<P> In humans, the duodenum is a hollow jointed tube about 25--38 cm (10--15 inches) long connecting the stomach to the jejunum . It begins with the duodenal bulb and ends at the suspensory muscle of duodenum . It can be divided into four parts . </P> <P> The duodenum is a 25--30 cm (10 - 12 inch) C - shaped structure lying adjacent to the stomach . It is divided anatomically into four sections . The first part of the duodenum lies within the peritoneum but its other parts are retroperitoneal . </P> <P> The first part, or superior part, of the duodenum is a continuation from the pylorus to transpyloric plane . It is superior to the rest of the segments, at the vertebral level of L1 . The duodenal bulb about 2 cm long, is the very first part of the duodenum and is slightly dilated . The duodenal bulb is a remnant of the mesoduodenum, a mesentery which suspend the organ from the posterior abdominal wall in fetal life . The first part of the duodenum is mobile, and connected to the liver by the hepatoduodenal ligament of the lesser omentum . The first part of the duodenum ends at the corner, the superior duodenal flexure . </P> <Ul> <Li> Anterior <Ul> <Li> Gallbladder </Li> <Li> Quadrate lobe of liver </Li> </Ul> </Li> <Li> Posterior <Ul> <Li> Bile duct </Li> <Li> Gastroduodenal artery </Li> <Li> Portal vein </Li> <Li> Inferior vena cava </Li> <Li> Head of pancreas </Li> </Ul> </Li> <Li> Superior <Ul> <Li> Neck of gallbladder </Li> <Li> Hepatoduodenal ligament (lesser omentum) </Li> </Ul> </Li> <Li> Inferior <Ul> <Li> Neck of pancreas </Li> <Li> Greater omentum </Li> <Li> Head of pancreas </Li> </Ul> </Li> </Ul>

What portion of the duodenum includes the duodenal bulb