CHAPTER XVIII 



THE CORTEX AND MEDULLARY CENTER OF THE CEREBRAL 



HEMISPHERE 



THE cerebral cortex forms a convoluted gray lamina, covering the cerebral 

 hemisphere, and varies in thickness from 4 mm. in the anterior central gyrus 

 to 1.25 mm. near the occipital pole. When sections through a fresh brain are 

 examined macroscopically, the cortex is seen to be composed of alternating 

 lighter and darker bands, the light stripes being produced by aggregations of 

 myelinated nerve-fibers (Fig. 212). 



Nerve-fibers. In addition to a very thin superficial white layer of tangential 

 fibers there are in most parts of the cerebral cortex two well-defined white bands, 

 the inner and outer lines of Baillarger 

 (Figs. 212, 215). These two bands con- 

 tain large numbers of myelinated nerve- 

 fibers running in planes parallel to the 

 surface of the cortex. In the region of 

 the calcarine fissure only the outer line 

 is visible; but this is very conspicuous 

 and is here known as the line of Gennari. 

 Myelinated fibers enter the cortex from 

 the white center in bundles that in 

 general have a direction perpendicular 

 to the surface of the cortex. These 

 bundles radiate into each convolution from its central white core and separate 

 the nerve-cells into columnar groups, thus giving the cortex a radial striation 

 (Fig. 215). 



Many of the fibers in these radial bundles are corticifugal, representing the 

 axons of the pyramidal and polymorphic cells of the cortex. Within the medul- 

 lary center they run (1) as association fibers to other parts of the cortex of the 

 same hemisphere, (2) as commissural fibers through the corpus callosum to the 

 opposite hemisphere, or (3) as projection fibers to the thalamus and lower .lying 

 centers. The others are corticipetal and are derived in part from the thalamic 

 radiation; but an even greater number of them are the terminal portions of as- 



283 



Fig. 212. Schematic sections of cerebral 

 gyri showing the alternate lighter and darker 

 bands which compose the cerebral cortex: 1 

 shows the layers as seen in most parts of the 

 cerebral cortex; 2, the layers as seen in the 

 region of the calcarine fissure. (Baillarger, 

 Quain's Anatomy.) 



