Poirot gave a brief explanation, and the doctor started examining our strange visitor, who seemed quite unconscious of his presence or ours.
"H'm!" said Dr. Ridgeway, when he had finished. "Curious case."
"Brain fever?" I suggested.
The doctor immediately snorted with contempt.
"Brain fever! Brain fever! No such thing as brain fever. An invention of novelists. No; the man's had a shock of some kind. He's come here under the force of a persistent idea—to find M. Hercule Poirot, 14 Farraway Street—and he repeats those words mechanically without in the least knowing what they mean."
"Aphasia?" I said eagerly.
This suggestion did not cause the doctor to snort quite as violently as my last one had done. He made no answer, but handed the man a sheet of paper and a pencil.
"Let's see what he'll do with that," he remarked.
The man did nothing with it for some moments, then he suddenly began to write feverishly. With equal suddenness he stopped and let both paper and pencil fall to the ground. The doctor picked it up, and shook his head.
"Nothing here. Only the figure 4 scrawled a dozen times, each one bigger than the last. Wants to write 14 Farraway Street, I expect. It's an interesting case—very interesting. Can you possibly keep him here until this afternoon? I'm due at the hospital now, but I'll come back this afternoon and make all arrangements about him. It's too interesting a case to be lost sight of."