Anatomy of eye movements
There are six external ocular muscles - two are supplied by nerves unique to them: the superior oblique is supplied by the trochlear nerve (IV), and the lateral rectus is supplied by the abducent nerve (VI). The other four external muscles, the internal muscles and the eyelid are supplied by the oculomotor nerve (III).

The four rectus muscles pull directly on the globe so that they move the eye in the direction of their name: superior, inferior, medial and lateral. The superior and inferior recti are not placed centrally so they have a tendency to move the eye medially but this is opposed by the two oblique muscles.

The oblique muscles move the eye outwards as well as up and down. Each hooks around a 'pulley' so that it moves the eye the opposite way to that which its name suggests. Hence, the superior oblique muscle moves the eye down and out (the tramp's muscle) and the inferior oblique muscle moves it up and out.