Cranial Nerve XII, The Hypoglossal Nerve
This nerve is tested by first observing the tongue (while in floor of mouth) for fasciculations. If present, fasciculations may indicate peripheral CNXII dysfunction. Next, the patient is asked to protrude tongue and move tongue in all directions. When protruded, tongue will deviate toward the side of a lesion (as the unaffected muscles push the tongue more than the weaker side).