Question at position 18 What is the main action of the medial rectus muscle?Depression of eyeballElevation of eyeballAbduction of eyeballAdduction of eyeball

Question

BlackTom AI · Accepted Answer

Medial rectus muscle adducts the eyeball toward the nose (moves the pupil medially).

1. Relevant knowledge points and concepts
- Extraocular muscles control eye movements: medial, lateral, superior, and inferior recti, plus superior and inferior oblique.
- Primary actions: medial rectus = adduction (moves eye toward midline); lateral rectus = abduction (moves eye away from midline); superior rectus/inferior oblique = elevation; inferior rectus/superior oblique = depression (with torsion components).
- Innervation: medial rectus is supplied by the oculomotor nerve (CN III).

2. Step-by-step reasoning process
- Identify the insertion: the medial rectus originates on the common tendinous ring and inserts on the medial aspect of the sclera.
- Determine the pull direction: contraction shortens the muscle and pulls the anterior sclera medially, rotating the eyeball around its vertical axis.
- Translate that rotation into the named action: rotation toward the nose is called adduction.

3. Intermediate steps (anatomical mechanics)
- Muscle vector: medial rectus exerts a horizontal inward vector (no significant vertical component), so its primary mechanical effect is horizontal movement.
- Compare to other recti: lateral rectus pulls opposite (outward), while superior and inferior recti have vertical and some torsional components—confirming medial rectus is purely medial movement.

4. Why the correct answer is correct
- Because the medial rectus's line of pull is directly toward the midline, its defined primary action is adduction of the eyeball. Its attachment and innervation (CN III) support this role in moving the globe medially.

5. Why the other options are incorrect
- Depression of eyeball: primarily by inferior rectus and superior oblique, not medial rectus.
- Elevation of eyeball: primarily by superior rectus and inferior oblique.
- Abduction of eyeball: performed by the lateral rectus (innervated by CN VI), opposite to medial rectus.

6. Conclusion and confirmation
- The correct answer is Adduction of eyeball, which matches the provided answer.

Question at position 18 What is the main action of the medial rectus muscle?Depression of eyeballElevation of eyeballAbduction of eyeballAdduction of eyeballSingle choice

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