Which muscle lies deep to the external intercostals?

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

Answer: (D)

The internal intercostal muscle is indeed the correct choice because it is located just beneath the external intercostal muscles in the thoracic region. The external intercostals are responsible for elevating the ribs during inhalation, while the internal intercostals assist in forced expiration by depressing the ribs. This anatomical arrangement highlights the layered structure of the rib cage muscles, where the internal intercostals serve a complementary but distinct function, making them the muscle that lies directly deep to the external intercostals.

The other muscles listed are not situated in the same region or layer as the external intercostals. The transversus abdominis and internal oblique muscles are part of the abdominal wall and are positioned inferior to the thoracic cavity, while the rectus abdominis is also part of the abdominal muscles and is anterior to the internal organs, further distinguishing it from the intercostal muscles. This spatial relationship of muscles is crucial for understanding their respective functions during respiration and core stability.