of the other muscles has primary, secondary, and tertiary functions that vary depending on the position of gaze (Fig 1-33). For further discussion and illustration of the EOMs and their actions, see BCSC Section 6, Pediatric Ophthalmology and Strabismus. Also see Chapter 7 in this volume for the clinical etiologies of EOM dysfunction.
Figure 1-33 Primary, secondary, and tertiary functions of the extraocular muscles, right eye.
(Illustration by Christine Gralapp.)
Excerpted from BCSC 2020-2021 series: Section 5 - Neuro-Ophthalmology. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.