Final answer:
The diaphragm is the structure that separates the thoracic cavity from the abdominal cavity. It is a dome-shaped muscle located at the bottom of the ribcage.
Step-by-step explanation:
The structure that separates the thoracic cavity from the abdominal cavity is the diaphragm.
The diaphragm is a dome-shaped muscle located at the bottom of the ribcage. It contracts and relaxes to facilitate breathing and also acts as a barrier between the thoracic and abdominal cavities.