RIBS ARE A PROMINENT FEATURE of the trunk in vertebrates and are often regarded as a shield protecting the vital organs of the thoracic cavity, in particular the heart and the lungs. In mammals, however, many ribs are exposed to pleural pressure, which is below atmospheric pressure. Moreover, ribs move cranially during the inspiratory phase of the breathing cycle.
This cranial movement is primarily related to the contraction of two sets of intercostal muscles, namely the internal intercostals of the parasternal area (the so-called parasternal intercostals) (2, 3, 7, 9) and the external intercostals of the rostral interspaces (1, 7, 11), and contributes significantly to the expansion of the lung. Thus, in addition to protecting the intrathoracic organs, the ribs would appear to provide two essential respiratory functions.
First, they would constitute the structural elements that carry the compressive stresses that balance the pressure difference across the chest wall. Second, the ribs would transform intercostal muscle shortening into lung volume expansion. Two series of experiments were designed to test these hypotheses, and they form the basis of the present report.
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