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 ramus of C7 continues as the middle trunk.  This passes infero-laterally, to divide just above or behind the clavicle into anterior and posterior divisions.