Final answer:
The vertebral arteries enter the cranium through the foramen magnum. They ascend protected by the cervical vertebrae's transverse foramina and contribute to the circle of Willis.
Step-by-step explanation:
The vertebral arteries are crucial in supplying blood to the central nervous system (CNS). They ascend through the transverse foramina of the cervical vertebrae in the neck, ensuring they are well-protected.
These arteries then make their way into the cranium through the foramen magnum, which is a large opening located in the occipital bone at the base of the skull.
Once inside the cranium, branches from the left and right vertebral arteries converge to form the anterior spinal artery, which nourishes the anterior aspect of the spinal cord. The vertebral arteries then join to become the basilar artery, from which branches extend to supply the brain stem and cerebellum.
Together with the internal carotid arteries, they contribute to the formation of the circle of Willis, an arterial ring at the base of the brain that ensures continuous blood flow even if one artery is obstructed.