takes a courageous person,” the poet Kay Ryan once wrote, “to leave spaces empty”—and Cajal, the draftsman-scientist, was anything but timorous. That space—about twenty to forty nanometers in distance—is left blank. It is tiny; you could wave it away. Perhaps it’s an artifact of microscopy or staining. But like the negative space in a Chinese painting, that space might represent the most important element of the whole drawing—and arguably, of the entire physiology of the nervous system. It immediately raises the question of why such a blank space exists: If you were building a nervous system
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