A 12-year-old boy develops fever, accompanied by occasional headaches, malaise, fatigue, and nausea a month after being bitten by a dog. One day later, he experiences episodes of rigidity, hallucinations, breath-holding, and difficulty swallowing because of uncontrollable oral secretions. Dr. Louis Pasteur is consulted. He writes: "The death of this child appearing to be inevitable, I decided, not without lively and sore anxiety, as may well be believed, to try ....... the method which I had found constantly successful with dogs. Consequently, 60 hours after the bites was inoculated under a fold of skin with half a syringeful of the spinal cord of a rabbit. In the following days, fresh inoculations were made. I thus made 13 inoculations." The boy survives. Which of the following histologic findings in the brain of the dog is most likely to be present?