558 



TEXT-BOOK OF EMBRYOLOGY.- 



question to be brought together in the upper part of Fig. 482. It is more prob- 

 able, though, that not only the first fibers cross in the lamina terminalis, but 

 that the later ones also cross in extensions of the latter. There are three views 

 regarding the further development of the corpus callosum. The first is that 

 all parts are represented at this stage, future growth being by intussusception of 

 fibers ; the second is that the part first formed represents the genu, the rest being 

 added caudally; the third (His) is that this first formed part represents the 

 middle portion of the callosum, both anterior (genu and rostrum) and posterior 

 (splenium) portions being subsequently added (Figs. 481 and 483). This 

 latter view is indicated in Fig. 483, the later additions being shaded darker. 



As the callosal fibers connect the limbi medullares, the limbus corticalis 

 and the arcuate fissure, corresponding to the gyrus dentatus and hippocampal 

 fissure of the temporal lobe, lie dorsal to the callosum. The limbus corticalis 

 is reduced to a mere vestige (indusium griseum and strice Lancisi) on the 

 dorsal surface of the corpus callosum the fissure becoming the callosal fissure. 

 The part of the limbus medullaris ventral to the corpus callosum, corre- 

 sponding to the fimbria of the temporal lobe, forms the posterior pillars and 

 body of the fornix. 



These relations are shown in the following table from His (slightly modi- 

 fied) : 



Upper callosal region 



Hippocampal region 



Fibers from the hippocampus enter the fimbria and pass forward in the pos- 

 terior pillars and body of the fornix. In or near the lamina terminalis these 

 fibers of the fornix descend, forming the anterior pillars of the fornix, and thence 

 pass back of the anterior commissure and caudally to the mammillary region. 



