Which muscle separates the chest from the abdomen?

• A. Serratus muscles
• B. Rib muscles
• C. Diaphragm
• D. Transverse abdominis

Answer: (C)

The boundary between the chest and the abdomen is formed by the diaphragm. This large, dome-shaped muscle sits between the thoracic cavity (chest) and the abdominal cavity and acts as the primary muscle of respiration. When it contracts, it flattens and moves downward, increasing chest volume and drawing air into the lungs; when it relaxes, it rises again, helping push air out. Because it physically separates the two cavities, it’s the structure that distinguishes the chest from the abdomen.

Serratus muscles are on the side of the chest wall and assist with shoulder and rib stability, not divide the chest from the abdomen. Intercostal (rib) muscles lie between the ribs and help move the rib cage during breathing, but they don’t form the boundary between the two cavities. The transverse abdominis is a deep abdominal muscle that supports the abdomen and spine, not the separation between chest and abdomen.