Much human behavior has by now been interpreted through the lens of neuroscience. Day after day, news outlets publish articles explaining how researchers have mapped some new part of the central nervous system — further correlating our thoughts, motions, and emotions to various “neural substrates” in our spinal cords and brains. The notion that our whole experience of the world is the result of a lot of little things called “neurons” has become, if not necessarily comprehensible, at least familiar to most of us.

Yet neuroscience is a very recent branch of biology. The “neuron doctrine”, which put forward the concept of the nervous system as a composite of discrete individual cells, did not gain traction until the late 1880s. In fact, Camillo Golgi — one of the scientists whose discoveries made the neuron doctrine possible — rejected it and instead clung to the “reticular theory”, which stated that the nervous system’s cells (what we would now call neurons) formed a single network, or reticulum.

The idea that living things are composed of cells is itself not yet two hundred years old. Though the English polymath Robert Hooke (1635–1703) and the Dutch businessman Antonie Van Leeuwenhoek (1632–1723) pioneered the construction and use of microscopes for scientific purposes, it was not until the 1820s that technology improved enough for microscopists to isolate cells. Around the same time, new dyes for staining animal tissue let scientists see that cell bodies (soma) could have only one nerve fibre (axon) but multiple “protoplasmic extensions” (dendrites) which appeared to allow cells to communicate.

Due in large part to technological limitations, even researchers with access to the most powerful nineteenth-century microscopes concluded that cells must communicate by fusing together and forming a network: this was the reticular theory that Golgi (1843–1926) brought to bear on his experiments.

Born the son of a doctor in a small town in Lombardy, Italy, Golgi studied at the University of Pavia, where he eventually specialized in the etiology of mental disorders. Eager to do research but in need of money, he took a job at a hospital, where, in a makeshift laboratory set up in the kitchen, he developed what he called the “black reaction”. Now known as the Golgi method, it involves immersing tissue specimens in silver nitrate, creating a black deposit on the soma, axon, and dendrites which, once placed under the microscope, become clearly visible against a yellow background. This method made it possible not only to trace the connections between neurons but also to visualize the complex networking structure of the brain and spinal cord.

Without the “black reaction”, Santiago Ramón y Cajal (1852–1934) might never have made the discoveries that still inform our understanding of the central nervous system today. Another son of a doctor born in a small town (Petilla de Aragon, in Zaragoza, Spain), Ramón y Cajal was a rebellious child who wanted to be an artist. This was not at all something his father encouraged, and there was a great deal of antipathy between them until, one day, the stern father invited the willful son to help him exhume human remains from local cemeteries. Sketching bones gave Ramón y Cajal a passion for anatomy that propelled him through his university years and medical school in Zaragoza. After a brief, unhappy, malaria-ridden stint as a medical officer for the Spanish army in Cuba, he returned to Spain, where he became a professor — one extremely passionate about the possibilities of modern microscopy.

All through the 1880s, Ramón y Cajal set about improving Golgi’s “black reaction”. Cutting thicker sections of tissue, staining more intensely, and using material from birds and young mammals (whose axons were not sheathed in the fatty substance myelin which interfered with the Golgi method), Ramón y Cajal established a more reliable staining procedure that allowed him, as Stanley Finger writes in Minds Behind the Brain, to “study the elements of the nervous system in a systematic way”.

Unable to find any scientific journal willing to publish his findings — which, like Golgi’s, included many illustrations — Ramón y Cajal founded a journal of his own. Though by no means rich (and, besides, a father of six), he used his own money to publish it and mailed sixty copies of each issue to anatomists around the world. His most revolutionary finding was the utter lack of evidence for either axons or dendrites fusing and forming networks like those described by Golgi. He observed that, on the contrary, it seemed neurons did not need to touch to communicate. They only had to be contiguous for signals to be transmitted from one to the other. (The term “synapse”, used to describe the structure that permits a neuron to pass on its signal, would not be coined, by Charles Sherrington, until 1897.)

In 1906, Golgi and Ramón y Cajal were jointly awarded the Nobel Prize in Physiology or Medicine and invited to share the stage in Stockholm. “The expectation was”, Finger writes, “that Golgi would talk about the stain that allowed scientists to see neurons better than ever before” and Cajal would “describe the studies that led him to neuron doctrine”. However, as soon as he stepped to the podium, Golgi began attacking the neuron theory (calling it “a fad already going out of favor”). Ramón y Cajal was, understandably, not exactly pleased by this attack on his ideas, but, unlike Golgi, he was gracious. He referred to the older man as his “illustrious colleague” and let time do its work.

Below, you can browse a number of Golgi and Ramón y Cajal’s beautiful illustrations of their findings — important depictions of the previously invisible branchings underlying our every thought and gesture.

For more of Cajal's wonderful imagery, also check out the 2017 book The Beautiful Brain: The Drawings of Santiago Ramon y Cajal.