The diaphragm is a skeletal muscle dividing the thoracic and abdominal cavities. Tendinous insertions arise from the circumference of the xiphisternum, costal margin and upper lumbar vertebrae. The tendons pass superiorly to form muscle fibres which merge to form tent-like domes on both the right and left sides. Medially and slightly inferiorly both domes merge into a central aponeurotic tendon. The central tendon has no bony attachment.
The acute angle that the tendinous diaphragmatic origins make with the body wall is termed the costophrenic angle. The maintenance of this angle is important to ensure that the diaphragm exerts a pull on the costal margin so as to move it superolaterally.