part of adult heart dorsal diaphragm A free-edged fibrous mesh along each side of the heart, bearing the pericardial cells and reaching its greatest width in the fifth abdominal segment. Attached to it are 4 pairs of delicate and inconspicuous muscle fibers. Along the first chamber, the dorsal diaphragm extends upwards toward the body wall and contains closely spaced longitudinal muscles that stretch from the ventral wall of the heart to the junction between thorax and abdomen. fly_anatomy.ontology In other insects, the dorsal diaphragm is more extensive and divides the body cavity into a dorsal sinus and perivisceral sinus. Attached muscle fibers may be the alary muscles. FBbt:00003171 multicellular structure