THE EVOLUTION OF THE CEREBRAL HEMISPHERES. 329 



several tracts of the fornix system which in monotremes entered 

 the hippocampus anterior to the lamina terminaUs are preserved 

 in small numbers and still enter the rudimentary hippocampus, 

 either running around the front edge (genu) of the corpus callosum 

 or piercing it {perforating fibers of authors, fornix longus of Forel). 

 These perforating fibers maintain the same position and course 

 which they have in all vertebrates, the corpus callosum is a new 

 structure which has run transversely through and mingled with 

 them, as explained above. 



The growth of the corpus callosum not only reduces the hippo- 

 campus to a rudiment but also changes its position. As the neo- 

 pallium enlarges and spreads backward the callosum spreads 

 in the same direction, pushing back part of the hippocampus 

 before it and stretching the rudimentary hippocampus which lies 

 on its dorsal surface. The corpus callosum is formed in the 

 lamina terminahs and its caudal border is mingled with the hip- 

 pocampal commissure. As the callosum tends to grow broader 

 antero-posteriorly its two borders are confined. In order to widen 

 it must either stretch the lamina terminahs backward or it must 

 bend and fold upon itself. It does both. In the simplest mammals 

 (bats) no change is seen from the marsupial condition, but in such 

 animals as the rabbit and hedgehog the corpus callosum has arched 

 upward in the form of an inverted U, stretching the rudimentary 

 hippocampus (Fig. i68). In higher mammals such as the cat, 

 and in man the callosum has grown much larger and has pushed 

 back over the hippocampal commissure. At the same time the 

 U-shaped bend in the callosum has become more and more sharp 

 until the two limbs have met and in man it appears on superficial 

 examination that the caudal border of the callosum is merely 

 thickened. In fact, this portion, known as the splenium, is formed 

 by an actual folding of the callosum due to its growth in width 

 while its borders remained fixed. The real border of the callosum 

 is beneath its body, some distance in front of the splenial border, 

 adjoining the psalterium (Fig. 169,/'''). Not only does the folded 

 callosum extend back over the hippocampal commissure, but it 

 stretches that part of the lamina terminahs containing that com- 

 missure backward far from its original position. This stretching 



