The middle meningeal artery: from clinics to fossils.

INTRODUCTION: Although research today ranges from molecular to universe scale, many issues regarding gross anatomy remain totally neglected. Within the framework of the endocranial morphogenesis and evolution, understanding the role and variation of the middle meningeal artery relies upon the very limited, scattered, and descriptive information available. The meninges are supplied by branches originating from both the internal and external carotid arteries, often converging in the same networks and hence raising questions on the homology and embryogenesis of these vessels. The middle meningeal artery is often ligated during craniotomies, with no apparent impairment of the cerebral functional processes. The exact physiological role of this extended vascular system, together with the adaptations and selective pressure associated with its evolutionary characterization, have generally been ignored. THE MIDDLE MENINGEAL VESSELS IN NONHUMAN PRIMATES: Anthropologists have made many attempts to quantify and qualify the differences and variation between and within human and nonhuman primates, with scarce results due to the blurry morphology of the vascular networks. Living apes and humans probably have meningeal vessels originating from different embryogenetic processes, further hampering easy phylogenetic comparisons. Generally, monkeys and apes display a larger component derived from the internal carotid artery and its ophthalmic branch. EVOLUTION AND MORPHOLOGICAL VARIATION IN FOSSIL HOMINIDS: The fossil endocasts partially show the traces of the middle meningeal vessels, allowing some hypotheses on the evolution of these structures. In contrast with modern humans, some extinct groups show a dominance of the posterior branch over the anterior one. The most interesting features are associated with the variation of the middle branch, which supplies the parietal areas. In any case, the most striking difference between the modern and non-modern humans regard the definite increase in the number and complexity of the anastomoses and reticulation in the former. This may be either the simple result of a larger percentage of traces left by the soft tissue or be associated with a more developed vascular network. PERSPECTIVES: Tools are needed to quantify and qualify the morphogenesis and variations of the middle meningeal artery. Supposing these vessels are not strictly necessary in the adult age, the evolutionary pressure shaping their structure may have been associated with early life stages. Apart from oxygenation, another function which deserves attention is thermoregulation, considering the metabolic loadings of the cerebral mass.