dorsomedial surface of the nerve, where they are anatomically vulnerable to compression. The uncus, which is the most medial aspect of the temporal lobe, is located just above the tentorium and the subarachnoid portion of CN III. Unilateral supratentorial mass lesions may force the uncus through the tentorial notch (uncal herniation) to compress the ipsilateral CN III.

At the back edge of the dura of the clivus and cavernous sinus, the nerve enters its own dural canal just above CN IV. Running forward in the superior lateral wall of the cavernous sinus, the nerve separates into a superior and an inferior division. These divisions enter the orbit through the superior orbital fissure within the annulus of Zinn. The superior division runs forward intraconally to innervate first the superior rectus muscle and then the levator palpebrae superioris muscle. The inferior division sends parasympathetic fibers to the ciliary ganglion in the orbital apex approximately 10 mm anterior to the annulus of Zinn and lateral to the optic nerve. Within the ciliary ganglion, the fibers destined for the pupillary sphincter and the ciliary body synapse. The fibers subsequently accompany the branch destined for the inferior oblique muscle. There are approximately 9–10 times as many fibers associated with accommodation innervating the ciliary body as there are fibers reaching the pupillary sphincter muscle. This disparity may be one reason for the development of light–near dissociation in Adie tonic pupil (see Chapter 10). The remaining branches of CN III within the orbit innervate the medial rectus and inferior rectus muscles.

Bhatti MT, Eisenschenk S, Roper SN, Guy JR. Superior divisional third cranial nerve paresis: clinical and anatomical observations of 2 unique cases. Arch Neurol. 2006;63(5):771–776.

Extraocular Muscles

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, Orbit, Eyelids, and Lacrimal System.)

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 a primary, secondary, and tertiary function that varies depending on the position of gaze (Fig 1-35). For further discussion and illustration of the EOMs and their actions, see BCSC Section 6, Pediatric Ophthalmology and Strabismus. Also see Chapter 8 for the clinical