The brachial plexus runs across the first rib in close relationship to the subclavian artery
On top of the first rib the branches of the brachial plexus are located postero-superior to the artery
The subclavian artery (green arrow) ends at the lateral margin of the first rib (blue arrow) where it becomes the axillary artery which dives under the clavicle (magenta arrow)
