THE NERVOUS SYSTEM. 



557 



further modifications, due principally to the development of commissural fibers 

 in this region. Some of these commissural fibers connect the representatives 

 on each side of the hippocampus (limbus corticalis) of this region, formino- the 

 fornix commissure, but most of them (corpus callosum) connect the rest of the 

 cortical areas (neopallial areas) of the two hemispheres. 



There are two views regarding the formation of these commissures. Ac- 

 cording to one view, the first commissural fibers appear in the upper (dorsal) 

 part of the lamina terminalis. The latter subsequently expands part passu 



Corpus callosum Callosal (continuation of hippocampal) fissure 

 Fornix (continuation of fimbria) ' 



Olfactory stalk I 



Optic commissure (chiasma) 



Lamina terminalis | 

 Anterior commissure 



Uncus 



ippocampal fissure 



FIG. 483. Graphic reconstruction of the mesial hemisphere wall of a 120 mm. foetus (end of four 



months). His, from Quain's Anatomy. 

 b, Fimbria; cs , cavity of septum pellucidum ("fifth" ventricle, ventricle of Verga); km, limbus 



corticalis (gyrus dentatus); P, stalk of hemisphere; v, outline of cavity of hemisphere (lateral 



ventricle). 



with the expansion of the corpus callosum. The commissural fibers are thus 

 confined to the original walls connecting the two hemispheres. According to 

 the other view, there is a secondary fusion of the mesial hemisphere walls and 

 in these fusions the fibers cross. The first fibers appear during the third month 

 and form at first a small band in the upper part of the lamina terminalis (Fig. 

 481). These fibers come partly from the limbus corticalis (fornix commissural 

 fibers) and partly from other parts of the cortex (callosal fibers), in either case 

 traveling along the intermediate layer. According to the fusion view, the 

 exposed intermediate layers (limbi medullares) fuse where the fibers cross. 

 This fusion can easily be imagined by conceiving the opposite surfaces in 



