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Tonic water glows blue under a black light because of quinine, a molecule this episode follows from the Andes to the bar cart. The Quechua people of Peru, Bolivia and Ecuador ground cinchona bark into sweetened water to stop shivering from cold. In the 1630s, Jesuit apothecary Agostino Salumbrino in Lima sent the bark to Rome, where malaria from the surrounding marshes was killing popes and cardinals, and it cured the disease. The episode calls this the first known use of a chemical compound to treat an infectious disease, and shows how quinine let European armies push into Africa and Asia.

Andean nations banned seed exports, but in 1865 Manuel Incra Mamani smuggled high yield seeds to Charles Ledger, who sold them to the Dutch, and Mamani later died after a beating. Dutch plantations on Java, controlled from 1913 by the Kina Bureau cartel, supplied 97 percent of the world's quinine by the 1930s, until Japan seized Indonesia in World War II. Today it survives in tonic water at 83 parts per million, while the World Health Organization prefers artemisinins.

• The US Cinchona Missions and why trees taking 10 to 12 years to mature came too late
• Japanese troops dying of malaria despite controlling the supply
• William Henry Perkin's failed 1856 synthesis that produced mauveine dye
• Woodward and Doering's 1944 synthesis and why bark is still cheaper
• How quinine blocks the parasite's heme crystallization, and the dangers of cinchonism

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Touching your nose with your eyes closed relies on proprioception, the sense of body position, movement and force that neurophysiologist Charles Scott Sherrington named in 1906. This episode explains the hardware: muscle spindles that track muscle length and stretch speed, Golgi tendon organs that measure tension, and joint receptors that fire near a joint's limit. Insects and crabs use chordotonal organs and campaniform sensilla for the same job. Most of this data never reaches awareness, handled by spinal reflex loops and the cerebellum, and the PIEZO2 gene encodes the stretch activated ion channel that turns mechanical pull into nerve signals.

The episode then covers what happens when the map fails. Growth spurts, gymnasts' twisties, alcohol and Ehlers-Danlos syndromes all disrupt it, and a vibrating device on the biceps tendon can make people feel their nose growing. Ian Waterman, Charles Freed and a patient described by Oliver Sacks lost proprioception from the neck down after rare autoimmune reactions and had to watch their limbs to move. Finally, plants turn out to sense their own curvature through actin and myosin to straighten without overshooting.

• The stretch reflex that steadies you when a bus lurches
• People with PIEZO2 mutations and profound deficits in joint position sense
• Sacks' patient gripping cups until her knuckles turned white
• Phantom limbs and the brain's hyperexcitable proprioceptive memory
• Why gravity sensing alone would leave a bent plant stem swinging back and forth

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The vagus nerve, the tenth cranial nerve, takes its name from the Latin for wandering. This episode traces its route from the medulla oblongata through the jugular foramen, down the neck in the carotid sheath, through the chest and diaphragm, and out over the stomach, liver and colon. The right vagus holds up to 105,000 fibers and the left upwards of 87,000, and 80 to 90 percent of them are sensory, carrying reports from the organs up to the brain. Otto Loewi showed how it speaks: fluid from a frog heart slowed by vagal stimulation slowed a second heart with no nerve attached, revealing acetylcholine, the first neurotransmitter.

As the brake pedal of the parasympathetic system, the vagus explains vasovagal fainting, feelings of fullness, the vomiting reflex triggered by serotonin, and Arnold's nerve, an ear branch that makes some people cough when cleaning their ears. Doctors now stimulate it to treat epilepsy and drug resistant depression and use the Valsalva maneuver for fast heart rhythms. Cutting it in old ulcer surgery caused B12 deficiency, and a Danish registry study from Aarhus University found full vagotomy cut Parkinson's risk in half.

• How acetylcholine opens potassium channels to hyperpolarize heart cells
• Why a fear spike followed by an overcorrection makes people pass out at the sight of blood
• Vagal signals telling the liver to stop releasing glucose after a meal
• Intrinsic factor and the slow, potentially fatal B12 deficiency after vagotomy
• Misfolded alpha-synuclein possibly climbing from the gut to the substantia nigra

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Oxytocin is sold in pop science as the cuddle hormone, but this episode argues it works more like a biological volume knob for tribalism. Its name comes from Greek words for swift birth. In 1906 British pharmacologist Henry Hallett Dale found that a pituitary extract made the uterus contract, and in 1953 American biochemist Vincent du Vigneaud sequenced and synthesized the nine amino acid hormone, winning the 1955 Nobel Prize in Chemistry. Made in the hypothalamus and released by the posterior pituitary, it drives labor through a rare positive feedback loop and triggers milk letdown, and evolution repurposed that pathway for bonding.

