A 75-year-old man has a history of transient attacks of loss of vision. The only abnormalities on physical examination are bruits over the carotids in the neck. Two days later he suddenly becomes hemiplegic and loses consciousness. He is rushed to the emergency room and a CT scan shows evidence of cerebral infarction. He is put on life support but dies 5 days later. At autopsy, there is an area of necrosis and microscopically these lesions are noted to have increased numbers of cells distributed around the central zone of necrosis. Which of the following cell types is most likely to have a phagocytic function in these lesions?