Composition of Encephalic Arteries and Origin of the Basilar Artery Are Different Between Vertebrates

Intracranial arterial vascular anatomy is well known in humans, but it is lacking for most mammalian and non-mammalian model species, especially concerning the origin of the basilar artery (BA). We have studied the encephalic arteries in three different species of birds and eight different species of mammals using formalin-fixed brains injected with arterial red latex. Our results and literature analysis indicate that, for all vertebrates, the internal carotid artery (ICA) supplies the brain and divides into two branches: a cranial branch and a caudal branch. The difference between vertebrates lies in the caudal branch of the ICA. For non-mammalian vertebrates, the caudal branch is the origin of the BA, and the vertebral artery (VA) is not involved in brain supply. For mammals, the VA supplies encephalic arteries, but in two different ways. In fact, there are two types of encephalic artery organization. In the first type of organization, mostly found in ungulates, the carotid rete mirabile supplies the encephalic arteries, the caudal branch is the origin of the BA, and the VA is indirectly involved in carotid rete mirabile blood supply. We believe that the carotid rete mirabile may be the result of occipitocervical biomechanics and climatic adaptations. The second type of encephalic artery organization for mammals is the same as in humans. The caudal branch of the ICA serves as the posterior communicating artery, and the BA originates from both VAs.