B Figure 6.15 (A) Serratus posterior superior and serratus posterior inferior. (B) Levatores costarum muscles.
(Fig. 6.15A). The levatores costarum (longus and brevis) muscles (Fig. 6.15B) elevate the ribs, and the latissimus dorsi (see Chapter 8) elevates the lower ribs if the arms are fi xed. The quadratus lumborum also stabilizes the lower ribs during diaphragm contraction, and portions of the erector spinae can increase intrathoracic volume by extending the trunk (see Chapter 5).
MUSCLES OF FORCED EXPIRATION
Quiet expiration is a passive process, as the diaphragm relaxes and returns to its dome position reducing the volume of the intrathoracic cavity and increasing the
pressure within the space. This increased pressure causes the air to move from the higher intrathoracic pressure to the lower external atmospheric pressure. Elastic recoil of thoracic and lung tissue also assists with the passive expulsion of air from the lungs.
Contrary to quiet passive expiration, forced expiration occurs when the body must rapidly reduce intrathoracic volume and increase pressure to expel air quickly. Forced expiration is the active process in which air is forced from the lungs by contracting muscles. The muscles involved in forced expiration include four abdominal muscles, the transversus thoracis, and the interosseous fi bers of the internal intercostals.
