Diaphragm

The diaphragm is the dome-shaped muscle that separates the thoracic cavity from the abdominal cavity. 

Gross anatomy

The diaphragm's muscular fibres originate around the circumference of the thoracic outlet and converge to a common insertion point of the central tendon.

The muscle slips can be grouped according to their origins:

• sternal: arises from two strips under the xiphoid process
• costal: arise from the inner surfaces of the costal cartilages of the 6th ribs on either side which interdigitates with the transversus abdominus muscles
• lumbar: arise from the aponeurotic arches (lubocostal arches) and from the lumbar vertebrae (crura)

There are two paired tendinouslumbocostal arches:

• medial lumbocostal arch (medial arcuate ligament): a tendinous arch that covers psoas major; continuous medially with left crura; attached to the L2 vertebral body and in the front of the transverse process of L1 and L2
• lateral lumbocostal arch (lateral arcuate ligament): covers quadratus lumborum; attached medially to the L1 transverse process and attached laterally to the tip of the 12th rib. This may be discontinuous on CT in up to 11% of people and hence may mimic diaphragmatic rupture⁸.

The crura are tendinous structures the blend with the anterior longitudinal ligament of the vertebral column:

• right crus is longer than the left, and arises from the anterior surfaces of the bodies of L1-3
• left crus arises from the corresponding portions of L1-2

The medial margins of the two crura pass forwards and medially. They meet in the midline to form an arch in front of the aorta called the median arcuate ligament. 

All these mucles insert into the central tendon, a thin but strong aponeurosis. It is situated immediately below and is fused to the pericardium. It is within this central tendon that vena caval hiatus is located, with the tendon allowing the IVC to remain patent during respiration.

Through the diaphragm are a series of 3 major and some minor apertures that permit the passage of structures between the thoracic and abdominal cavities:

• aortic hiatus
• oesophageal hiatus
• venal caval foramenhiatus
• lesser apertures
• sternocostal foramina

Blood supply

• arterial supply 
  • costal margins: lower five intercostal; subcostal arteries
  • abdominal surface: right and left inferior phrenic arteries (from abdominal aorta) ⁴
  • supply is augmented by superior phrenic artery and musculophrenic artery ⁵

Nerve supply

• eachphrenic nerve (C3-5) supplies the ipsilateral hemidiaphragm with motor fibres
• lower intercostal nerves supply proprioceptive fibres to the margins ⁴

Variant anatomy

• muscle slips
• scalloped or serrated appearance: due to individual muscle slips ⁷
• dromedary diaphragm (hump)
• accessory diaphragm

Related pathology

• diaphragmatic eventration
• diaphragmatic herniation
• diaphragmatic rupture