Gyral and Sulcal Patterns in Multiple Hemispheres. A, Part of the frontal and parietal lobes has been removed while preserving the pre- and postcentral gyri separated by a central sulcus. The central sulcus commonly opens directly into the interhemispheric fissure but is separated from the sylvian fissure by a gyral bridge, the subcentral gyrus. It has a gently convex posterior upper curve and a gently convex anterior lower curve. The shallow indention on the subcentral gyrus is not a sulcus but is related to an artery crossing the gyrus. B, Gyral bridges frequently interrupt the precentral and postcentral gyri but not the central sulcus (Table 1). The postcentral sulcus in this hemisphere is continuous and not interrupted by any gyral or sulcal bridge. The central sulcus opens into the interhemispheric fissure but is separated from the sylvian fissure by a subcentral gyrus. Two complex gyral bridges (red arrows) connect the superior and middle frontal gyri to the lower part of the precentral gyrus. C and D, Medial surface of two right hemispheres.  C, The paracentral lobule is formed by the extension of the pre- and postcentral gyri onto the medial surface of the hemisphere. The paracentral lobule is limited posteriorly by the ascending ramus of the cingulate sulcus and anteriorly by the paracentral ramus of the cingulate sulcus. The paracentral sulcus was present in approximately half of hemispheres. The ascending ramus was found in nearly all hemispheres and is easily seen on MRI at the posterior edge of the paracentral lobule. The precuneus is located between the ascending ramus and the parietooccipital sulcus. D, The paracentral sulcus, which was absent in approximately 50% of hemispheres, does not reach the upper margin of this hemisphere. The anterior limit of the paracentral lobule can be determined by the overlap of the central sulcus onto the medial surface. E, Superior view. On the left hemisphere there is a large gyral bridge (red arrow) connecting the upper edges of the pre- and postcentral gyri and separating the upper end of the central sulcus from the interhemispheric fissure. The “hand knob” on the precentral gyrus is seen in both hemispheres (yellow stars). F, Lateral surface. The lower third of the central sulcus usually courses lateral and parallel to the central insular sulcus. (Images courtesy of AL Rhoton, Jr.)