THE EVOLUTION OF THE CEREBRAL HEMISPHERES. 331 



contain a few nerve cells and fibers of the fornix system and 

 constitute the two leaves of the septum pellucidum. The space 

 between the two leaves of the septum pellucidum is merely a part 

 of the great sagittal fissure of the brain which by this process 

 becomes roofed in by the corpus callosum and is called the "fifth 

 ventricle". While these changes in the commissures are taking 

 place the part of the hippocampus which retains its full development 

 is pushed back until it no longer touches the lamina terminalis 

 in higher mammals (Fig. 169). 



While the expansion of the neopallium and its commissure has 

 been so profoundly affecting the form of the hemisphere and the 

 relations of the archipallium, the hippocampal formation itself 

 has undergone changes of form and become more complex in 

 structure. In monotremes and marsupials the hippocampus 

 occupies the mesial wall of the hemisphere and is slightly infolded 

 into the lateral ventricle, the line of infolding being marked by 

 the hippocampal fissure. Along the lower border of the hippo- 

 campus the cells multiply and proliferate from the ventricular 

 layer to form the fascia dentata. As the hippocampus folds more 

 strongly and the fissure deepens the thick fascia dentata which 

 forms the lower limb of the fold retains its position on the surface 

 and the upper limb, or hippocampus proper, is drawn deeper in 

 and wrapped around the fascia dentata (Fig. 170) until the greater 

 part of it is submerged within the hippocampal fissure. In most 

 mammals the in- rolling goes so far that a larger or smaller part of 

 the ventricular surface of the hippocampus is brought out upon 

 the external surface of the brain where it is exposed except for a 

 thin layer of fibers belonging to the fimbria. This inverted por- 

 tion is the only part of the hippocampus, except the fascia dentata, 

 which is exposed to view in the human brain. Of the two parts 

 of the hippocampal formation the fascia dentata seems to serve 

 wholly for the reception of olfactory impulses and to send its 

 fibers into the hippocampus proper, while the hippocampus 

 receives fibers from olfactory and other centers and sends out com- 

 missural fibers and fibers df projection to the corpus mammillare 

 and nucleus habenulae. 



This long and complex history of the evolution of the cerebral 



