What is the name of the muscle at the lower base of the ribcage?

The muscle at the lower base of the rib cage is the diaphragm. This sheet of muscle separates the chest cavity from the abdomen, and it stretches across the back and front of the body.

The diaphragm is important for respiration; when a person breathes in, the diaphragm contracts, which increases the size of the thoracic cavity to accommodate the air. Exhaling leads to relaxation of the diaphragm as the air exits the body.

The diaphragm consists of both muscle and tendon. The tendon connects various muscle fibers in the diaphragm, and it helps ensure fluid motion of the muscle during breathing.