any structural anomaly of the layer of loose smooth muscle bundles (tunica muscularis) located beneath the mucosa of the gallbladder wall and composed of fibers that are not arranged in distinct layers but lie in circular, longitudinal, and oblique directions; when these fibers contract, the gallbladder’s contents are released through the cystic duct into the bile duct and eventually emptied into the duodenum to aid in the digestion of fats