Describe the origin, insertion, and main action of the external intercostals.

• A. Origin: superior border of the rib below; Insertion: inferior border of the rib above; Action: depresses ribs.
• B. Origin: inferior border of the rib above; Insertion: superior border of the rib below; Action: elevates the ribs during inspiration to enlarge the thoracic cavity.
• C. Origin: internal intercostals; Insertion: adjacent ribs; Action: compresses rib cage.
• D. Origin: costal cartilage; Insertion: sternum; Action: expands thoracic cavity.

Answer: (B)