The canine parasternal and external intercostal muscles drive the ribs differently

1. In the dog, the elevation of the ribs during inspiration results from th e combined actions of the parasternal and external intercostal muscles. In the present studies, the hypothesis was tested that co-ordinated activity a mong these two sets of muscles reduces the distortion of the rib cage. 2. During spontaneous inspiration before or after section of the phrenic ne rves, the ribs moved cranially and outward in the same way as they did duri ng passive inflation. However, whereas the sternum moved cranially during p assive inflation, it was displaced caudally during spontaneous inspiration. 3. When the parasternal intercostal muscles were selectively denervated, bo th the sternum and the ribs moved cranially, but the rib outward displaceme nt was markedly reduced. In contrast, when the external intercostals were e xcised and the parasternal intercostals were left intact, the sternum conti nued to move caudally and the outward displacement of the ribs was augmente d relative to their cranial displacement. 4. These observations establish that the external intercostal muscles drive the ribs primarily in the cranial direction, whereas the parasternal inter costals drive the ribs both cranially and outward. They also indicate, in a greement with the hypothesis, that co-ordinated activity among these two se ts of muscles displaces the ribs on their relaxation curve. 5. However, this co-ordinated activity also displaces the sternum caudally. Although this distortion requires an additional energy expenditure, it enh ances the outward component of rib displacement which is more effective wit h respect to lung expansion.