A, The right thalamus has been removed. The posterior part of the floor of the third ventricle is formed by the upper surface of the midbrain located behind the mamillary bodies. The tentorial edges join at the tentorial apex located in the quadrigeminal cistern behind the aqueduct. The choroidal fissure in the body of the ventricle is located between the body of the fornix and the upper surface of the thalamus. The floor between the optic chiasm and mamillary bodies is located above the chiasmatic cistern. The most common site for a third ventriculostomy is located just in front of the mamillary bodies. B, Anterior part of the left thalamus has been removed to expose the cerebral peduncles and upper midbrain on both sides of the third ventricle. The oculomotor nerves arise below the posterior part of the floor of the third ventricle. The infundibular recess is located behind the optic chiasm. The pons is exposed below the mamillary bodies and infundibular recess. C, Both thalami have been removed. The third ventricular floor extends from the optic chiasm to the aqueduct. The choroidal fissure in the body of the ventricle is located between the body of the fornix and the thalamus, in the atrium it is between the crus of the fornix and the pulvinar, and in the temporal horn it is between the fimbria and lower surface of the thalamus. D, Enlarged view. The upper midbrain and pons are located below the floor of the third ventricle. The oculomotor nerves exit the midbrain below the floor. The aqueduct and posterior commissure are positioned in the posterior wall of the third ventricle in front of the tentorial apex and quadrigeminal cistern. (Images courtesy of AL Rhoton, Jr.)