Mechanical coupling between the ribs and sternum in the dog

We measured the axial (cranio-caudal) displacements of the sternum and the second and seventh bony ribs using linear displacement transducers in five supine anesthetized dogs during passive inflation and deflation, during quiet breathing and static inspiratory efforts before and after bilateral phrenicotomy, and during tetanic stimulation of either the sternocleidomastoids or the sternal fibers of the rectus abdominis. Quiet inspiration before and after phrenicotomy was always associated with a caudal displacement of the sternum and a cranial displacement of the seventh rib; the second rib, however, was either motionless or also showed an inspiratory caudal displacement. During static inspiratory efforts, the second rib was always moving in concert with the sternum in the caudal direction, while the seventh rib, in particular after phrenicotomy, usually moved in the cranial direction. Finally, for any given axial (cranial or caudal) displacement of the sternum, stimulation of the sternocleidomastoid or rectus abdominis muscles invariably caused the second rib to move disproportionately more than the seventh. These results indicate that the upper ribs are more tightly linked to the sternum than the lower ribs. This presumably results from the fact that the costal cartilages increase in length from above downwards, and it implies that the upper portion of the rib cage behaves more as a unit with the sternum than the lower portion.