René Descartes' diagram of the brain and the pineal gland in his work De Homine Figuris. Image credit: Wellcome Library, London // CC BY 4.0


 René Descartes once described the pineal gland as “the principal seat of the soul.” Though medical knowledge has vastly progressed since then, here are a few things you might not have known about this critical organ.

1. IT’S BEEN RECOGNIZED AS IMPORTANT SINCE THE ANCIENT GREEKS.

Though the pineal gland wouldn’t be fully understood until the 20th century, descriptions of its anatomical location are included in the writings of Galen (ca. 130-ca. 210 CE), a Greek doctor and philosopher.

2. ITS SHAPE INFLUENCED ITS NAME.

This itty-bitty little gland, located very deep in the center of the brain, gets its name from its pinecone-like shape, most recently from the French (pinéal, or "like a pinecone"), itself from the Latin for pinecone (pinea). However, at about one-third of an inch long in adults, it's smaller than your average pinecone.

3. THE PINEAL GLAND IS PART OF YOUR ENDOCRINE SYSTEM.

Though located in your brain, the pineal gland is actually a crucial part of your endocrine system, which regulates major bodily processes such as growth, metabolism, and sexual development through the release and control of hormones.

4. IT CONNECTS THE ENDOCRINE AND NERVOUS SYSTEMS.

The gland translates nerve signals from the sympathetic nervous system into hormone signals.

5. THE PINEAL GLAND WAS LONG CONSIDERED MYSTERIOUS.

Because the pineal gland was the last of the endocrine structures to be discovered, scientists considered it a mysterious organ. Today, we know that unlike much of the rest of the brain, the pineal gland is not isolated from the body by the blood-brain barrier system.

6. DESCARTES WAS WRONG ABOUT ITS RELATIONSHIP TO THE MIND, THE SOUL …

The 16th-century French philosopher and mathematician René Descartes was fascinated with the pineal gland, considering it “the place in which all our thoughts are formed.” Scientists now credit that function to the neocortex.

7. …AND TINY ANIMAL SPIRITS IN THE BRAIN.

Descartes thought that within the pineal gland, tiny animal spirits were like “a very fine wind, or rather a very lively and pure flame,” feeding life into the many small arteries that surround the gland. This was likely due to his abysmal understanding of anatomy and physiology.

8. IT'S BEEN CALLED THE "THIRD EYE."

The pineal gland was commonly dubbed the “third eye” for many reasons, including its location deep in the center of the brain and its connection to light. Mystic and esoteric spiritual traditions suggest it serves as a metaphysical connection between the physical and spiritual worlds.

9. IN REALITY, IT PRODUCES A SINGLE—BUT KEY—HORMONE.

As scientists have learned more about the functions of the pineal gland, they’ve learned it synthesizes the hormone melatonin from the neurotransmitter serotonin. Melatonin production determines your sleep-wake cycles and is purely determined by the detection of light and dark. The retina sends these signals to a brain region known as the hypothalamus, which passes them on to the pineal gland. The more light your brain detects, the less melatonin it produces, and vice versa. Melatonin levels are highest at night to help us sleep.

10. MELATONIN IS ALSO CRITICALLY INVOLVED IN REPRODUCTION.

Melatonin inhibits the release of pituitary reproductive hormones, known as gonadotropins, from the pituitary gland, affecting male and female reproductive organs. In this way, melatonin—and therefore the pineal gland—regulates sexual development.