A 58-year-old man with a 20-year history of poorly controlled hypertension presents with acute onset of weakness affecting his right face, arm, and leg equally, with preserved sensation and no speech disturbance. Imaging reveals a small lacunar infarct in the distribution of the lenticulostriate branches of the middle cerebral artery. The structure marked **C** in the diagram is responsible for this clinical presentation. Which of the following best explains why motor deficits in this location characteristically spare sensory function and language?