The news of a newly discovered part of the human body made a big splash earlier this summer: a never-before-noted set of lymphatic vessels in the central nervous system. New discoveries like this one can alter our understanding of anatomy.

But also important? The historic un-discoveries scientists make: for example, the human rete mirabile. Spoiler alert: it doesn't exist. 

A rete mirabile—Latin for "wonderful network"—is a complex circulatory system structure found in some vertebrates. It's a closely interwoven system of arteries and veins whose nearness to each other allows for the exchange of gases, heat, and ions. Retia perform diverse functions. In fish, they allow for the exchange of gases in order to inflate the swim bladder. In birds, body temperature is managed via a series of retia in the legs. Retia are also found in some mammals: the necks of giraffes, for example, contain a blood pressure–stabilizing rete mirabile, and many deep-diving species, like dolphins, use them to regulate oxygen supply.

Galen's Goof

The existence of a rete mirabile located in the human head was first described by the influential Greek physician Galen. In the 2nd century CE, Galen served as physician to the imperial court of the Roman emperor Marcus Aurelius. Thanks to Galen, the contemporary understanding of physiology grew by leaps and bounds. His work was so revered that it became the curriculum for medical students for centuries.

But there was a major impasse in Galen’s oeuvre; dissecting a human cadaver in ancient Rome was not only an unthinkable taboo, it was illegal. So Galen conducted his studies of human anatomy on animal corpses. Of course, we still use animals in human research today; but as you may have noticed, humans bear at least a few anatomical differences from our fellow mammals. Galen’s dissection of a sheep head rather than a human one led to centuries of anatomists dutifully propagating the myth that a human rete mirabile existed.

Galen believed this human rete transformed "vital spirits" (essentially, oxygenated blood) into "animal spirits" (the material of imagination and intellect) that lived in empty spaces inside the brain. To him, the brain itself was just a dull mechanical participant in the activities of the mind, pumping these animal spirits around the body. And because this world-changing physician was so revered, and his word taken so staunchly as fact, the next 1400 years saw advances in neurology come to a near standstill, as the brain itself was ignored in favor of this imaginary system.

Vesalius's Visionary Incisions

By the 1500s, cultural attitudes around death had shifted, and dissecting a cadaver was an acceptable practice. That's how an anatomist named Andreas Vesalius (1514–1564) noticed that this crucial network Galen had described didn’t actually exist, thanks to his study of human specimens provided by a local executioner in Padua, Italy. (Even if you've unfamiliar with his name, it's likely you've seen the beautiful—and often unsettling—anatomical drawings from his groundbreaking, multi-volume work De humani corporis fabrica (On the Fabric of the Human Body). When he cracked open those crania, he couldn’t find the structures Galen had described.

Other academics, however, weren’t impressed by Vesalius’s anatomic heresy. They preferred to believe that their eyes were fallible rather than that the great Galen could be wrong. But the missing rete wasn't the only anatomical error that Vesalius uncovered, and it wasn't even the only mistake localized to the head. Galen believed that the human mandible was made up of two separate bones instead of just one, as the physician had assumed by examining the jawbones of dogs. It would be decades before Vesalius’s view of the brain fully supplanted Galen’s teachings.

Why Don't We Have Our Own Rete Mirabile?

The needs that could have been met by one are fulfilled in other ways. Humans have mechanisms for temperature regulation (sweating, shivering), we don’t spend a lot of time submerged deep in the ocean, and our necks are, thankfully, much shorter than a giraffe's. Of course, at least one human has been born with his very own rete. Doctors thought at first that the structure was a set of malformed blood vessels, but it proved to be a rare anatomic anomaly instead. So let’s give Galen a little posthumous credit where credit is due.