Which muscle forms the boundary between the thorax and abdomen and courses from anterior to posterior?

• A. Diaphragm
• B. Intercostal muscles
• C. Pectoralis major
• D. Rectus abdominis

Answer: (A)

The diaphragm is the muscle that forms the boundary between the thorax and the abdomen, acting as the chief divider between the two cavities. Its fibers originate from anterior attachments along the inner surfaces of the lower ribs and the xiphoid process, and from posterior attachments via the crura to the lumbar vertebrae, converging on a central tendon. This arrangement makes the muscle fibers run from front to back as they connect to posterior structures, creating a continuous muscular sheet that spans the thoracoabdominal boundary. During inspiration, the diaphragm contracts and moves downward, increasing thoracic volume. The other muscles listed do not form this boundary: intercostal muscles lie between the ribs, pectoralis major is a chest wall muscle, and the rectus abdominis lies along the anterior abdominal wall.