I’ve just found a fascinating article in the History of Psychiatry about a type of delusion that was widely reported in the 15th to 17th centuries but rarely occurs in modern times. The reports were of patients who believed that they were made of glass and thought they might shatter if they suffered even the lightest of knocks.
In some of the more unusual forms, people struck with this form of madness might even consider themselves to be an oil lamp, a drinking vessel or even trapped in glass bottle.
The belief could even be specific to certain parts of the body:
Reports of glass bones, arms, and legs appeared much later, but Early Modern accounts were particularly rich in allusions to glass hearts/chests, and fragile heads. Tommaso Garzoni, an Italian monk,wrote a series of character sketches of mentally-disturbed people in 1586. In one of these cameos, drawn from Galen, the fragile delusion presents as a man who thought that his body consisted of only a large head, which he protected from injury by avoiding all contact with his fellows.
The delusion was reported in medical and the proto-scientific literature of the time, but also shows up in plays and literature.
Reportedly, one famous sufferer was King Charles VI of France, who allegedly refused to allow people to touch him, and wore reinforced clothing to protect himself.
While we tend to be most interested in how new delusional themes arise in response to cultural developments, we pay much less attention about delusions which were once common but now rarely occur.
This is a lovely example of a very well researched look at the history of no-longer popular delusions.
It’s also worth noting that Wikipedia has a page on the delusion where someone has briefly summarised some of the main points of the article.
Link to ‘Reflection of the Glass Delusion of Europe’.
Link to DOI entry for same.
Link to glass delusion page on Wikipedia.
Mind Hacks 
Glass was rare. It must have seemed very odd to people and some psychological condition must have latched onto that. Mass production of glass windows didn’t begin until the early 20th c. I know that people had glass windows prior to that, but it was reserved for the well-off. In the period you’re writing about even castles didn’t have glass windows. However with industrialization mass production of all kinds of goods became more common. I can’t in a quick search find any info about glass items. But it would be interesting to see if that became more common after the 17th c even though windows were still relatively uncommon.
Malnutrition, even among the wealthy, was fairly common in ancient/medieval times (remember, the causes of scurvy weren’t even known, officially, until the early 20th century). I think it’s entirely possible that the “glass delusion” might actually be a reference to some form of vitamin D deficiency, osteogenesis imperfecta, osteoporosis, or any number of other bone diseases which hadn’t yet been officially discovered or diagnosed. That would explain why the “glass delusion” seems to have “fallen by the wayside”.
In addition to the sources in the paper, the English physician Thomas Willis also wrote about some related cases in the mid-1600s. “Some have believed themselves to be dogs or wolves, and have imitated their ways and kind by barking or howling; others have thought themselves dead, desiring presently to be buried, others imagining that their bodies were made of glass, were afraid to be touched lest they be broke to pieces.” (For more details, see my book Soul Made Flesh: http://carlzimmer.com/books/soulmadeflesh/index.html )
Popular delusions are often related to recent technological developments (messages are coming from the electrical outlets, aliens control my body from outer space, etc.). The 15th-17th centuries were important in the history of the popularization of glass. A website on the history of glass making at http://inventors.about.com/od/gstartinventions/a/glass.htm say: “During the 15th century in Venice, the first clear glass called cristallo was invented and then heavily exported. In 1675, glassmaker George Ravenscroft invented lead crystal glass by adding lead oxide to Venetian glass.”