A, The dissection is begun by examining the relationships in the anterior transcallosal approach to the third ventricle. The right frontal lobe, between the large middle and posterior frontal bridging veins, has been retracted away from the falx to expose the anterior cerebral arteries coursing on the upper surface of the corpus callosum. The inset shows the relationship to the coronal suture. There is usually an area just in front of the coronal suture that is relatively devoid of bridging veins entering the superior sagittal sinus. The bone flap for the transcallosal approach is placed two-thirds in front and one-third behind the coronal suture. B, Enlarged view. The falx and frontal lobe have been retracted to expose the anterior cerebral arteries above the corpus callosum. The veins draining the medial surface of the hemisphere often join the veins from the lateral surface to form large bridging veins that empty into the sagittal sinus. C, The corpus callosum has been opened to expose the fornix coursing anterior and superior to the foramen of Monro. The transcallosal opening has been completed without sacrificing a bridging vein. D, Enlarged view. The anterior caudate and superior choroidal veins join the anterior end of the thalamostriate vein. The column of the fornix passes anterior and superior to the foramen of Monro. The choroidal fissure begins at the posterior edge of the foramen of Monro where the choroid plexus is attached by the tenia fimbria and tenia thalami to the fornix and thalamus. The floor of the frontal horn is formed by the rostrum of the corpus callosum, the medial wall by the septum pellucidum, and the lateral wall by the caudate nucleus. (Images courtesy of AL Rhoton, Jr.)