Inferior oblique muscle
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- innervation: inferior branch of the oculomotor nerve (CN III)
- origin: orbital surface of the maxilla
- insertion: globe (posterior, lateral surface)
- primary function: one of two ocular external rotators
- secondary function: one of the two ocular elevators
- tertiary function: one of the three ocular abductors
Inferior oblique arises from the medial orbital surface of the maxilla, and is unique among the extraocular muscles in not originating from the apex of the orbit.
The muscle has an oblique course throughout its length, in comparison to the other extraocular muscles which all begin by passing anteriorly from the orbital apex. It passes between the inferior rectus and the orbital floor then wraps round the inferolateral aspect of the eye, deep to the lateral rectus, before inserting in the posterior surface of the sclera between the superior rectus and lateral rectus muscles 1.
In the neutral position, the primary action of the inferior oblique is to externally rotate the eye (see figure 1) 2. However, because the apex of the orbit is placed medially in the skull, the orbital axis does not correspond with the optical axis of the eye in its neutral position. This means that the inferior oblique has secondary actions of ocular elevation and abduction (see figures 2 and 3).
If the eye is abducted by the lateral rectus such that the optical axis lines up with the orbital axis, the inferior oblique produces external rotation only, as it is acting roughly perpendicular to the orbital axis.
If the eye is adducted by the medial rectus, the optical axis runs roughly perpendicular to the orbital axis, and as a result is lined up with the course of the inferior oblique muscle. Therefore, in adduction the inferior oblique muscle produces ocular elevation, and is solely responsible for this movement.
Thus, when the physician testing eye movements first asks the patient to follow their finger medially then superiorly in the familiar H-shape, the inferior oblique muscle (and the inferior branch of the oculomotor nerve that supplies it) are being directly tested 2.
As the inferior oblique muscle has a short, entirely oblique course, and is closely opposed to the eye for much of its length, it is less well demonstrated than the other extraocular muscles which are all clearly seen end-on surrounded by orbital fat in the coronal plane.
- 1. Gray's basic anatomy. Churchill Livingstone. ISBN:1455710784. Read it at Google Books - Find it at Amazon
- 2. Moore KL, Agur AMR, Dalley AF. Clinically Oriented Anatomy. (2013) ISBN: 9781451119459
- 3. Imaging of the Head and Neck. Thieme. (2012) ISBN:3131505311. Read it at Google Books - Find it at Amazon