Which muscle is primarily responsible for depressing ribs 1-11?

• A. Transversus abdominis
• B. Internal intercostal muscle
• C. Internal oblique
• D. Rectus abdominis

Answer: (B)

The internal intercostal muscle is primarily responsible for depressing ribs 1-11 because it runs between the ribs and plays a crucial role in the mechanics of respiration. This muscle is oriented in such a way that its contraction pulls the ribs downward, thus aiding in forced expiration, like during heavy breathing or coughing. By depressing the ribs, the internal intercostals help to reduce the volume of the thoracic cavity, which increases the intrapulmonary pressure and facilitates the expulsion of air from the lungs.

This action is essential for effective breathing, particularly in circumstances requiring active expiration. While other muscles, such as the transversus abdominis or the abdominal muscles, may assist overall breathing and abdominal compression, they are not primarily responsible for rib depression during respiration. Each muscle has a distinct role, and in this case, the internal intercostal muscle is specifically designed for rib movement related to the respiratory process.