A, Superior view of the lateral ventricles. The choroidal fissure is the cleft between the fornix and the thalamus along which the choroid plexus is attached. The frontal horn is located anterior and the ventricular body behind the foramen of Monro. The thalamus forms the floor of the body of the lateral ventricle and the anterior wall of the atrium. B, Enlarged view. The columns of the fornix form the anterior and superior margins of the foramen of Monro. The choroid plexus in the body extends through the posterior margin of the foramen of Monro and is continuous with the choroid plexus in the roof of the third ventricle. The right thalamostriate vein passes through the posterior edge of the foramen of Monro and the left thalamostriate vein passes through the choroidal fissure behind the foramen. The floor of the frontal horn is formed by the rostrum, and the anterior wall is formed by the genu of the corpus callosum. The lateral wall is formed by the caudate nucleus. The septum pellucidum is attached to the upper edge of the body of the fornix. C, Enlarged view of the foramen of Monro. The columns of the fornix form the anterior and superior margins of the foramen. An anterior septal vein passes backward along the septum pellucidum and crosses the column of the fornix. The thalamostriate vein passes forward between the caudate nucleus and thalamus and turns medially to pass through the posterior margin of the foramen of Monro to empty into the internal cerebral vein. The choroid plexus is attached medially by the tenia fornix to the body of the fornix and laterally by the tenia thalami to the thalamus. D, The transchoroidal exposure is begun by dividing the tenia fornix that attaches the choroid plexus to the margin of the fornix. The tenia thalami that attaches the choroid plexus to the thalamus is not opened. (Images courtesy of AL Rhoton, Jr.)