Thoracic Cage


The diaphragm is a sheet of muscle which is dome in shape. It separates the thoracic and abdominal cavities.  The change in volume of the thoracic cavity during breathing is due to the alternate contraction and relaxation of the diaphragm. 

Another fun etymology – the diaphragm was named after the Greek word diáphragma which means partition. 

The superior surface of the diaphragm is convex, creating the elevated floor of the thoracic cavity. The inferior surface is concave, creating the curved roof of the abdominal cavity.


The diaphragm can be divided into three parts (based on their peripheral attachments):

  1. Sternal part – originates from the posterior aspect of xiphoid process.
  2. Costal part – originates from the internal surfaces of the lower costal cartilages and ribs 7-12.
  3. Lumbar part – originates from L1-L3 vertebral bodies and invertebral discs, the anterior longitudinal ligament and the arcuate ligaments (medial, lateral).

The lumbar attachments are tendinous in structure, and forms the right (L1 – L3) and left crura (L1 – L2).

All the peripheral muscle fibres converge to form a central tendon which has no bony attachments. This tendon fuses with the fibrous pericardium and branches laterally to form the right and left domes.

When at rest, the right dome lies slightly higher than the left due to the presence of the liver.


Motor innervation:  phrenic nerves (C3-C5). These nerves innervate the diaphragm from its abdominal surface.

Sensory innervation (pain and proprioception):

  • central tendinous part – phrenicnerves
  • peripheral muscular portions – 6th to 11th intercostal nerves.

C3, 4, 5 keeps the diaphragm alive! (this is for motor innervation)

Irritation of the diaphragm from intraperitoneal pathologies such as liver abscess, spleenic rupture or cholecystitis may be referred to the shoulder. This can be explained by the supraclavicular nerve (which supplies the skin of the upper and posterior parts of the shoulder) has the same cervical nerve origin as the phrenic nerve i.e. C3 and C4.

Diaphragm Opening/Hiatus

There are three main openings that act as passageway of structures:

  1. Vena Cava opening (T8) –
      • IVC
      • RIGHT phrenic nerve
  2. Oesophageal hiatus (T10) –
      • Oesophagus
      • RIGHT and LEFT vagus nerve
      • Oesophageal branch of LEFT gastric artery/vein
  3. Aortic Hiatus (T12) –
      • Aorta
      • Thoracic duct
      • Azygous Vein

*** This is a famous exam question! ***

Vena Cava (8 letters) =

Oesophagus (10 letters) =

Aortic Hiatus (12 letters) =

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