THE CENTRAL NERVOUS SYSTEM. 365 



of the fornix (Fig. 143, v; Fig. 150, e), which cross the 

 anterior commissure (Fig. 143, c\ Fig. 150, /) 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 spleniurn (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 flmbria (Fig. 148, beneath e). 



The anterior commissure (Fig. 143, c; Fig. 150, /) is a 

 transverse band of white fibres which stretches from one hemi- 

 sphere to the other about half way between the interventricular 

 foramen or foramen of Monroe and the floor of the third ven- 

 tricle, 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 

 laminae 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. 



The hippocampus (Fig. 148, d; Fig. 152, d} is an elon- 

 gated rounded elevation of the floor of the lateral ventricle. 

 It is continuous mediad with the fornix (Fig. 148, d] and 



