Now that we’ve looked at the diaphragm, we’ll move on to look at the muscles that produce movements of the ribs. First we’ll look at the principal muscles that produce inspiration, the external intercostals, and the scalene muscles.
Here are the external intercostal muscles. They’re thin sheets of muscle, that connect each rib to its neighbor. Each external intercostal runs from here on the rib above, to here on the rib below. They extend from the tubercles of the ribs behind, round to the middle of the costal cartilages in front. The fibers of the external intercostals run forward, from above, downward.
To understand how the external intercostals act, we’ll look at a simplified model of two ribs. When we apply a pulling force in the direction of the external intercostal fibers, the ribs move upwards. As the ribs move upwards, their ends, together with the sternum, move forwards. So the action of the external intercostals produces an upward and forward movement of the anterior chest wall.
Next we’ll look at the scalene muscles, which assist in inspiration by raising the first and second ribs. Here’s the manubrium, here's the first rib, here’s the second rib. Here are the scalene muscles: anterior, middle, and posterior.
The anterior scalene muscle arises from the anterior tubercles of the transverse processes from C3 to C6. It inserts here on the first rib. The middle scalene muscle arises from the posterior tubercles of the transverse processes from C2 to C6, and inserts here on the first rib. The posterior scalene muscle arises from the posterior tubercles, from C4 to C6, and inserts down here, on the second rib.
The action of the scalene muscles raises the first and second ribs, and the manubrium, in deep inspiration.