Posterior Incisural Space with Anatomical Relationships. A, The roof of the posterior incisural space is formed by the lower surface of the splenium and the terminal part of the crus of the fornix. The posterior incisural space extends inferiorly into the cerebellomesencephalic fissure. The posteroinferior surface of the temporal lobe (parahippocampal gyrus and lingual gyrus) surrounds the posterior incisural space. B, The medial part of the anterior wall below the colliculi is formed by the lingula of the vermis. Between the postcentral fissure and the primary fissure lie the culmen medially and the quadrangular lobule laterally, which constitute the floor of the posterior incisural space. C, The superior surface of the superior vermis is divided, by short deep fissures, into the lingula, central lobule, and culmen. The white matter of the lingula is continuous with that of the superior medullary velum. D, The anterior wall consists of the posterior third ventricle, pineal body, and quadrigeminal plate (rostral to caudal). The lateral parts of the anterior wall below the colliculi are formed by the superior cerebellar peduncles. E, The segments of the PCA are classified according to our classification. The P2 segment extends from the posterior communicating artery to the point at which the PCA enters the quadrigeminal cistern. The P2 segment is subdivided into equal anterior (P2a) and posterior (P2p) halves. The P3 segment begins at the posterior midbrain, courses within the quadrigeminal cistern, and ends at the anterior limit of the calcarine fissure. F, The internal cerebral and basal veins and many of their tributaries converge at the vein of Galen in the posterior incisural space. The vein of the cerebellomesencephalic fissure crosses the quadrigeminal cistern anterior to the central lobule to drain into the vein of Galen, either directly or through the superior vermian vein. (Images courtesy of AL Rhoton, Jr.)