lobe of the pituitary, and the inferior surface of the optic nerve and chiasm. These small arteries anastomose with their counterparts from the opposite internal carotid artery and with the inferior hypophysial arteries form a vascular plexus around the pituitary stalk. Posterior Communicating Artery
The next major branch of the internal carotid artery is the posterior communicating artery. It originates from the inferior lateral wall of the supraclinoid internal carotid artery, 2 to 7 mm distal to the anterior clinoid process, and exits from the carotid cistern by penetrating the arachnoid posteriorly and inferiorly to enter the interpeduncular cistern. This portion of the posterior communicating artery is covered by arachnoid. As the artery runs posteriorly, it is adjacent to the posterior clinoid process, to which it is occasionally adherent. In approximately 60 percent of patients, the posterior communicating artery is 2 mm or less in diameter. Its diameter is larger in children than in adults. In approximately 10 percent of patients, the posterior communicating artery is hypoplastic or absent. Rarely, it can have a duplication or fenestration. After it leaves the internal carotid artery, the posterior communicating artery runs medially and, thus, is hidden from view if the surgeon uses a subfrontal approach to this region. If a more lateral or temporal approach along the axis of the sphenoid wing is used, the artery can be followed further into the interpeduncular cistern. Approximately 3 mm from its origin, the posterior communicating artery gives rise to 2 to 10 branches that run posteriorly, inferiorly, and medially into the interpeduncular cistern to supply the optic chiasm and tract, tuber cinereum, mammillary bodies, hypothalamus, and inferior thalamus. These branches are long and variable in caliber. Anterior Choroidal Artery
13 Neurosurgery Books