The cleidomastoideus is a thick, strap-like muscle that together with the cleidocervicalis forms the proximal part of the brachiocephalicus. It lies beneath the cleidocervicalis. Its origin is the clavicular tendon with its insertion being at the mastoid process of the temporal bone. It is distinctly narrow and is united with the cleidocervicalis and cleidobrachialis by the clavicular tendon. Together with the other two components of the brachiocephalicus muscle, its primary function is to advance the free limb, however, it can also act as an shoulder extensor. Acting bilaterally, it fixes and depresses the neck, while by its unilateral action, it allows the head and neck to be drawn to the side. Its blood supply is the inferior cervical, carotid and vertebral arteries and its nerve supply is the spinal accessory, cervical and axillary nerves. [ MURDOCH:379 ]