Now we’ll add the clavicles and the scapulae to the picture, and go round to the back again to look at three large muscles that shape the back of the neck: semispinalis, splenius, and trapezius. We’ll add semispinalis to the picture first. Here's semispinalis. It arises by many tendons of origin from the articular processes of C4 to C7, and from the transverse processes of T1 to T6.
Semispinalis runs almost vertically, to insert here on the occiput, just behind the two rectus muscles. The action of semispinalis is to extend the head. In addition, when we’re upright, or leaning forward the tonic action of semispinalis prevents gravity from flexing of the head.
Next we’ll add splenius to the picture. Here’s splenius. It’s a broad strap of muscle, which arises from the spinous processes of T3 to C7, and from the lower half of the nuchal ligament. Splenius passes upward and laterally, to insert on the lateral half of the superior nuchal line, and on the back of the mastoid process. Splenius assists in rotating the head, toward the same side. This muscle beside splenius is levator scapulae, which is shown in Volume 1 of this atlas.
Lastly we’ll add trapezius to the picture. Here’s trapezius. Trapezius is a large and complex muscle. As shown in Volume 1, its lower part extends all the way down to T12. Here we’re concerned only with its upper part.
The upper part of trapezius arises from the medial part of the superior nuchal line, and from the nuchal ligament. Its fibers fan out downward and laterally, to insert on the spine of the scapula, the acromion, and the lateral third of the clavicle.
The trapezius muscles largely define the shape and outline of the neck, both from behind (here are the two trapezius muscles) and from in front. This is trapezius again.
Trapezius is thought of mainly as a shoulder muscle. Its upper part raises the scapula. In addition, when the scapula is held steady by the action of other muscles, trapezius acts in the same way as semispinalis, in extending the head, and in keeping the head upright when we lean forward.
The last muscle to add to our picture is the sternocleidomastoid. Here it is. It arises from here on the mastoid process and just behind it. The sternocleidomastoid muscle runs downwards, forwards and medially to insert partly on the medial end of the clavicle, and partly on the manubrium.
Contraction of one sternocleidomastoid muscle produces rotation of the head toward the opposite side. Contraction of both sternocleidomastoids together produces flexion of the head and cervical spine. When we’re leaning backwards, their tonic action prevents gravity from extending the head and neck.
The tendons of insertion of the two sternocleidomastoid muscles, together with the medial ends of the clavicles, define this hollow in the lower part of the neck.