The musculature of the gall bladder and biliary pathways in the guinea-pig.

A study of the gall bladder and the biliary pathways was carried out in the guinea-pig by light and electron microscopy. The musculature of the distended gall bladder measures only about 220 microns in thickness and consists of muscle bundles running in various directions. The muscle cells are loosely packed and are much smaller than the muscle cells of the intestinal muscularis externa. They resemble, in structure and arrangement, the cells of the muscularis mucosae of the intestine. In the cystic duct, the hepatic duct and the upper third of the bile duct, the musculature is similar to that of the gall bladder but is sparser and mixed with abundant connective tissue (fibromuscular layer). In the middle and lower portions of the bile duct there is a progressive increase in the amount of musculature. While the outer diameter of the organ remains constant (about 1.3 mm), the amount of muscle in a transverse section of the duct increases more than ten times along the length of the duct. Here the muscle cells are large and densely packed and closely resemble those of the duodenal muscularis externa. The bile duct forms with the duodenum an angle of about 30 degrees and it opens into a large cavity within the duodenal wall, the ampulla, which has a complex muscular system of its own. The ampulla is drained into the duodenal lumen through a duct within the papilla, around which lies a ring of circular musculature. The pancreatic duct opens independently into the duodenum about 6 cm aboral to the bile duct.