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. The intrinsic patterns in both the opssum and armadillo are essentially like those observed in all vertebrates. They are remarkably constant and are easily recognized from one form to another. The arterioles penetrating the armadillo brain terminate in a continuous capillary plexus like the intrinsic plexuses observed in most of the submammals as well as in all mammals except marsupials. Arterioles penetrating the brain of the American opossum, as well as of all other marsupials, do not form a capillary plexus but terminate in capillary loops. Every penetrating arteriole is accompanied by a venule; each pair of vessels terminates in one or more capillary loops, thus constituting a true end‐system. However, as in the rest of its body, capillaries in the infundibular process and stalk, choroid plexuses, area postrema, supraoptic crest and subfornical organ are not paired but form freely anastomosing plexuses. The major developmental advance, i.e., the “mammalian shift,” in the blood supply to brains of primitive mammals is the addition of the vertebral arteries to supply the brainstem and an inferior cerebellar branch going to the cerebellum. The implications of these phylogenetic changes are discussed and related to the functional development.