This muscle is situated in front of the first rib, and extends upwards as high as the fourth neck bone. It is composed of two unequal parts, the lower of which is the larger. Between them the axillary plexus of nerves passes on its way to the under part of the scapula, to be distributed to the fore extremity, and parts about the chest, breast, and shoulder. The axillary artery, in leaving the chest to be distributed to the fore-limb, winds round the anterior border of the first rib immediately beneath this muscle.

Origin. - From the transverse processes of the last four cervical vertebrae.

Insertion. - Into the anterior border and outer surface of the first rib.

Action. - By drawing the first rib forward, it assists inspiration. When the rib is fixed and the muscle contracts, the neck would be drawn downward and to one side.