Which cranial nerve is the only one that exits from the back of the human brainstem?

The trochlear nerve is the only cranial nerve that exits from the back of the human brainstem.

It is the fourth cranial nerve, commonly written as cranial nerve IV. After emerging from the dorsal surface of the midbrain, its fibres wrap around the brainstem before reaching the orbit. The trochlear nerve supplies the superior oblique muscle, which helps move and rotate the eye.

The nerve’s unusual route is a useful anatomical landmark. It is also the only cranial nerve whose fibres completely cross within the brainstem before emerging. The superior oblique muscle contributes to movements including depression and intorsion of the eye, especially when the eye is turned inward. Trochlear-nerve injury can cause vertical or torsional double vision, and people may tilt their heads to compensate. The other cranial nerves emerge from different brainstem surfaces or from the forebrain.