The three trunks of the brachial plexus originate from the roots, pass laterally over rib I, and enter the axilla (Fig. 7.52):
The superior trunk is formed by the union of C5 and C6 roots.
The middle trunk is a continuation of the C7 root.
The inferior trunk is formed by the union of the C8 and T1 roots.
The inferior trunk lies on rib I posterior to the subclavian artery; the middle and superior trunks are more superior in position.