Final answer:
The hypoglossal nerve, or twelfth cranial nerve, controls the tongue muscles and is tested by observing the direction the tongue protrudes when a patient is asked to 'stick out your tongue.'
Step-by-step explanation:
The hypoglossal nerve, also known as the twelfth cranial nerve, is responsible for the contraction of the tongue muscles except for the palatoglossus muscle, which is innervated by the vagus nerve.
The extrinsic muscles of the tongue are connected to other structures, whereas the intrinsic muscles are fully contained within the tongue itself. Testing the function of the hypoglossal nerve includes asking a patient to 'stick out your tongue,' which is an examination of the genioglossus muscle and allows for the assessment of potential nerve impairment. Normal function is shown by the tongue sticking straight out, while deviation to one side may suggest damage to the nerve on that side, possibly also indicating issues in adjacent anatomical structures.