Situated on the side of the neck beneath the splenitis. It is a long muscle, composed of two fleshy divisions which pass from the head downwards to the anterior extremity of the hack.
Origin. - It takes its origin from the transverse processes of the first two dorsal vertebrae, and from the articular tubercles of the last six cervical vertebrae.
Insertion. - Into the mastoid process of the temporal bone, and to the wing of the atlas by a flat tendon common to the splenius and mastoido-humcralis.
Action. - When acting alone, the trachelo-mastoideus draws the head and neck to one side. When acting with the corresponding muscle of the other side, it raises the head.