ORIGIN AND INSERTION
Scalenus anterior– it is the largest of all scalene muscles and has its origin from the transverse processes of third, fourth, fifth and sixth cervical vertebrae (above), and inserts into the inner border of first rib (below).
Scalenus medius– it originates from the transverse process of axis (second cervical vertebrae), some of it also originates from the transverse process of lower five cervical vertebrae. It inserts into the upper surface of first rib.
Scalenus posterior – it is the smallest of all and originates from the transverse processes of fourth, fifth and sixth cervical vertebrae. It has its insertion into the outer surface of second rib.
All of the scalene muscles get their blood supply from the ascending cervical branch of inferior thyroid artery. Superficial cervical artery is an additional supply for the scalene posterior muscle.
Scalenus anterior – the nerve supply of this muscle is from the branches of ventral rami (anterior branches) of fourth, fifth and sixth cervical spinal nerves.
Scalenus medius – it is innervated by the ventral rami of C3 to C8 (C=cervical) spinal nerves.
Scalenus posterior – this muscle gets its nerve supply from the ventral rami of the lower three cervical spinal nerves.
The function of the anterior and the medius scalene muscles is to raise the first rib and bend the neck laterally to the same side. The scalene posterior muscle elevates the second rib and bends the neck to the same side.
These muscles also function as accessory muscles for respiration (sternocleidomastoid being the other accessory muscle).
The nerves of the brachial plexus (nerves of arms, forearms and hands) pass through the space between scalenus anterior and scalenus medius. This region is therefore used by the anesthetics for regional anesthesia prior to the surgery of arm or shoulder.