Now we’ll look at the principal muscles that produce expiration: the internal intercostals, and the muscles of the abdominal wall. The internal intercostals lie just beneath the external ones, which we’ll remove.
Here are the internal intercostal muscles. Each internal intercostal runs from here on the rib below, to here on the rib above. The internal intercostals extend from the angles of the ribs behind, to the end of the intercostal spaces in front.
The fibers of the internal intercostals run forward, from below, upward. To show how they act, we’ll look at the model again, with the force now applied in the direction of the internal intercostal fibers.
As the force is applied, the ribs move downwards. As the ribs move downwards, their ends, together with the sternum move backwards. The action of the internal intercostals moves the anterior chest wall downwards and backwards.
The other important muscles of expiration are the muscles of the abdominal wall. They have two important effects. We’ve noted already that they raise the intra-abdominal pressure, and so push the diaphragm up. In addition to this, abdominal wall muscles pull the lower ribs downward, assisting the action of the internal intercostals. We’ll be seeing these muscles in detail in the next section. Here, we’ll just take a quick preview of them.
On each side of the midline are the rectus abdominis muscles, which go from the fifth, sixth and seventh ribs, down to the pubis. Between the rectus muscles in front, and the posterior abdominal wall behind, there are three sheets of muscle, one inside the other. This one, the innermost, is the transversus abdominis. Its fibers run horizontally, the uppermost ones go from the lowest four ribs, to insert on this sheet of tendon which goes to the midline.
Outside transversus is the internal oblique Its fibers arise from the iliac crest and fan out in many directions, the highest ones inserting on the lowest three ribs. Outside the internal oblique is the external oblique. It arises from the lower seven ribs, and inserts partly on the iliac crest, partly into this broad tendinous sheet, the external oblique aponeurosis.
The most important contribution that the abdominal wall muscles make to the movements of respiration is in the powerful action of forced expiration, as in coughing or sneezing.
In addition to the muscles of respiration that we've seen, there are some minor ones that we're going to leave out, since they're unimportant. These are the levators of the ribs and the serratus posterior muscles on the outside, and the transversus thoracis and innermost intercostal muscles on the inside.
In addition to being an expandable container for the heart and lungs, the thorax also forms the foundation from which the upper extremity arises. The muscles of the shoulder region, which cover up most of the ribs in front, and all the ribs behind, are shown in Volume 1 of this Atlas.