The Diaphragm is a dome-shaped musculofibrous septum which separates the thoracic from the abdominal cavity, its convex upper surface forming the floor of the former, and its concave under surface the roof of the latter. Its peripheral part consists of muscular fibers which take origin from the circumference of the thoracic outlet and converge to be inserted into a central tendon.
The muscular fibers may be grouped according to their origins into three parts—sternal, costal, and lumbar. The sternal part arises by two fleshy slips from the back of the xiphoid process; the costal part from the inner surfaces of the cartilages and adjacent portions of the lower six ribs on either side, interdigitating with the Transversus abdominis; and the lumbar part from aponeurotic arches, named the lumbocostal arches, and from the lumbar vertebrae by two pillars or crura. There are two lumbocostal arches, a medial and a lateral, on either side.