The dorsal portion of the hemispheres has been dissected away and the corpus callosum removed. _a_, fornix; _b_, columns or pillars of the fornix; _c_, crura of the fornix; _d_, hippocampus; _e_, choroid plexus of the lateral ventricles overlying the fimbria (the choroid plexus shows an artery); _f_, corpus striatum; _g_, corpora quadrigemina; _h_, position of the interventricular foramen (foramen of Monroe).] The =fornix= (Fig. 148, _a_; Fig. 143, _u_, _v_; Figs. 150-152, _b_) consists of an arched tract of longitudinal fibres near the medial border of each hemisphere, ventrad of the corpus callosum. Each tract begins in the mammillary bodies, and passes dorsad, the two converging until they run side by side, forming a cylindrical dorsoventral bundle known as the columns or pillars of the fornix (Fig. 143, _v_; Fig. 150, _e_), which cross the anterior commissure (Fig. 143, _c_; Fig. 150, _f_) caudad of the latter. Caudad of the pillars of the fornix, between these and the thalamus, lies on each side the interventricular foramen or foramen of Monroe, a small opening which connects the lateral ventricles with the third ventricle. Dorsad of the foramen the fornix turns caudad, the two fibre-tracts of each hemisphere lying side by side and closely connected (Fig. 148, _a_), forming thus another secondary union between the medial surfaces of the two hemispheres. This portion of the fornix is the =corpus= or =body= (Fig. 148, _a_); it lies dorsad of the roof of the third ventricle and passes to the splenium (Fig. 143, _s_) of the corpus callosum, and its dorsal surface unites with the ventral surface of the latter (Fig. 143). Caudad the two halves of the fornix diverge, forming the =crura= of the fornix (Fig. 148, _c_); these and the body are continuous laterally with the =hippocampus= (Fig. 148, _d_) and the =fimbria= (Fig. 148, beneath _e_). The =anterior commissure= (Fig. 143, _c_; Fig. 150, _f_) is a transverse band of white fibres which stretches from one hemisphere to the other about half way between the interventricular foramen or foramen of Monroe and the floor of the third ventricle, and just craniad of the pillars of the fornix. This tract of fibres is developed in the original wall of the third ventricle, so that it does not form a _secondary_ connection between the halves of the cerebrum, as do the fornix and corpus callosum. It lies dorsad of the lamina terminalis (Fig. 143, _d_) and is continuous with it. The =septum pellucidum= (Fig. 143, _t_) is a vertical partition which separates the lateral ventricles and fills the interval between the corpus callosum dorsad and the fornix ventrad. It is triangular and translucent. It is formed from the medial walls of the two hemispheres and therefore is made up of two laminæ which embrace between them a space which originally was a part of the fissure separating the hemispheres. This space has been called the fifth ventricle.