The human body is full of weird, gross and awe-inspiring stuff as we know it—but for people who lived when ideas were unbound by strict anatomical correctness, it was even more so. Here are 10 things people thought, and in some cases still think, were inhabiting their bodies.

1. THREE SOULS

The existence of a physical soul inside the body was widely accepted by Greek philosophers and physicians. Plato had posited in the Timaeus that there were three souls: the Immortal Soul in the head, and a Mortal Soul divided into two parts by the diaphragm. The upper Mortal Soul was the Irascible Soul which assisted reason against desire. Beneath the diaphragm was the Appetitive Soul, which just plain desired. The diaphragm was an essential barrier keeping this hungry proto-id from being a bad influence on the Immortal Soul.

Claudius Galenus (130-199 CE), doctor to the gladiators of Pergamon, had a great deal of practical experience in the treatment of injury. However, he only ever dissected animals (mainly monkeys), so as far as he knew, there was all kinds of space inside the human body for souls to inhabit. Following in Plato's footsteps, Galen defined the three anatomical souls thus: Rational, seated in the head; Spirited, seated in the heart; and Desiderative, seated in the liver. These spirits were composed of matter, corporeal even if ethereal.

2. PNEUMA

Five systems of the body—arterial, venous, skeletal, nervous, muscular—as described by Galen. From the "Salomon Glossaries," Germany, 1158 and 1165, Bayerische Staatsbibliothek, Munich.

Hippocrates (460-370 BCE), the father of Western medicine and he of oath fame, and his successors, most notably Praxagoras (born ca. 340 BCE), believed that the arteries carried a vital force called pneuma, a sort of heated air that was the source of both life and spirit.

Cadaver dissection was taboo, so most of the ancient Greek doctors got their ideas about the soft tissues of bodies from observation of the living or the vivisection and dissection of animals. Because the arteries of dead animals are virtually empty, Praxagoras concluded that arteries and veins were separate circulatory systems, and that the former carried pneuma while the latter carried the blood. In other words, arteries delivered vital heat to the organs. When the pneuma reached the brain and heart, it generated thought and action; veins just delivered nourishment.

It took almost 2000 years for the existence of pneuma to be seriously challenged. Around 1508, Leonardo da Vinci wrote, “All the veins and arteries arise from the heart. And the reason is that the largest veins and arteries are found at their conjunction with the heart, and the further they are removed from the heart, the finer they become, dividing into very small branches.” In 1628 William Harvey (1578-1657) demonstrated that arteries and veins were both part of the same circulatory system, disproving pneuma once and for all.

3. COMMON SENSE

Now it's a metaphor, but common sense started out as a physical organ. Aristotle believed there was a spot near the heart where all the data from the senses was interpreted and adjudicated. The sense organs perceived information about the person's environment. That information was carried, probably by the blood, to a single central faculty called the sensus communis. The sensus communis then processed the information and converted it into a reaction or understanding.

Galen agreed with the concept, but he thought the sensory information was transmitted by the pneuma, not the blood, through hollow sensory nerves to the brain, not to the heart. The brain would then pump the pneuma into motor nerves and from there into muscles where the information became movement. Since Galen was held to be an unimpeachable source on medical matters well into the Renaissance, people kept on looking for the sensus communis in the brain until anatomists in the 17th century studied enough actual human brains to conclude that there was no such thing as the common sense.

4. AND 5. DEMONS AND GHOSTS

The earliest surviving medical texts are Sumerian cuneiform tablets engraved around 2100 BCE. By then, Mesopotamian medical precepts that disease (as opposed to injury) was caused by an irate god or demon possession were already firmly established. There were two kinds of healers: the ashipu, or exorcist, and the asu, or physician/pharmacist. The ashipu diagnosed the patient by determining which god or demon was causing the illness, and performed the incantations necessary to drive out the possessing spirit. The asu treated wounds and prescribed herbal remedies. Sometimes they worked together.

Demonic or ghostly possession as a physical invasion of the body causing a panoply of symptoms from seizures to self-harm to glossolalia carried forward into Jewish and later Christian tradition. There are exorcisms in the Dead Sea Scrolls and the New Testament. Jewish writings from the Middle Ages and early modern era report demons or dybbuks (spirits of the dead) physically leaving the body of the possessed through bloody fingernails or toenails, or in worst case scenarios, from the throat, vagina, or rectum.

6. HOMUNCULUS

Drawing of a homunculus inside a sperm cell byNicolaas Hartsoeker, published in "Essay de dioptrique," 1694.

Antonie van Leeuwenhoek, the father of microbiology, was the first person to examine seminal fluid under a microscope and discovered spermatozoa in 1677. He postulated that "a human being originates not from an egg but from an animalcule that is found in the male semen." Animalcules was the old-timey word for single-celled animals that people suddenly realized the world was crawling with once microscopes became popular.

