Cut the corpus callosum transversely through near its posterior edge and bend the anterior portion of it forwards and sideways. The rear edge (_splenium_) left _in situ_ bends round and downwards and becomes continuous with the _fornix_. The anterior part is also continuous with the fornix, but more along the median line, where a thinnish membrane, the _septum lucidum_, triangular in shape, reaching from the one body to the other, practically forms a sort of partition between the contiguous portion of the lateral ventricles on the two sides. Break through the _septum_ if need be and expose the upper surface of the fornix, broad behind and narrow in front where its _anterior pillars_ plunge down in front of the third ventricle (from a thickening in whose anterior walls they were originally formed), and finally penetrate the corpus albicans. Cut these pillars through and fold them back, exposing the thalamic portion of the brain, and noting the under surface of the fornix. Its diverging _posterior pillars_ run backwards, downwards, and then forwards again, forming with their sharp edges the _corpora fimbriata_, which bound the cleft by which the ventricle lies open. The semi-cylindrical welts behind the _corpora fimbriata_ and parallel thereto in the wall of the ventricle are the _hippocampi_. Imagine the fornix and corpus callosum shortened in the fore-and-aft direction to a transverse cord; imagine the hemispheres not having grown backwards and downwards round the thalamus; and the corpus fimbriatum on either side would then be the upper or anterior margin of a split in the wall of the hemispheric ventricle of which the lower and posterior margin would be the posterior border of the corpus striatum where it grows out of the thalamus. The little notches just behind the anterior pillar of the fornix and between them and the thalami are the so-called _foramina of Monro_ through which the plexus of vessels, etc., passes from the median to the lateral ventricles. See the thick _middle commissure_ joining the two thalami, just as the corpus callosum and fornix join the hemispheres. These are all embryological aftergrowths. Seek also the _anterior commissure_ crossing just in front of the anterior pillars of the fornix, as well as the _posterior commissure_ with its lateral prolongations along the thalami, just below the pineal gland. On a median section, note the thinnish _anterior wall_ of the third ventricle and its prolongation downwards into the _infundibulum_.