The abdomen is the part of the body between the thorax (chest) and pelvis, in humans and other vertebrates. It is the front part of the abdominal segment of the torso and is enclosed by the abdominal muscles at the front and sides, and by part of the vertebral column at the back. The area occupied by the abdomen is called the abdominal cavity, which is continuous with, and above, the pelvic cavity. The abdomen contains many vital organs, including the stomach, small intestine (jejunum and ileum), large intestine (colon), liver, spleen, gallbladder, pancreas, uterus, fallopian tubes, ovaries, kidneys, and adrenal glands. The urinary bladder may be seen as either an abdominal or pelvic organ. The abdomen ultimately serves as a cavity to house vital organs of the digestive, urinary, endocrine, exocrine, circulatory, and parts of the reproductive system. The alimentary tract in the abdomen consists of the digestive system, which is where most of the absorption and digestion of food occurs. Embryologically, the gastrointestinal system develops as the foregut, midgut, and hindgut. Clinically, the abdomen is roughly divided into nine regions by two sagittal planes from the midclavicular lines to the mid inguinal lines, and two transverse planes, one at the subcostal line and one at the iliac tubercles. The umbilicus serves as the center of the nine regions.