[TA] a triangular area in the diaphragm between its lumbar and costal parts and superior to the lateral arcuate ligament that is devoid of muscle fibers; covered by pleura superiorly and by peritoneum inferiorly; when it fails to form congenitally (a closure defect of the fetal pleuroperitoneal hiatus), the consequent foramen of Bochdalek is the most common site of diaphragmatic hernia of abdominal viscera. SYN: trigonum lumbocostale diaphragmatis [TA], Bochdalek gap, vertebrocostal trigone.