The abdomen (also called belly), in vertebrates, is the region of the body between the chest and the pubic area. From a skeletal point of view, it is a large cavity (called abdominal cavity) framed by the (rib cage, spine, and the pelvis. It contains the digestive organs (stomach, small intestine, large intestine, liver, gallbladder, pancreas), the urinary system (kidneys and ureters) and the spleen.
Shape and variations
In humans and other mammals, the navel sits at about the midpoint of the abdomen's surface. In muscular people with little subcutaneos fat, the recti muscles may be visible, colloquially called "six pack".
The abdomen can vary considerably in shape and volume. Slim or underweight people have an "hourglass" figure where the front of the abdomen is flat or slightly concave and the sides are clearly concave. In normalweight people, the front is slighty convex and the sides are slightly concave. In overweight or obese people, the front is strongly convex and the sides are slightly convex. An obese person's belly may be so big they can no longer see their genitals without a mirror. The body mass index and the waist-to-hip ratio are means of measuring under- and overweight.
Influencing the appearance
As a rule, losing weight (reducing body fat) results in a reduced waist circumference, and putting on weight (gaining body fat) the opposite. The abdominal muscles can be trained by exercises such as sit-ups.
Tightlacing and corsets are technical means to reduce the waist circumference when worn to create an artificial "hourglass" shape. They do not reduce the weight though, and tightlacing is known to come with serious health risks.
Drawing the abdomen
When drawing the human torso, the abdomen is one of the two big volumes, and the rib cage the other. Due to the flexibility of the spine, the rib cage and pelvis may twist and bend in relation to another, and the abdomen stretches or compresses as needed to always attach to the top of the pelvis and the bottom of the rib cage. The abdomen's volume, fat or muscular appearance depends much on the individual figure. A vertical center line helps to get the shape of the abdomen right, and to correctly locate the navel and the "six pack", if desired. When the character is sitting, the abdomen is compressed and horizontal folds may show, such that e.g. the navel is hidden in such a fold.