Now bend up the posterior edge of the hemispheres, exposing the corpora quadrigemina (of which the anterior pair are dubbed the _nates_ and the posterior the _testes_), and noticing the _pineal gland_, a small median organ situated just in front of them and probably, like the pituitary body, a vestige of something useful in premammalian times. The rounded posterior edge of the corpus callosum is visible now passing from one hemisphere to the other. Turn it still farther up, letting the medulla, etc., hang down as much as possible and trace the under surface from this edge forward. It is broad behind but narrows forward, becoming continuous with the _fornix_. The anterior stem, so to speak, of this organ plunges down just in front of the _optic thalami_, which now appear with the fornix arching over them, and the median _third ventricle_ between them. The margins of the fornix, as they pass backwards, diverge laterally farther than the margins of the corpus callosum, and under the name of _corpora fimbriata_ are carried into the lateral ventricles, as will be seen again. It takes a good topographical mind to understand these ventricles clearly, even when they are followed with eye and hand. A verbal description is absolutely useless. The essential thing to remember is that they are offshoots from the original cavity (now the third ventricle) of the anterior vesicle, and that a great split has occurred in the walls of the hemispheres so that they (the lateral ventricles) now communicate with the exterior along a cleft which appears sickle shaped, as it were, and folded in. The student will probably examine the relations of the parts in various ways. But he will do well to begin in any case by cutting horizontal slices off the hemispheres almost down to the level of the corpus callosum, and examining the distribution of gray and white matter on the surfaces of section, any one of which is the so-called _centrum ovale_. Then let him cut down in a fore-and-aft direction along the edge of the corpus callosum, till he comes 'through' and draw the hemispherical margin of the cut outwards--he will see a space which is the ventricle, and which farther cutting along the side and removing of its hemisphere-roof will lay more bare. The most conspicuous object on its floor is the _nucleus caudatus_ of the _corpus striatum_. [Illustration: FIG. 36.--Horizontal section of human brain just above the thalami.--_Ccl_, corpus callosum in section; _Cs_, corpus striatum; _Sl_, septum lucidum; _Cf_, columns of the fornix; _Tho_, optic thalami; _Cn_, pineal gland. (After Henle.) ]