A 25-year-old man was stuck in the face during a fight. He is brought to the emergency room where he can no longer close his mouth. Pain is intense in the right side of his jaw, and bloodstained saliva drips from his mouth. The patient indicates that he cannot feel the skin of his chin or lower lip on the side of the fracture. Radiology revealsa mandibular fracture. The posterior part of the mandible may be displaced superiorly by contractions of which muscle?