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Blood supply to primitive mammalian brains

Brains of mammals are supplied by paired internal carotid arteries and also by paired vertebral arteries. The internal carotids distribute to the forebrain with its developing neopallium. The hindbrain with the expanding cerebellum and newly acquired middle cerebellar peduncle are supplied by the vertebral‐basilar system. In the opossum and armadillo the caudal rami of the internal carotid arteries resemble the patterns seen in submammals. The major source of blood to the posterior cerebral arteries is variable, coming from the basilar, the internal carotids or both. The ventral arterial circle (circulus arteriosus) is open as there is no anterior communicating artery in the two forms described. With the increase in size of the lateral lobes, the cerebellum of lower mammals is sufficiently larger and better developed to require a pair of inferior cerebellar arteries in addition to the phylogenetically older superior cerebellar arteries which originate from the basilar trunks. The latter are formed by the bifurcation of the basilar artery. The tectum in these forms is supplied by branches from the posterior cerebral and superior cerebellar arteries as well as tectal rami arising from the basilar trunks. Branches of the basilar artery going to the brainstem have a branching pattern similar to that seen in the submammals.