THE BRAIN. 



505 



and pyramidal, having a diameter of about 10 //, with their 

 long axes vertical to the surface of the convolutions. Their 

 dendrites pass into the first layer; their axis-cylinder processes 

 passing off from the base give off collaterals and form projection- 

 fibers which go to the corpus striatum. 



3. Large Pyramid-cell Layer. This layer is characterized by 

 being made up of pyramidal cells larger than those of the second 

 layer, and increasing in size 



from above downward, reach- 

 ing a diameter of 40 /JL. The 

 breadth of this layer is shown 

 in the illustration. The axis- 

 cylinder processes of these 

 cells give off collaterals and 

 pass into the white substance 

 of the brain, where they be- 

 come medullated. 



4. Polymorphous-cell Layer 

 (Fig. 295). The cells of this 

 layer are irregular in shape, 

 each giving off several den- 

 drites and an axis-cylinder 

 process. Some of these proc- 

 esses pass into the white cen- 

 ter, while others pass to the 

 first layer and are contained 

 in one of its fibers. 



5. Fusiform-cell Layer. 

 In this layer are spindle- 

 shaped or fusiform cells. In 



the inner half thev are nu- 



FIG. 297. Principal types of cells in the 

 cerebral cortex : A, medium-sized pyrami- 

 meroilS and arranged parallel dal cell of the second layer ; B, large pyram- 



to the snrfarp Thp rlni/tfri/m idal cel1 of third la y er "> ^ polymorphous 

 ,ce. ine ciaustrum cell of fourth layer; D cell of wnich the 



IS made Up OI this layer, sepa- axis-cylinder process is ascending ; E, neu- 



rated by white substance from rg lia c / n ; ^ cel1 ? f the & *> or molecular, 



layer, forming an intermediate cell-station 

 between sensory fibers and motor cells. 

 Notice the tangential direction of the nerve- 



the other gray matter. 



The different varieties of 



-.-,. fibers ; G, sensory fibers from the white 

 cells are well shown m Fig. matt er; H, white matter; J, collateral of 

 297. The number of cells in * h e white matter (Ramon y Cajal). 



the cortex has been estimated 



at 1,200,000,000 by Donaldson, and 9,200,000,000 by Thompson : 

 the latter regards 1 59,960 of these as motor ; this number would 

 therefore represent the largest pyramidal cells or " giant-cells." 



White Matter. The medullated nerve-fibers of the white 

 center are traced through the deeper layers of the gray matter. 

 Some are continuous with the axis-cylinder processes of the 

 pyramidal and polymorphous cells; others arborize around the 



