Nerve · Head

Trochlear nerve

Nervus trochlearis

The trochlear is the odd one out among the cranial nerves in three separate ways: it is the smallest by fibre count, it is the only one to leave the brainstem from the back rather than the front, and it is the only one whose fibres cross completely inside the brainstem — so the left nucleus drives the right eye. All of that effort is for a single muscle, superior oblique, whose tendon runs through a cartilaginous pulley on the frontal bone before turning back to the eyeball, which is what lets it roll the eye inward and pull it down when the eye is turned toward the nose. Having emerged behind the midbrain it has the longest course inside the skull of any cranial nerve, curving right around the brainstem before it can start heading forward.