A 27-year-old man had pericoronitis of 38 treated by irrigation 5 days ago. He now has severe trismus, a temperature of 39.1 C, pain on swallowing, and the right lateral pharyngeal wall and uvula are pushed toward the left. The floor of the mouth is not raised and there is no swelling below the mandible. Spread into which space best explains these findings, and why is it urgent?