Diaphragm, phrenic nerve

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labeled CT: 

Explanation

The structure outlined in red is the diaphragm, or the thoracic diaphragm (to distinguish it from other diaphragms, such as the urogenital or pelvic diaphragm).  It is the primary muscle of respiration, pulling the lungs downward like a piston when it contracts, leading to inspiration.  The innervation of the diaphragm is primarily the phrenic nerve, which arises from C3, C4 and C5, passing along the lateral margins of the mediastinum to reach the lower chest.  Some innervation is also supplied from intercostal nerves to the peripheral portions of the diaphragm.