The intercostal muscles provide about 25% of the total force of breathing, and may help stabilize the chest wall The intercostals are three layers of muscles that pass between the ribs. Each external outer layer intercostal attaches to the lower border of one rib, and, on a diagonal line, to the upper border of the rib below. The lowest intercostals merge with the external oblique muscles. The internal intercostals connect each rib from the cartilage to the upper border of the rib below it. There is also a deeper, innermost intercostal layer. The intercostal nerve runs between it and the more...