I love the Head Museum! How did they manage to get their lively pre 30th century exibits?
As The Infosphere states:
The fluid that preserves the heads contains a very very small amount of powder made from opals. Such powder is capable of maintaining the heads by creating a temporal stasis, where they are kept with the same youth as they had in their respective eras.
The manner in which all the heads at the museum were collected is never explained, but due to the temporal effects of the opal powder it is quite likely that a skull could simply be dug up from a grave and then placed in a jar containing the powder, thus returning the head to the state it was in when still alive.