Morphologically, the posterior segment of the cerebral arterial circle (CAC) consist of two posterior communicating arteries (ACoPs) and precommunicating parts of the posterior cerebral arteries. In fetuses and adult cadavers, an excess (variable) artery is found, which is a cause for macromorphological, micromorphological and pharmacological investigation in this study, as well as the conditions under which it occurs. Macromorphological study was conducted on 388 brains of human cadavers up to 95 years of age. Micromorphological examination included hematoxylin–eosin and modified Movat pentahrome staining. Pharmacological study involved the examination of the vascular response of the arteries in the posterior segment of the CAC on the endogenous and some synthetic substances. An excess artery in the posterior segment of the CAC, named as a. communicans intermedia (ACoI) occurs in two forms―morphologically stated (ACoI*), or as a pseudo form (p–ACoI*). The both forms of ACoI were obser...ved in 12.6% of the examined cadavers. The investigation did not proved statistically significant relationship between gender and the occurrence of the (p–)ACoI*. An excess artery was noted significantly more frequent in cases with asymmetrical posterior segment of the CAC, more often as a p–ACoI*. ACoI* was noted significantly more frequent in cases with ipsilateral adult, and p–ACoI* in fetal configuration of the ipsilateral posterior segment of the CAC. An excess artery had the smallest diameter and the smallest deviation in diameter compared to the adjacent arteries. Side branches was recorded in half of the (p–)ACoI* and twice as often on the left side. There were no differences in the structure of the arterial wall of the (p–)ACoI* in comparison to other arteries of the posterior segment of the CAC. All changes in the structure of arterial walls, except in the level of microaneurismal enlargements, could be explained by ageing. During the pharmacological study, vasomotion was noted and differences in the vascular responses of the examined arteries were found, particularly during treatments by vasoconstrictors. Predominant unilateral appearance of the ACoI* in the posterior segment of the CAC with adult configurations, or p–ACoI* in the posterior segment with fetal configuration, as well as common pharmacological response, more like the carotid arteries of origin, indicate the influence of embryological vascular and hemodynamic factors during their formation.
