Final answer:
The fourth cranial nerve, also known as the trochlear nerve, is a motor nerve that controls the superior oblique muscle of the eye, aiding in downward and inward movement. It is tested by asking a patient to track an object with their eyes.
Step-by-step explanation:
The fourth cranial nerve, also known as the trochlear nerve, is primarily a motor nerve that controls one of the muscles of the eye, specifically the superior oblique muscle. This muscle is responsible for the downward and inward movement of the eyeball. The trochlear nerve is unique because it is the only cranial nerve that emerges from the dorsal aspect of the brainstem.To test the functionality of the trochlear nerve, clinicians may perform an exam in which they ask the patient to follow an object with their eyes, looking for smooth tracking movements, particularly in the downward and inward directions. Any difficulty in performing these specific eye movements may indicate damage or impairment of the trochlear nerve.