The abdomen (less formally called the belly, stomach, or tummy), in vertebrates such as mammals, constitutes the part of the body between the thorax (chest) and pelvis.

The region enclosed by the abdomen is termed the abdominal cavity. In arthropods it is the posterior tagma of the body; it follows the thorax or cephalothorax. Anatomically, the abdomen stretches from the thorax at the thoracic diaphragm to the pelvis at the pelvic brim. The pelvic brim stretches from the lumbosacral angle (the intervertebral disk between L5 and S1) to the pubic symphysis and is the edge of the pelvic inlet.

Wikipedia: Abdomen

history | excerpt history