INFERENCES ON PASSIVE DIAPHRAGM MECHANICS FROM GROSS-ANATOMY

The diaphragm is a relatively thin curved structure that is categorize d in mechanics as a membrane. Tension in the membrane is given by the product of muscle thickness and stress parallel to the fiber bundles. If all muscle fibers were cylindrical and extended from origin to inse rtion, the ratio of thickness near the chest wall (CW) to thickness ne ar the central tendon (CT) would vary inversely with the ratio of CW t o CT perimeters. In freshly excised diaphragms of 36 mongrel dogs, the ratios of the perimeters (CT/ CW) in the right and left costal diaphr agm were 0.63 +/- 0.04 and 0.62 +/- 0.04, respectively. The means of t he ratio of thickness near CW to that near CT in the right and left co stal regions were 0.96 +/- 0.07 and 0.95 +/- 0.05, respectively, consi stent with a nearly constant relationship between costal diaphragm mem brane tension and muscle stress in the direction of the fibers. In the crural diaphragm, the average ratio of the perimeters of the insertio ns on CT to CW was 1.16 +/- 0.10. The average ratio of thickness of cr ural CW to CT was 1.25 +/- 0.11. The discrepancy between the perimeter ratio and thickness ratio in the costal diaphragm is incompatible wit h the muscle consisting of uniform fibers extending from CW to CT, Our data suggest that muscle fibers are either in series with a smaller n umber along the smaller perimeter or that they terminate by tapering w ithin the muscle bundle. Both arrangements are consistent with previou s anatomic studies (Gordon et al. J. Morphol. 201: 131-143, 1989). Hav ing a nonuniform number of fibers mechanically in series is compatible with uniform stress in the fibers if the membrane is sufficiently cur ved as in a domed structure.