The evidence spans species: virgin sheep given oxytocin into cerebrospinal fluid mothered foreign lambs, knockout mice without receptors failed to recognize familiar mice, and wild chimpanzees showed higher urinary oxytocin after sharing food than after grooming. In humans, nasal spray studies raised trust in strangers. Then the darker turns: oxytocin promoted suspicion of outsiders, raised separation anxiety, and increased lying when the lie benefited a person's group. A starfish relative, asterotocin, even made starfish evert their stomachs.

• Oxytocin spray making participants twice as likely to show the highest level of trust
• Partnered men keeping 10 to 15 centimeters more distance from an attractive stranger
• Testosterone inhibiting oxytocin release in mouse studies
• Why plasma oxytocin stays high, and anxiety rises, when couples are separated
• A blister study linking supportive conversation, higher oxytocin and faster wound healing

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In late November 1803 the ship Maria Pita left A Coruña, Spain, carrying 22 orphan boys aged three to ten as living containers for Edward Jenner's cowpox vaccine. This episode tells the story of the Royal Philanthropic Vaccine Expedition led by military physician Francisco Javier de Balmis under a mandate from King Charles IV, whose own daughter had suffered smallpox. With no refrigeration, the vaccine had to stay alive in human hosts, so two boys at a time were infected and, after about nine or ten days, fluid from their pustules was passed arm to arm to the next pair by lancet. Orphanage rectoress Isabel Zendal Gómez traveled with them to keep the chain intact.

In Puerto Rico the team found the vaccine had already arrived from Danish St. Thomas. In Venezuela the expedition split: deputy José Salvany spent seven years carrying it through the Andes, losing an eye before dying in Cochabamba in 1810, while Balmis went north through Havana to Mexico and then across the Pacific with 25 Mexican orphans. In Manila, Governor General Rafael Aguilar vaccinated his own five children to win public trust, and Balmis later reached Macau and Canton.

• Smallpox killing about 400,000 Europeans a year and causing a third of all blindness
• The ethics of using unconsenting orphans as wards of the state
• Three enslaved girls given to Balmis in Cuba to carry the vaccine to Mexico
• Vaccinating the British population of St. Helena in 1806 during the Anglo-Spanish War
• Jenner's praise and Julia Alvarez's novel Saving the World about Isabel Zendal

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The weight you feel right now is not gravity but the floor pushing up against you. This episode starts there and builds a working explanation of g-force as a measure of acceleration, felt only when one surface presses against another. It walks through why upward acceleration feels identical to a heavier planet, why an unconditioned person sitting upright loses consciousness around four to five Gs, and how gray-out, tunnel vision, blackout and G-LOC follow as blood drains from the eyes and brain. G suits and straining breathing let trained pilots sustain nine Gs, while negative Gs of two to three cause red-out because jugular veins lack one way valves.

The story then moves to the extremes. In 1954 John Stapp rode a rocket sled to Mach 0.9 and stopped in just over a second, enduring a peak of 46.2 Gs and living to 89. Racing driver Kenny Bräck survived 214 Gs in a 2003 IndyCar crash because the spike lasted a fraction of a second. Beyond humans, the mantis shrimp strikes at 10,400 Gs, jellyfish stingers fire at over 5 million Gs, and bacteria kept growing at 403,627 Gs in a Japanese ultracentrifuge.

• Eyeballs in versus eyeballs out: why lying down changes tolerance
• Sustained acceleration versus jerk, and how crumple zones stretch out a crash
• A hearty back slap at 4.1 Gs and a sneeze at 2.9 Gs, compared with 3 Gs on the space shuttle
• Cavitation bubbles that let a mantis shrimp stun prey even when it misses
• Nematodes surviving 400,000 Gs for an hour and what that suggests for panspermia

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In the early 1990s, neurophysiologists at the University of Parma were recording single neurons in area F5 of macaque monkeys when a motor neuron fired as a monkey simply watched a researcher grab food. Nature rejected the finding for lack of general interest, yet mirror neurons became one of the most debated ideas in neuroscience. This episode explains that they are ordinary neurons classified only by behavior, firing both when an animal acts and when it watches the same action, and how a 2010 UCLA study by Roy Mukamel used electrodes already implanted in 21 epilepsy patients to record human mirror cells directly.

From there the claims grew: Christian Keysers' rat studies tied pain mirror neurons in the anterior cingulate cortex to emotional contagion, a charades experiment linked mirroring to the gestural origins of language near Broca's area, and V.S. Ramachandran proposed a role in self awareness. Then came the backlash. The broken mirror theory of autism faded after Ilan Dinstein's fMRI work found intact mirror activity, Patricia Churchland critiqued the hype in Braintrust, and Cecilia Heyes argued mirror properties are learned through association.

