Spinalis capitis, or an accessory paraspinous muscle?

A unilateral muscle, the location and dimensions of which do not exactly conform to existing descriptions, was found during dissection of the suboccipital region. The muscle in question extended from the spine and transverse process of the 6th cervical vertebra to the base of the skull. At its rostral attachment it blended with the insertion of the left rectus capitis posterior minor muscle on the inferior nuchal line. The caudal attachment arched over the semispinalis cervicis, separated from that muscle by an extensive venous complex. Medially, along the length of the muscle, weak fascial attachments to the ligamentum nuchae were present. Arterial branches from the occipital artery entered the muscle near its rostral end and nerve fibres and vascular channels from the lower cervical region entered the deep surface of the muscle.