9. How does the anatomy of the superior oblique muscle 
    influence its primary and secondary actions in different 
    positions of gaze?

The superior oblique muscle originates from the body of the sphenoid bone outside the tendinous ring. 

It courses forward for about 40 mm along the medial wall of the orbit to the trochlea which is a V-shaped fibrocartilage attached to the frontal bone.

The muscle becomes tendinous about 10 mm posterior to the trochlea and is encased in a synovial sheath through the trochlea.

From the trochlea, it courses posteriorly, laterally and downwards, forming an angle of 54⁰ with the visual axis of the eye in the primary position.

It passes beneath the superior rectus and inserts on the upper temporal quadrant of the globe. This insertion is fanned out in a curved line 10-12 mm in length. 

The muscle is innervated by the fourth cranial nerve which enters the muscle on the upper surface.

Because of this anatomy, the action of the superior oblique muscle differs according to the position of gaze of the eye.
 

• The primary action of the superior oblique muscle is intortion. This action is maximal when the eye is abducted.

 
• The secondary action is depression and is maximal when the globe is adducted.

 
• The tertiary action is abduction and this is maximal when the globe is abducted.