Which cranial nerve is responsible for the ability to move eyes downward and inward?

The trochlear nerve is responsible for the ability to move the eyes downward and inward, specifically by innervating the superior oblique muscle of the eye. This muscle plays a crucial role in controlling eye movement, particularly in actions such as looking down or toward the nose.

When the trochlear nerve is functioning properly, it enables smooth and coordinated movements of the eyeball, which is essential for activities like reading or looking at objects that are close. The primary function of this nerve highlights its importance in binocular vision and depth perception.

In contrast, the oculomotor nerve primarily controls other eye muscles and is responsible for movements such as elevating the eyelid and constricting the pupil. The abducens nerve mainly controls lateral eye movement, allowing for outward gaze. The facial nerve is not involved in eye movement but rather governs facial expression and taste sensations. Therefore, the trochlear nerve's specific role in descending and internal eye movement makes it the correct answer to the question.