The bony borders of the superior thoracic aperture (thoracic outlet) consist of the manubrium anteriorly, the first thoracic vertebra posteriorly, and the first ribs. Note the 45-degree front-to-back incline of the opening. The first rib is flattened from top to bottom.
The cervical nerves descend from their respective foramina and drape over the middle of the first ribs as they converge to form the brachial plexus (see brachial plexus anatomy). Note the T-1 nerve root emerging between the heads of the first and second ribs to contribute to the brachial plexus.
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