Medial Surface of Brain

Temporal Lobe. The temporal lobe is divided by superior and inferior temporal sulci into superior, middle, and inferior temporal gyri. The sulci run backward and slightly upward, in the same general direction as the posterior ramus of the lateral sulcus, which lies above them. The superior sulcus ends in the lower part of the inferior parietal lobule, and the superjacent cortex is called the angular gyrus. The superior temporal gyrus contains the auditosensory and auditopsychic areas.

Insula. The insula is a sunken lobe of cortex, overlaid by opercula and buried by the exuberant growth of adjoining cortical areas. It is ovoid in shape and is surrounded by a groove, the circular sulcus of the insula. The apex is inferior, near the anterior (rostral) perforated substance, and is termed the limen of the insula. The insular surface is divided into larger and smaller posterior parts by the central sulcus of the insula, which is roughly parallel to the central sulcus of the cerebrum. Each part is further subdivided by minor sulci into short and long insular gyri. The claustrum and lentiform nucleus lie deep to the insula.

MEDIAL SURFACE OF CEREBRAL HEMISPHERES

The medial surfaces of the cerebral hemispheres are flat, and, although separated for most of their extent by the longitudinal fissure and falx cerebri, they are connected in parts by the cerebral commissures and by the structures bounding the third ventricle.

Corpus Callosum. The corpus callosum is the largest of the cerebral commissures, and forms most of the roof of the lateral ventricle. In a median sagittal section, it appears as a flattened bridge of white fibers, and its central part, or trunk, is convex upward. The anterior end is recurved to form the genu, which tapers rapidly into the rostrum. The expanded posterior end, or splenium, overlies the midbrain and adjacent part of the cerebellum. The corpus callosum is about 10 cm long and 2.5 cm wide between the points where it sinks into the opposing hemispheres in the depths of the corpus callosal sulcus. Its fibers diverge to all parts of the cerebral cortex.

Fornix. Below the splenium and trunk of the corpus callosum are the symmetric arching bundles (crura of the fornix) that meet to form the body of the fornix and separate again to become the columns of the fornix, curving downward to the mammillary bodies. The body of the fornix lies in the roof of the third ventricle, and the tela choroidea is subjacent; the lateral fringed margins of this double fold of pia mater are the choroid plexuses of the central parts of the lateral ventricles, while an extension from the underside of the fold in the midline forms the choroid plexus of the third ventricle.

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