It is continuous below with the pelvis (the two are often spoken of as the abdomino-pelvic cavity). The abdomen is largely bounded by a muscular (rather than a bony) wall. This means that its shape can vary within the same individual much more than (for example) the thorax, and the abdomen alters with distension of the alimentary canal, with breathing movements and (most spectacularly) with pregnancy.
The major organs inside the abdomen include the stomach, small and large intestine, liver, pancreas, kidneys and spleen.
Flexor muscles of the hip (psoas major and iliacus) have their origins inside the abdomen, as do nerves arising from the lumbar plexus such as the femoral and obturator nerves.