Trochlear nerve

The fourth of twelve cranial nerves, the trochlear nerve controls the function of the superior oblique muscle, which rotates the eye away from the nose and also moves the eye downward. It is the only cranial nerve to originate from the dorsal part of the brainstem. It is also the smallest of the cranial nerves.

A brief survey of neurology and physiology texts and websites reveals much confusion about the role of the superior oblique muscle, with many sources claiming that its role is to move the eye towards the nose. In fact, because of its positioning, it is able to rotate the eye away from the nose so that when the eye is already adducted (looking directly "inwards") its rotational action turns the pupil downwards to look towards the mouth, which many texts misinterpret as its primary action.

This nerve is sometimes called the 'pathetic' nerve as loss of its function paralyses the superior oblique muscle giving the eyes a pathetic look.

Etymology
The term trochlea means "pulley" in Latin. Specifically, the trochlea referred to is a loop inside the orbit of the eye, through which the tendon of the superior oblique muscle passes. The nerve is called trochlear because of its association with this muscle.