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In the middle of the 16th century,

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a talented young anatomist named Andreas
Vesalius made a shocking discovery:

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the most famous human anatomy texts in 
the world were wrong.

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They not only failed to account for many
details of the human body,

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they also described the organs of apes
and other mammals.

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While Vesalius knew he was right,

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announcing these errors would mean
challenging Galen of Pergamon–

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the most renowned physician 
in medical history.

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But who was this towering figure?

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And why did doctors working more than
1,300 years later so revere and fear him?

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Born in 129 CE,

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Galen left home as a teen to scour the
Mediterranean for medical wisdom.

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He returned home a gifted surgeon with a
passion for anatomy

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and a penchant for showmanship.

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He gleefully entered public anatomy 
contests,

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eager to show up his fellow physicians.

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In one demonstration,

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he caused a pig to lose its voice by tying
off one of its nerves.

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In another, he disemboweled a monkey and
challenged his colleagues to repair it.

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When they couldn’t, he did.

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These grizzly feats won him a position as 
surgeon to the city’s gladiators.

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Eventually, he would leave the arena 
to become the personal physician

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to four Roman Emperors.

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While his peers debated symptoms and 
their origins,

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Galen obsessively studied anatomy.

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He was convinced that each organ had a
specific function.

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Since the Roman government largely 
prohibited working with human cadavers,

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Galen conducted countless dissections 
of animals instead.

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Even with this constraint,

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his exhaustive investigations yielded 
some remarkably accurate conclusions.

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One of Galen’s most important 
contributions

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was the insight that the brain,
not the heart, controlled the body.

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He confirmed this theory by opening the
cranium of a living cow.

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By applying pressure to different 
parts of the brain,

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he could link various regions 
to specific functions.

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Other experiments allowed him to 
distinguish sensory from motor nerves,

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establish that urine was 
made in the kidneys,

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and deduce that respiration was 
controlled by muscles and nerves.

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But these wild experiments also produced 
extraordinary misconceptions.

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Galen never realized that blood cycles
continuously throughout the body.

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Instead, he believed the liver constantly
produces an endless supply of blood,

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which gets entirely depleted on its 
one-way trip to the organs.

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Galen is also credited with solidifying 
the popular theory of the Four Humours.

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Introduced by Hippocrates 
centuries earlier,

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this misguided hypothesis attributed most
medical problems

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to an imbalance in four bodily fluids 
called humours.

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To correct the balance of these fluids, 
doctors employed dangerous treatments

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like bloodletting and purging.

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Informed by his poor understanding 
of the circulatory system,

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Galen was a strong proponent 
of these treatments,

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despite their sometimes lethal 
consequences.

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Unfortunately, Galen’s ego 
drove him to believe that

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all his discoveries were 
of the utmost importance.

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He penned treatises on everything from
anatomy to nutrition to bedside manner,

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meticulously cataloguing his writings 
to ensure their preservation.

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Over the next 13 centuries,

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Galen’s prolific collection dominated 
all other schools of medical thought.

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His texts became the standard works 
taught to new generations of doctors,

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who in turn, wrote new essays extolling 
Galen’s ideas.

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Even doctors who actually dissected 
human cadavers

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would bafflingly repeat Galen’s mistakes,

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despite seeing clear evidence 
to the contrary.

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Meanwhile, the few practitioners bold 
enough to offer conflicting opinions

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were either ignored or ridiculed.

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For 1,300 years, Galen’s legacy 
remained untouchable–

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until renaissance anatomist Vesalius 
spoke out against him.

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As a prominent scientist and lecturer,

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his authority influenced many young 
doctors of his time.

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But even then, it took another
hundred years

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for an accurate description 
of blood flow to emerge,

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and two hundred more for the theory 
of the Four Humours to fade.

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Hopefully, today we can reap the benefits
of Galen’s experiments

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without attributing equal credence 
to his less accurate ideas.

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But perhaps just as valuable

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is the reminder that science is an 
ever-evolving process,

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which should always place 
evidence above ego.