Working alone at the turn of the 20th century in Spain, Santiago Ramón y Cajal (1852-1934) ventured into science as both an artist and a pathologist, and became the first person to see a neuron. Working by gaslight, he made thin slices of brain tissue and subjected them to the same silver-nitrate chemistry he used to capture images on photographic plates.

Peering through a microscope at the silver-stained tissue, Cajal saw a thicket of bizarre black shapes resembling swarms of spiny insects embedded in translucent amber. Other scientists examining similar preparations perceived only a bewildering tangle of continuous fibers, which they presumed transmitted nervous energy throughout the brain, like vibrations through a spiderweb. But Cajal observed his slides with an artist’s keen eye for discerning form and function amid chaos, and he saw neurons — individual cells, each one a separate, unique jewel of intricate beauty.

Moreover, Cajal saw that the neuron is not a knot in a network that broadcasts signals in every direction: The neuron, he concluded, must pass electrical information in only one direction. Simply from their form, Cajal deduced that nervous signals enter the neuron through its elaborate rootlike dendrites and exit through its single slender axon, and that one neuron relays messages to the next by passing information across a gap of separation, the synapse.

Cajal’s two brilliant insights — that every neuron in the brain is separate and that neurons communicate across synapses — came to be known as the neuron doctrine. Because that gap between neurons is too small to see through a light microscope, Camillo Golgi and other rigorous scientists of Cajal’s day at first dismissed the neuron doctrine as a fantasy. It would take another half-century until a new instrument, the electron microscope, could finally confirm what Cajal had seen in his mind’s eye — and carefully sketched out in thousands of stunning pen-and-ink diagrams.

But long before the synapse was visible, Cajal’s neuron doctrine had transformed scientists’ understanding of the nervous system and formed the bedrock upon which neuroscience is built. For that reason, Cajal was awarded the 1906 Nobel Prize in physiology or medicine (an honor that, in an ironic twist, he shared with Golgi, who had invented the silver-staining technique that made Cajal’s observations possible). And Cajal’s exquisite, meticulous drawings of neurons in the brain and spinal cord proved to be powerful tools for persuasively communicating his vision to the scientific world. Even today, they continue to inspire neuroscientists.

The strange thing is that every one of Cajal’s immortal drawings is marred by an odd bit of deliberate vandalism: a blue cataloging stamp, often placed directly in the middle of the artwork. The first time I saw one of his drawings of neurons, when I was a neuroscience graduate student at the University of California, San Diego, I was struck by its beauty and intricacy, but I thought it was a postcard because it was defaced by what looked like an ugly postmark. My mentor, the neuroethologist Theodore H. Bullock, assured me that all of Cajal’s drawings had that stamp on them. Why? He didn’t know. In the decades since, when I’ve talked about this with others in neuroscience, no one could solve the mystery.