**Core Concept**
The superior oblique muscle is one of the six extraocular muscles responsible for controlling eye movements. It plays a crucial role in the rotational movement of the eyeball, specifically in the depression and intorsion of the eye.

**Why the Correct Answer is Right**
The primary action of the superior oblique muscle is to depress the eyeball, particularly when the eye is adducted (moved towards the midline of the body). This muscle also causes intorsion (inward rotation) of the eyeball, meaning it rotates the top of the eye towards the nose. The superior oblique muscle achieves this action through its tendon, which wraps around the trochlea (a small bony structure in the orbit) before inserting into the sclera of the eyeball.

**Why Each Wrong Option is Incorrect**
* **Option A:** The superior oblique muscle does not primarily abduct the eyeball, as this action is primarily carried out by the lateral rectus muscle.
* **Option B:** The superior oblique muscle does not primarily elevate the eyeball, as this action is primarily carried out by the superior rectus muscle.
* **Option C:** The superior oblique muscle does not primarily extort (outward rotate) the eyeball, as this action is primarily carried out by the inferior oblique muscle.

**Clinical Pearl / High-Yield Fact**
The superior oblique muscle is often weakened or paralyzed in conditions such as Brown's syndrome, where there is an abnormality in the superior oblique tendon or its insertion. This can lead to impaired depression and intorsion of the eyeball.

**Correct Answer: C. Intorsion (inward rotation) and depression of the eyeball.**