1. The lower boundary of the front of the thorax: Greg read in his biology book about the abdominothouacic arch including the part of the human body between the neck and abdomen, which is enclosed by the ribs and containing the heart and lungs.
2. A bell-shaped line along the lowest of the false ribs and the lower end of the sternum, making a rough boundary line between the abdomen and thorax: Janet learned more about the abdominothouacic arch in humans by viewing a drawing in her biology book.