The =duodenum= is that part of the small intestine which follows the stomach. At the pylorus (Fig. 97, _f_) the alimentary canal makes a rather sharp turn so that the first part of the duodenum forms an angle with the pyloric portion of the stomach, and extends caudad and slightly toward the right, soon becoming directed almost entirely caudad and lying along the right side. About eight or ten centimeters caudad of the pylorus it makes a U-shaped bend, extending thus craniosinistrad for four or five centimeters. Here it passes without definite limit into the jejunum, the duodenum being considered to end at the next turn caudad. The entire duodenum is about fourteen to sixteen centimeters in length. Between the two limbs of the U-shaped bend formed by the duodenum, the duodenal half of the pancreas is enclosed (Fig. 102, _a_). The walls of the duodenum are composed of the serous (peritoneal) investment, a muscular coat which is made up of an outer thin, longitudinal layer of fibres and an inner thick, circular layer, a submucous coat, and inside this a mucous coat. The mucosa is thrown up into numerous delicate finger-like villi which give to it a velvety appearance. On the dorsal wall of the duodenum, about three centimeters distad of the pylorus, the mucosa presents a slight papilla, at the apex of which is seen the oval opening of the =ampulla of Vater=. This is an ovoid space in the wall of the duodenum. The space is encroached upon by numerous folds of the walls. The common bile-duct and the pancreatic duct open into it, the former extending from the bottom of the ampulla nearly to its mouth, and the latter extending from the bottom about half-way to the mouth. Two centimeters caudoventrad of the opening of the ampulla of Vater is the opening of the accessory pancreatic duct. It can usually be demonstrated only by passing a bristle into the duodenum through an opening in the duct. The =jejunum= is the part of the small intestine following the duodenum. It is not separated from the part of the small intestine following it by any sharp line. In man it constitutes two-fifths of the small intestine exclusive of the duodenum, and is characterized by its emptiness after death and by the absence from it of Peyer’s agminated glands (Peyer’s patches).