The Morphology of the Forebrain Arteries, With Especial Reference to the Evolution of the Basal Ganglia

Abstract

THE morphology of the cerebral arteries has been investigated in particular by Hofmann(1) and de Vriese (2), who regarded the vessels merely as an isolated morphological unit and failed to correlate changes in vascular pattern with any evolutionary progress in the brain itself. Shellshear, in many papers (3, 4, 5, 6, 7, 8), has claimed that the arteries together with the brain constitute a single functional unit, a neurovascular mechanism of which neither component is independent of the other. He has shown that the arteries are constant in their terminal supply; consequently the evolution of the vessels is dependent upon, and proceeds hand in hand with, the evolution of their field of distribution.A survey of the literature relating to cerebral blood supply reveals a great amount of variation and apparent contradiction of the principle of vascular constancy. Much of this apparent incompatibility is due to the failure to appreciate the significance of the principles of economy of distribution and convenience of source. These have been discussed briefly in previous papers (9, 10); here they will be exemplified more fully. We may say that the brain and its blood supply are not independent variables, the arteries of the brain are as much a part of the neurovascular mechanism as is the brain itself; any change in brain structure is reflected in an equivalent alteration in its blood supply. Thus the two components of this mechanism evolve in harmonious co-operation. As the brain expands to meet new situations, its arteries extend to satisfy the increased functional demand. Cerebral evolution would not be possible were it not accompanied by a corresponding evolution in the vascular pattern.The following work was undertaken to determine the blood supply of the brain in as complete a phylogenetic series as was available. It The arteries of the various brains were first studied in their surface distribution and subsequently dissected in the frog, crocodile, Marsupial, sheep, reindeer, lemur, ape and Man. Both hemispheres were used, and some of the hemispheres of each type were cut horizontally and others coronally in sections to ensure accuracy of localisation. In the dissections the arterial branches were followed individually to their destinations with the aid of a binocular microscope.The vessels of the brain of the dogfish, frog, tuatara, sheep and Man will be described in detail. Those of the other types will be discussed only in so far as they conform to, or appear to contradict, the theory of constancy in the vascular pattern of the fore-brain. DOGFISH (SQUALUS ACANTHiAS)Hofmann(1) and Kappers (13) have both studied the cerebral vessels of Fishes and describe a medial and a lateral olfactory artery passing forwards to the fore-brain from the cranial division of the internal carotid artery. In fig. 1 these vessels can be seen running along the medial and lateral borders of the hemisphere to the anterior aspect; they are related more to the ventral than the dorsal half of the hemisphere and, by small anastomotic c...

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