The inferior oblique crosses the floor of the orbit in a posterolateral direction between the inferior rectus and the floor of the orbit, before inserting into the outer posterior quadrant just under the lateral rectus.
Contraction of the inferior oblique directs the pupil up and out (Fig. 8.97A).
The inferior branch of the oculomotor nerve innervates the inferior oblique.
To isolate the function of and to test the inferior oblique muscle, a patient is asked to track a physician’s finger medially to bring the axis of the eyeball into alignment with the axis of the muscle and then to look up, which tests the muscle (Fig. 8.97B).
Six of the seven extrinsic muscles of the orbit are directly involved in movements of the eyeball.
For each of the rectus muscles, the medial, lateral, inferior, and superior, and the superior and inferior obliques, a specific action or group of actions can be described (Table 8.8). However, these muscles do not act in isolation. They work as teams of muscles in the coordinated movement of the eyeball to position the pupil as needed.
For example, although the lateral rectus is the muscle primarily responsible for moving the eyeball laterally, it is assisted in this action by the superior and inferior oblique muscles.
The arterial supply to the structures in the orbit, including the eyeball, is by the ophthalmic artery (Fig. 8.99). This vessel is a branch of the internal carotid artery, given off immediately after the internal carotid artery leaves the cavernous sinus. The ophthalmic artery passes into the orbit through the optic canal with the optic nerve.
In the orbit the ophthalmic artery initially lies inferior and lateral to the optic nerve (Fig. 8.99). As it passes forward in the orbit, it crosses superior to the optic nerve and proceeds anteriorly on the medial side of the orbit.