Usnea, also known as beard lichens, is a group of pale, grayish-green lichens which like to grow on anything from bark and twigs to gravestones and skulls. In late Medieval times, the latter was considered to be worth its weight in gold as doctors prescribed skull lichens for various diseases, and the nature of the skull-owner’s death was thought to dictate the potency of the “medicine”.

Usnea, “the periwig of a dead cranium”, or muscus ex craneo humano (true moss of a dead man’s skull)

Should a human head find itself abandoned in a cool, wet environment, such as the United Kingdom, it will break down and the resulting bony skull can become an anchor for Usnea. While battles have yielded lots of lost heads to grow skull lichens, the ingredient was so covetable that people actually cultivated it.

Exactly when skull lichens began being used as medicine is something that’s hard to pin down, wrote Christopher J Duffin in his article ‘The periwig of a dead cranium’: medicinal skull moss. Written records put it back to 1493, as detailed by Swiss physician Philippus Aureolus Theophrastus Bombastus von Hohenheim, but it’s likely it emerged as a traditional medicine earlier than this.

In 1590, the ”father of German Botany” Tabernaemontanus (also referred to as Jacobus Theodorus or Jacob Diether) published a volume of 2,255 plant illustrations entitled Eicones plantarum seu stirpium. It contains a famous image of skull lichens growing on human remains and details how medics and apothecaries would purposefully leave skulls in damp places where they’d be colonized by woodland mosses. The Usnea would “cling” and “crawl back and forth like a long rough worm” over the skull’s surface, he wrote.

As for how they got those heads? Well, that’s a whole other can of worms.

How death and the delivery of heads influenced Usnea’s worth

When skull lichens were a popular product on the pharmacist’s shelf, it was widely believed that within the human body existed a soul, or spirit. When a person died naturally, it was thought that that soul extinguished but people who died by the sword or some other violent means were considered to be chock-full of profitable spirit.

That our bones don’t rot away like flesh told the doctors of the time that they were the store of this life force and that it could be harvested to extend the lives of the living. The head was of particular interest, especially in cases where someone died by strangling or hanging as this was believed to basically squeeze the life force up into the head. The bones of the skull also allegedly benefited from chugging on our brain goo.

“The Belgian chemist Jean Baptise Van Helmont… explained that, after death, all the brain is consumed and dissolved in the skull’. It is, he believed, by ‘the continual… imbibing of [this] precious liquor’ of dissolved brains, that ‘the skull acquires such virtues',” wrote Dr Richard Sugg In Mummies, Cannibals And Vampires: The History Of Corpse Medicine From The Middle Ages To The Falun Gong.

“The skull seems here to have been all but marinaded in its own brains. Steeped in the ‘precious liquor’ which refines and improves with time like some rare old wine, the brain-sodden skull acts as a kind of natural laboratory or alchemist vessel.” Try pairing that with some artisanal cheeses.

But a violent death and brain mush marinade wasn’t the only thing that dictated the worth of a skull for usnea cultivation. At least, not for the British.

“Despite [usnea’s] widespread availability, there was only one type of skull which English doctors considered to be appropriate: the Irish,” wrote James Watson and William Regan in their paper Usnea and the Commodification of Irish Bodies in 16th and 17th Century London.

“After stories of large-scales massacres and beheadings committed by both sides of the conflict were fed back to the London public, English doctors and pharmacists came to look to the land across the Irish Sea as a profitable reservoir of corporeal medicine. As Francis Bacon wrote, the wars had left ‘heaps of slain bodies’ unburied and ripe for harvesting.”

Butchers and bandits were incentivized to go out and fetch heads both for show and medicine in the 16^th century, and the delivery of a skull dictated its value. While good “hedd monie” was paid for public figures (the Earl of Desmond’s head earned a butcher named Kelly £93 in the 1600s) it seems extra was given for style (Captain Thomas Cheston earned £120 for a skull around the same time as Kelly, simply for handing it over on the tip of his sword).

How was Usnea from skull lichens used and consumed?

Skulls in shop windows adorned with lichens were seen as a symbol of wealth and status in late Medieval London. Consumers would take those skulls and scrape them for their lichens which were ground up and used to promote wound healing, treat whooping cough, and manage epilepsy, among other things.

Van Helmont, as well as promoting the benefits of brain slushies, had a tale of a nobleman who wore a lock of skull lichen sewn inside his skin, wrote Sugg. You might gasp, but according to Van Helmont, the seemingly brutal act would save the nobleman’s life when, during a duel, he received a blow to the head with a sword which, despite cutting through his hat and hair, failed to break the skin.

And it gets stranger still. Skull lichens were also the main ingredient of Unguentum armariun, or weapon salve, a liniment used in the “magnetic cure of wounds”. The practice involved healing wounds by applying a treatment to the weapon that inflicted it, rather than the wounds themselves.

“The weapon could be located very far from the wounded, and sometimes it could even be different from the one which had caused the wound, provided that it was of similar shape; it could even be a stone, a stick or a dagger,” wrote Paolo Modenesi in their review Skull lichens: A curious chapter in the history of phytotherapy.  

“The weapon had to be put into the wound first, in order to open it and also to stain the weapon with blood. The weapon, or its dummy, was then anointed and bandaged until the wound was perfectly healed. Weapon salve was renowned and very popular even in intellectual circles, and commonly used by physicians, who were convinced of its efficacy.”

So, next time you’re out walking, and you see a strip of Usnea growing on a wet branch, doff your cap to its complex history. In our time, humans have done some pretty weird things with the humble lichen and our own, brain-drenched skulls.