THE CENTRAL NERVOUS SYSTEM. 395 
of the fornix (Fig. 143, v; Fig. 150, ¢), which cross the 
anterior commissure (Fig. 143, ¢; 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 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, @) and the fimbria (Fig. 148, beneath ¢). 
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, @) and is 
continuous with it. 
The septum pellucidum (Fig. 143, 2) 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 
laminze 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, @) is an elon- 
gated rounded elevation of the floor of the lateral ventricle. 
