The diaphragm is a curved musculofibrous sheet that separates the thoracic from the , it is mainly convex on the upper surface, and concave inferior surface. The muscle attachments are at a lower level posteriorly and laterally, but high anteriorly. The musclular part can be divide to three parts—sternal, costal and lumbar—based on the regions of peripheral attachment. The sternal part from the back of the xiphoid process; it is not always present. The costal part arises from the internal surfaces of the lower six costal cartilages and their adjoining ribs on each side. The lumbar part arises from two aponeurotic arches, the medial and lateral arcuate (sometimes termed lumbocostal arches) and from the lumbar vertebrae by two pillars or crura (sing. crus).

The lateral arcuate , a band in the that covers quadratus lumborum, arches across the upper part of that muscle. It is attached medially to the front of the transverse process of the first lumbar vertebra, and laterally to the lower margin of the twelfth rib near its midpoint.

The medial arcuate ligament is a tendinous arch in the fascia that covers the upper part of psoas major. Medially, it is continuous with the lateral tendinous margin of the corresponding crus, and is thus attached to the side of the body of the first or second lumbar vertebra; laterally, it is fixed to the front of the transverse process of the first lumbar vertebra.

The crura are tendinous at their attachments, and blend with the anterior longitudinal ligament of the vertebral column. The right crus is broader and longer than the left. It arises from the anterolateral surfaces of the bodies and intervertebral discs of the L1- L4 lumbar vertebrae. The left crus arises from the corresponding parts of the upper two lumbar vertebrae. The medial tendinous margins of the crura meet in the midline to form an arch across the front of the at the level of the thoracolumbar disc; this is often poorly defined.

From these circumferential attachments the fibres of the diaphragm converge into a central tendon. The central tendon of the diaphragm is a thin but strong aponeurosis of closely interwoven fibres situated near the centre of the muscle, but closer to the front of the , so that the posterior muscular fibres are longer. In the centre it lies immediately below the pericardium, with which it is partially blended. Its shape is trifoliate. The anterior, leaf is directed towards the xiphoid process. The right and left folia curve laterally and backwards, the left being a little narrower. The central area of the tendon consists of four well-marked diagonal bands fanning out from a thick central node where compressed tendinous strands decussate in front of the oesophagus and to the left of the vena cava.

Structures pass between the thorax and through apertures in the diaphragm. There are three large openings—for the aorta, the oesophagus and the vena cava and a number of smaller ones.

The aortic aperture is the lowest and most posterior of the large openings; it is at the level of the lower border of the twelfth thoracic vertebra and the thoracolumbar intervertebral disc, slightly to the left of the midline. Strictly speaking, it lies behind the diaphragm and its median arcuate ligament (when present). The aortic opening transmits the aorta and the thoracic duct and sometimes the azygos and hemiazygos ; some lymphatic trunks also descend through it from the lower posterior thoracic wall.

The oesophageal aperture is located at the level of the tenth thoracic vertebra, above, in front, and a little to th