The joint between the costal margin and sternum lies roughly in the same horizontal plane as the intervertebral disc between vertebrae TIX and TX. In other words, the posterior margin of the inferior thoracic aperture is inferior to the anterior margin.
When viewed anteriorly, the inferior thoracic aperture is tilted superiorly.
The musculotendinous diaphragm seals the inferior thoracic aperture (Fig. 3.4B).