THE gallbladder acts as a concentrating reservoir. It lies on the under surface of the liver and is attached to the side of the bile ducts. The right and left hepatic bile ducts, leaving their respective lobes of the liver, join to form the hepatic duct. This, in turn, joins with the cystic duct to form the common bile duct. The cystic duct extends from the gallbladder connecting it to the duct system. The common bile duct extends downward to the duodenum. The secretions of the biliary tract are poured into the second portion of the duodenum to aid in the digestion of fat. The presence of fatty foods in the duodenum causes the formation of a hormone, cholecystokinin, which is formed in the duodenal mucous membrane. This hormone circulates through the blood stream and is usually responsible for the contraction of the gallbladder. Nervous stimulation can also cause it to contract. When the gallbladder contracts, concentrated bile is propelled down the common bile duct and into the duodenum, where it is mixed with the fatty substances of the diet and produces complete digestion of the fatty elements. Between meals and during the night, a constant thin secretion of watery bile from the liver is discharged into the extrahepatic biliary tract and flows down the common hepatic duct as far as the cystic duct where it enters the gallbladder and is concentrated to ten times the strength of liver bile. Here the bile remains until an adequate stimulus from the gastrointestinal tract causes the gallbladder to contract and empty itself into the bile ducts and duodenum. The conditions in which surgery on the biliary tract is indicated may be divided into two major groups. There is the chronic group-affecting a majority of the patients-in which surgery is elective, and there are the acute or urgent cases which may require emergency surgery. In between these two are the semi-urgent conditions, which may not require emergency surgery, but should be treated surgically as soon as the patient can stand the operative procedure.