The internal or mucous coat (tunica mucosa vesicæ felleæ) is loosely connected with the fibrous layer. It is generally of a yellowish-brown color, and is elevated into Mucosal folds (Rugae; minute rugæ). Opposite the neck of the gall-bladder the mucous membrane projects inward in the form of oblique ridges or folds, forming a sort of spiral valve.
The mucous membrane is continuous through the hepatic duct with the mucous membrane lining the ducts of the liver, and through the common bile duct with the mucous membrane of the duodenum. It is covered with columnar epithelium, and secretes mucin; in some animals it secretes a nucleoprotein instead of mucin.