Course

It arises, in most cases, from the costocervical trunk

Costocervical trunk

The costocervical trunk arises from the upper and back part of the subclavian artery, behind the scalenus anterior on the right side, and medial to that muscle on the left side....

, and is analogous to the posterior branch of an aortic intercostal artery: occasionally it is a separate branch from the subclavian artery

Subclavian artery

In human anatomy, the subclavian arteries are two major arteries of the upper thorax , below the clavicle . They receive blood from the top of the aorta...

.

Passing backward, above the eighth cervical nerve and between the transverse process of the seventh cervical vertebra and the neck of the first rib, it runs up the back of the neck, between the semispinalis capitis

Semispinalis capitis

The Semispinalis capitis is situated at the upper and back part of the neck, deep to the Splenius, and medial to the Longissimus cervicis and capitis....

 and semispinalis cervicis

Semispinalis cervicis

The semispinalis cervicis , thicker than the semispinalis dorsi, arises by a series of tendinous and fleshy fibers from the transverse processes of the upper five or six thoracic vertebræ, and is inserted into the cervical spinous processes, from the axis to the fifth inclusive.The fasciculus...

, as high as the axis vertebra, supplying these and adjacent muscles, and anastomosing with the deep division of the descending branch of the occipital, and with branches of the vertebral.

It gives off a spinal twig which enters the canal through the intervertebral foramen between the seventh cervical and first thoracic vertebrae

Thoracic vertebrae

In human anatomy, twelve thoracic vertebrae compose the middle segment of the vertebral column, between the cervical vertebrae and the lumbar vertebrae. They are intermediate in size between those of the cervical and lumbar regions; they increase in size as one proceeds down the spine, the upper...

.