This nerve supplies muscles of the tongue. To test the nerve ask the patient to protrude the tongue. In a normal person the protruded tongue lies in the midline. If the nerve is paralysed the tongue deviates to the paralysed side. The explanation for this is as follows.
Protrusion of the tongue is produced by the pull of the right and left genioglossus muscles. The origin of the right and left genioglossus muscles lies anteriorly (on the hyoid bone), and the insertion lies posteriorly (on to the posterior part of the tongue). Each muscle draws the posterior part of the tongue forwards and medially. Normally, the medial pull of the two muscles cancels out, but when one muscle is paralysed it is this medial pull of the intact muscle that causes the tongue to deviate to the opposite side.
Deviation of the tongue should be assessed with reference to the incisor teeth, and not to the lips. Remember that in facial paralysis the tongue may protrude normally, but may appear to deviate to one side because of asymmetry of the mouth.
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