This is the gland in which black adders keep their venom. Some of you may say that these things are actually called sacs, but that's because you don't understand how much funnier things are if they rhyme.
This is a further reference to the show Blackadder, in which Blackadder is occasionally derisively referred to as 'Slack Bladder'. Also, the opening musical lyrics in series one include "Beware the deadly flashing blade."
