Which muscle inserts at the inferior borders of ribs 8-12?

Which muscle inserts at the inferior borders of ribs 8-12?

• A. Serratus posterior inferior
• B. Serratus posterior superior
• C. Latissimus dorsi
• D. External intercostals

Answer: (A)

The key idea is how a muscle’s insertion matches its role in moving the rib cage. Serratus posterior inferior specifically attaches to the inferior borders of ribs 8–12, so its action is to pull those ribs downward, aiding expiration. It originates from the lower thoracic and upper lumbar vertebrae (spinous processes and thoracolumbar fascia) and inserts along the inferior borders of ribs 8–12. Its nerve supply comes from the intercostal nerves (T9–T12), which fits with its role in the thoracic wall. In contrast, serratus posterior superior attaches to the upper ribs (2–5) and elevates them, helping inspiration, not expiration. Latissimus dorsi mainly inserts on the humerus, not ribs, so it doesn’t attach to rib borders. External intercostals run between the ribs and insert on the superior border of the rib below, not on the inferior border of the ribs themselves, so they don’t fit the described insertion either. So, the muscle that inserts at the inferior borders of ribs 8–12 is the serratus posterior inferior, matching its name, location, and function in depressing those lower ribs during expiration.

The key idea is how a muscle’s insertion matches its role in moving the rib cage. Serratus posterior inferior specifically attaches to the inferior borders of ribs 8–12, so its action is to pull those ribs downward, aiding expiration. It originates from the lower thoracic and upper lumbar vertebrae (spinous processes and thoracolumbar fascia) and inserts along the inferior borders of ribs 8–12. Its nerve supply comes from the intercostal nerves (T9–T12), which fits with its role in the thoracic wall.

In contrast, serratus posterior superior attaches to the upper ribs (2–5) and elevates them, helping inspiration, not expiration. Latissimus dorsi mainly inserts on the humerus, not ribs, so it doesn’t attach to rib borders. External intercostals run between the ribs and insert on the superior border of the rib below, not on the inferior border of the ribs themselves, so they don’t fit the described insertion either.

So, the muscle that inserts at the inferior borders of ribs 8–12 is the serratus posterior inferior, matching its name, location, and function in depressing those lower ribs during expiration.