A 62-year-old man is brought to the emergency department after developing sudden double vision and difficulty walking. On exam, he has right-sided ptosis with the right eye deviated "down and out." The right pupil is dilated and minimally reactive to light. He also has a coarse, low-frequency tremor and ataxia of the right upper extremity. MRI shows a focal infarct in the rostral midbrain. Which of the following structures is most directly responsible for the dilated pupil in this patient?