Leeuwenhoek's student Nicolaas Hartsoeker, later inventor of the screw-barrel microscope, claimed to have made the discovery a few years earlier. There was a dispute between them over who got there first, but Leeuwenhoek was the boss, so he gets the credit. Hartsoeker didn't actually see any wee people curled up in sperm, but unlike Leeuwenhoek, he came to advocate the preformatist spermist position, i.e., that the miniature baby is already in the sperm before it gets anywhere near a uterus and that the woman only contributes the growing environment. He called it the "homunculus," from an alchemical term for a tiny full-grown person created by arcane means.

7. BOSOM SERPENT

Dr. Gardner advertises his animal removal services, LeedsIntelligencer, July 20, 1801.

The term "bosom serpent" was made famous by Nathaniel Hawthorne as the title of an 1843 short story about a man convinced that there's a snake living inside his chest. The idea that a snake, lizard, or assorted amphibians could be squirming around inside a person's body far predates Hawthorne, however, in folklore from all over the world.

In 1910, folklorist Thomas Johnson Westropp recorded a story from Clonlara, County Clare, Ireland, of a "worm with legs" running down a sleeping man's throat. Over time his appetite increased to alarming levels until he was compelled to consult a "wise person." The wise man told him to drink nothing and eat only bacon for two days. Then he was taken near a stream and his mouth pried open. When a rasher of crispy bacon was held to his mouth, the worm with legs ran out of his mouth and jumped on the bacon. The wise person threw the lizard bacon into the water and the patient was cured.

A certain Dr. Gardner, "Inventor of the Universal Medicines," announced his arrival in the Leeds Intelligencer of July 20, 1801, with guarantees that his nostrums could cure any number of ailments, including parasitic worms. Nothing remarkable there, but he also promised to show visitors more unusual beasts that he removed from afflicted patients: "One like a Lizard, the other has a Mouth like a Place, a Horn like a Snail, Two Ears like a Mouse, and its Body covered with Hair, was destroying the Man's Liver, a Portion of which it has brought off with it."

8. COMBINATION GUT CRITTERS AND DEMON POSSESSION

TheodorusDöderlein vomits lizards, frogs, insects. Print published in "De Incantamentis" by Georg Abraham Mercklin, 1715.

Theodorus Döderlein was 12 years old when he was stricken with terrible stomach cramps. The boy—who was the son of a pastor in Berolzheim, Germany—soon began to vomit streams of critters. He started off with insects and other invertebrates, and would go on to upchuck 21 newts, four frogs, and several toads. His doctor doubted it was really possible for so many animals to live comfortably inside the human stomach, but the local pastors were convinced it was real and caused by demonic possession. They didn't change their minds even when a doctor dissected one of the frogs and found partially digested insects in its stomach, suggesting the poor creature had eaten a fresh meal out of doors in the recent past.

The exorcists took over. There was report of a snake's head coming out of the boy's mouth during the ritual only to rush back down to the comfort of his gut when they tried to pull it out. With Theodorus still mired in invasive reptiles, amphibians, and insects, the exorcists decided to employ a surefire remedy for animals in the stomach: Horse urine. And lots of it. Accompanied by prayers and hymns, they poured multiple bottles of horse piss down naughty Theo's gullet. It worked like a charm. Theodorus never vomited up a single animal ever again.

9. BULUK'SIT ("BULGING EYE WORM")

The Tzeltal people in the highlands of Chiapas, Mexico, have an ancient medical tradition that goes back to the Maya. Body invaders play a significant role in this belief system. The Buluk'sit, or bulging eye worm, is one of the most insidious. It's a caterpillar about 3 inches long with horns and a large mouth for sucking like a leech or lamprey. It enters a woman's body through the vagina and takes up residence in the uterus. Every midnight it suckles from the womb, drawing nourishment from its unwitting "mother" as if it were a human fetus. This condemns the woman to infertility because all of the life force that would otherwise go to her possible baby is sucked away by the horned caterpillar. But some Tzeltal say that the caterpillar eats semen, and prevents pregnancy that way.

10. POKOK ("FROG")

In the same family of Tzeltal is the pokok syndrome, in which a frog is implanted by sorcery into the uterus of a woman where it grows as if it were a genuine pregnancy, only to end in the miscarriage of a malformed frog-like fetus.

BONUS: OK, THIS ONE MIGHT HAVE BEEN FOR REAL, THOUGH

Russian agriculturist and prolific memoirist Andrey Bolotov (1738-1833) told how a Russian peasant woman came to him with a bloated stomach. She claimed a koldun, or male sorcerer, had put a toad in her stomach. Bolotov wrote off her story as superstition and gave her an emetic to help her bring up whatever was causing her bloat. She vomited a toad. A live toad. Astounded, Bolotov examined the animal and found it was blind with atrophied rear legs as if it had lived in a dark, confined space for a long time.