• Why fMRI could not prove human mirror neurons existed at the single cell level
• Deactivating the rat anterior cingulate cortex and reducing shared distress
• A Deakin University review finding weak links between mirror activity and emotional empathy
• Lindsay Oberman's 2005 mu wave suppression studies in autistic children
• The chicken or egg problem: how a mirror neuron could know an action's goal before firing

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Michel Lotito, the French entertainer known as Monsieur Mangetout, could swallow a steel bicycle chain without distress, yet a banana gave him severe heartburn. This episode follows his life from his birth in 1950 in Villard-Bonnot, France, to the teenage years when he began eating strange objects, a habit doctors diagnosed as pica. Gastroenterologists found an unusually thick lining in his stomach and intestines and extremely powerful stomach acids, and he prepared every stunt with care: objects cut into small pieces, swallowed whole like pills, eased down with mineral oil and large volumes of water.

Records estimate he consumed about one kilogram of material a day, roughly nine tons over a career running from 1959 to 1997. His most famous claim, eating a Cessna 150 between 1978 and 1980, draws scrutiny: a 2022 Snopes investigation by Madison Dapcevich confirmed he ate dangerous objects on stage but found no hard proof he consumed the whole plane. He died of natural causes in Grenoble in 2007, aged 57, and inspired a novel by Ben Sherwood and a 2016 children's picture book.

• Conflicting accounts of whether he started at age 16 or as early as age nine in 1959
• A menu of 18 bicycles, 15 shopping carts, seven television sets, six chandeliers and a pair of skis
• A waterbed, a coffin with its handles, a computer and a textbook on gravitation
• How he avoided chewing metal and relied on mineral oil and water to protect his throat
• Receiving a Guinness World Records title for strangest diet and eating the brass plaque

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For centuries, anxiety, fainting, insomnia, fluid retention, and even a tendency to cause trouble could all be diagnosed as hysteria, a word taken from the Greek for uterus. This episode traces the diagnosis from Egypt's Kahun Papyri around 1900 BC, which linked many complaints to the uterus, to the Hippocratic texts and Plato's Timaeus, which described a wandering womb seeking moisture. Aretaeus of Cappadocia called it an animal within an animal, and doctors tried to herd it back with foul smells at the nose, sweet scents at the pelvis, and forced sneezing. Galen rejected the wandering womb but blamed retained female seed that turned venomous, calling it the widow's disease.

Medieval writers recast symptoms as demonic possession, and Renaissance physicians such as Abraham Zacuto returned to Galen and prescribed marriage. Philippe Pinel freed patients at the Salpêtrière, Franz Mesmer used charged metal rods, Jean-Martin Charcot photographed patients, George Beard listed 75 pages of symptoms, and Edward Hammond Clarke warned in 1875 that education drained women's reproductive organs. In the 1910s L. E. Emerson at the Boston Psychopathic Hospital found many patients were survivors of sexual violence. Hysteria left the DSM in 1980.

• A brief 18th century period when hysteria was seen as a nervous collapse and diagnosed in men too
• Rachel Maines's vibrator hypothesis and the challenge to it from Hallie Lieberman and Eric Schatzberg
• How police dismissed women reporting assault by labeling them hysterics
• The EEG separating epilepsy from cases once filed as hysteria
• Catherine Clément and Hélène Cixous recasting hysteria as rebellion, and Charlotte Perkins Gilman's The Yellow Wallpaper

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The winged staff wrapped by two snakes on ambulances, uniforms, and drug ads is the caduceus, and this episode explains why it historically has nothing to do with medicine. Its roots go back to Mesopotamia between 4000 and 3000 BC, where the underworld god Ningishzida bore a staff with two intertwined snakes, carved on the libation vase of the Sumerian ruler Gudea around 2100 BC. The symbol also appears in temples of India's Western Ghats and on punch marked coins linked to the Maurya Empire and King Ashoka. In Greek myth Hermes received the golden wand from Apollo in exchange for the lyre he had made from a tortoise shell.

Hermes was the god of messengers, boundaries, merchants, and thieves, and a guide of souls to the underworld, so his staff came to mark Roman diplomats, Renaissance printers such as Johann Froben, the planet and element mercury, and customs agencies today. The symbol of healing is the rod of Asclepius: one snake, a rough branch, no wings. The publisher John Churchill's mark on 19th century medical books blurred the two, and in 1902 the U.S. Army Medical Corps adopted the caduceus.

• The myth of Tiresias, turned into a woman and back again after striking mating snakes with his staff
• Hermes separating two fighting serpents, and Roman envoys called caduceatores
• The caduceus on customs seals in China, Belarus, Bulgaria, and Russia and on Slovakia's Financial Administration emblem
• A survey finding 62 percent of professional health associations use the rod of Asclepius while 76 percent of commercial health organizations use the caduceus
• Stuart L. Tyson's 1932 critique in the Scientific Monthly of a trickster god's emblem on doctors