A 26-year-old man has had pain around a partly erupted mandibular left third molar for 4 days. He can open his mouth only 12 mm and it hurts to swallow, yet there is almost no extraoral swelling and the floor of the mouth is soft. The anterior tonsillar pillar on the left is slightly red but the pharyngeal wall is not displaced. Between which structures has the pus most likely collected?