The opening of cystic duct is guarded by which of the following valve ?

Cystic duct is about 3 to 4 cm long. It begins at the neck of the gall bladder, runs downwards, backwards and to the left, and ends by joining the common hepatic duct at an acute angle to form the bile duct. The mucous membrane of the cystic duct forms a series of 5 to 12 crescentic folds , arranged spirally to form the so-called spiral valve of Heister. This is not a true valve. Ref : B D Chaurasia's Human Anatomy , seventh edition , volume 2 , pg . no., 316.