The thoracic cavity and the abdominal cavity are separated by the?

The insuflation, deflation of the lungs produces great variations of pressure in the torax. If those variations were produced in a cavity including abdominal organs, some processes wouldn't take...

The diaphragm separates the chest from the abdomen and pelvis

diaphragm

This is your diaphragm. It is the muscle most important in your breathing. It also is used to help expel feces, and is similarly used to aid childbirth.

diaphragm