The vertebral artery ascends vertically along the lateral border of the longus cervicis to the level of the cricoid cartilage and there, in the angle between the scalenus anterior and longus cervicis, enters the transverse foramen of the sixth cervical vertebra.
Source: Boileau Grant JC. A Method of Anatomy: Descriptive and Deductive. 5th ed. Baltimore, Md: Williams & Wilkins Co; 1952:681.
May 2003. Arch Surg. 2003;138(5):546. doi:10.1001/archsurg.138.5.546