trochlear nerve

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun Either of the fourth pair of cranial nerves that originates in the midbrain and innervates a muscle of the eyeball.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun anatomy A motor nerve that innervates the superior oblique muscle of the eye.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun either of the two cranial nerves on either side that control the superior oblique muscles of the eyes