The abdomen and its external surface, divided into nine regions by four imaginary planes: two horizontal, one at the level of the ninth costal cartilage (or the lowest point of the costal arch) and the other at the level of the highest point of the iliac crest; two vertical, through the centers of the inguinal ligaments (or through the nipples or through the centers of the clavicles) or curved and coinciding with the lateral borders of the two abdominal rectus muscles.