Microsurgical Anatomy of the Orbit: The Rule of Seven
Figure 3
(a) The roof and lateral wall of the left orbit have been removed. Beyond the optic canal and superior orbital fissure, the bony orbit is covered by an extension of the cranial dura mater, the periorbit. The periorbit or orbital dura envelops and holds the intraorbital structures, especially the orbital fat. (b) With the exception of the orbicularis oculi muscle (Figure 1(a)), located along the base of the orbital pyramid, seven are the intraorbital muscles: levator palpebrae, superior, inferior, lateral, and medial rectus, and superior and inferior oblique muscles. Of these, only the inferior oblique muscle is attached to the medial orbital wall. The four recti and the superior oblique muscles attach along the orbital apex in and around a common annular tendon or fibrous ring, the annulus of Zinn. (c) The annulus inserts along the orbital apex, encircling the orbital opening of the optic canal and the central part of the superior orbital fissure. Structures coursing through the annulus are the optic nerve (II) and ophthalmic artery (OA), the oculomotor (III) and abducens (VI) nerves, and the nasociliary part (N) of the ophthalmic division of the trigeminal nerve. The troclear nerve (IV) the frontal (F) and lacrimal (L) divisions of the trigeminal nerve course outside the annulus, on the lateral sector of the superior orbital fissure.
