The trochlear nerve (/ˈtrɒklɪər/), also called the fourth cranial nerve or CN IV, is a motor nerve (a somatic efferent nerve) that innervates just one muscle: the superior oblique muscle of the eye, which operates through the pulley-like trochlea. The trochlear nerve is unique among the cranial nerves in several respects: Homologous trochlear nerves are found in all jawed vertebrates. The unique features of the trochlear nerve, including its dorsal exit from the brainstem and its contralateral innervation, are seen in the primitive brains of sharks.