The brachial plexus extends from the neck to the axilla and supplies the upper limb.  It is formed from the ventral rami of C5, C6, C7, C8 and T1; branches from the ventral rami of C4 and T2 may also contribute.  These five ventral rami emerge from between the anterior and middle scalene muscles.

The ventral rami of C5 and C6 unite at the lateral border of scalenus medius to form the upper trunk.  This passes infero-laterally, dividing just above or behind the clavicle into anterior and posterior divisions.