1. In higher animals, the abdomen is the large space that contains the stomach, intestines, liver, gall bladder, spleen, and pancreas. In women, the abdomen also includes the uterus, Fallopian tubes, and ovaries. It lies between the chest area and the pelvic area (hips). The abdominal cavity is lined with the peritoneum, which also provides partial or complete covering for the organs within the cavity.
2. In insects and other arthropods, the abdomen is the rearmost (posterior) segment of the body, which contains part of the digestive tract and all the reproductive organs.
The human abdomen
The abdominal cavity in humans is subdivided into the abdomen proper and the pelvic cavity. The abdomen proper is bounded above by the diaphragm; below it is continuous with the pelvic cavity; posteriorly it is bounded by the spinal column, and the back muscles; and on each side it is bounded by muscles and the lower portion of the ribs. In front, the abdominal wall is made up of layers of fascia and muscles.
The abdomen is divided into nine regions whose boundaries may be shown by lines drawn on the surface. The mid-section above the navel between the angle of the ribs is known as the epigastric region; that portion around the navel, as the umbilical; below the navel and above the pubic bone, as the hypogastric region. It is further divided into right and upper left quadrants on each side above the navel, and right and left lower quadrants on each side below the navel. The lumbar region extends on either side of the navel posteriorly and laterally.
The principal organs of the abdominal cavity are the stomach, duodenum, jejunum, ileum, part of the large intestine, liver, gall bladder and biliary system, spleen, pancreas and their blood and lymphatic vessels, lymph glands, and nerves, kidneys and ureter. The pelvic portion of the abdomen contains the sigmoid colon and rectum, part of the small intestine, the urinary bladder, in the male the prostate gland and seminal vesicles, in the female the uterus, Fallopian tubes, and ovaries.