Digestive tract

Historians who had been allowed access to the "Monster Vaults" of the databanks in the Doctor's TARDIS observed that the heads of the Abzorbaloffs' absorbed victims remained alive and conscious within the Abzorbaloff's digestive tract and only when it had completely drained their brains were the heads fully assimilated into its "fleshy corpulence". (PROSE: The Monster Vault)