Scalenus Anticus.
Scalenus Medius.
Scalenus Posticus.

The Scalenus Anticus is a triangular muscle, situated deeply at the side of the neck, behind the Sterno-mastoid. It arises by a narrow, flat tendon from the tubercle on the inner border and upper surface of the first rib, and ascending vertically upwards, is inserted into the anterior tubercles of the transverse processes of the third, fourth, fifth, and sixth cervical vertebra. The lower part of this muscle separates the subclavian artery and vein; the latter being in front, and the former, with the brachial plexus, behind.

Relations, By its anterior surface, with the Sterno-mastoid and Omo-hyoid muscles, the transversalis Colli, and descending cervical arteries, and the phrenic nerve. By its posterior surface, with the subclavian artery, and brachial plexus of nerves. It is separated from the Longus colli on the inner side by the subclavian artery.

The Scalenus Medius, the largest and longest of the three Scaleni, arises, by a broad origin, from the upper surface of the first rib, behind the groove for the subclavian artery, as far back as the tubercle, and ascending along the side of the vertebral column, is inserted, by separate tendinous slips, into the posterior tubercles of the transverse processes of the six lower cervical vertebrae. It is separated from the Scalenus anticus by the subclavian artery below, and the cervical nerves above.

Relations. By its external surface, with the Sterno-mastoid; it is crossed by the clavicle and Omo-hyoid muscle. To its outer side, is the Levator anguli scapulae and the Scalenus posticus muscle.

The Scalenus Posticus, the smallest of the three Scaleni, arises by a thin tendon from the outer surface of the second rib, behind the attachment of the Serratus magnus, and enlarging as it ascends, is inserted, by two or three separate tendons, into the posterior tubercles of the transverse processes of the two or three lower cervical vertebra. This is the most deeply-placed of the three Scaleni, and is occasionally blended with the Scalenus medius.

Nerves. The Rectus capitis anticus major and minor are supplied by the sub-occipital and deep branches of the cervical plexus; the Rectus lateralis by the sub-occipital; and the Longus colli and Scaleni by branches from the lower cervical nerves.

Actions. The Rectus anticus major and minor are the direct antagonists of those placed at the back of the neck, serving to restore the head to its natural position when drawn backwards by the posterior muscles. These muscles also serve to bow the head forwards. The Longus colli will flex and slightly rotate the cervical portion of the spine. The Scaleni muscles, taking their fixed point from below, draw down the transverse processes of the cervical vertebras, flexing the spinal column to one or the other side. If the muscles of both sides act, the spine will be kept erect. When taking their fixed point from above, they elevate the first and second ribs, and are, therefore, inspiratory muscles.