328 NERVOUS SYSTEM OF VERTEBRATES. 



fiber tracts which have been described above as forming the 

 fornix system. It is very important that this interminghng of the 

 transverse fibers of the corpus callosum with the longitudinal 

 fibers which connect the hippocampus with the olfactor}- lobe, 

 the base of the brain and the hypothalamus should be held in 

 mind. The effect of the corpus callosum piercing the hippocampus 

 is to cause it to degenerate in its middle part, adjacent to the 

 lamina terminahs. In more highly developed (mammahan) brains 

 the hippocampus from this point forward degenerates to a mere 



Fig. 168. — Schemes to explain the expansion of the corpus callosum and the 

 formation of the septum pellucidum. After G.Elliot Smith. A, reptile; B.mono- 

 treme; C, marsupial; D, bat; E, higher mammal, c.a., anterior commissure; ex., 

 corpus callosum; c.h., hippocampal commissure; fi., fimbria; hipp., hippocampus; 

 ind., indusium; p., precommissural body- 



rudiment, the position of the original hippocampus being occupied 

 by the enlarged corpus callosum. The rudiment of the hippo- 

 campus remains on the dorsal surface of the corpus callosum and 

 extends forward from it along the line of demarcation between 

 the precommissural body and the neopallium. The fibers of the 



