A number of these muscles attach to ribs and function as accessory respiratory muscles; some of them also stabilize the position of the first and last ribs.
Completely surrounded by skeletal elements, the superior thoracic aperture consists of the body of vertebra TI posteriorly, the medial margin of rib I on each side, and the manubrium anteriorly.
The superior margin of the manubrium is in approximately the same horizontal plane as the intervertebral disc between vertebrae TII and TIII.