184 



ANATOMY OF THE CENTRAL NEHTOUS SYSTEM. 



The knife held horizontally passes simultaneously through both hemi- 

 spheres, and removes sections two to three millimeters in thickness. The 

 first and second of these sections contain very much gray cortex and rela- 

 tively little of the inclosed white matter; but even in the third section a 

 large, white, medullary field is uncovered in the middle of each hemisphere. 

 This is the centrum semiovale. 



In it run all the tracts which pass down from the cortex and a portion 

 of the fibers which unite the various regions of the brain with one another. 



Sim 



Ccl 



Fig. 124. — Forebrain from above. The hemispheres have been removed down 

 to the level of the corpus callosum {Gel). The white space between Gol and 

 the cortex is the centrum semiovale. Lt, Ligamentum tectum or stria longit. 

 Lancisi: a part of the cortex which borders on the corpus callosum. Sim, Striae 

 longitudinales mediales: longj white bundles of fibers which interlace on the 

 middle of the corpus callosum. (After Henle.) 



On examining Fig. 22 we should expect that on farther section only a thin 

 epithelial layer would be found lying over the ventricles in the median line, 

 between the hemispheres. This is not the case, however. During a later 

 embryonic period dense masses of fibers have grown transversely over the 



