A 24-year-old man has had pain around a partly erupted 38 for 5 days. He can now open only 12 mm. There is firm, tender fullness over the angle and lower ramus of the mandible, deep to the masseter, but little visible swelling. Intraorally the pterygomandibular fold, soft palate and tonsillar pillars look normal, the floor of the mouth is soft and he swallows normally. His temperature is 37.9 C. Spread into which space best explains these findings?