THE CORTICAL GRAY MATTER. 187 



from the commissural, the associative and the projection systems. 

 It is very well marked in the motor area (Fig. 58), not so well in 

 the common sensory area (Fig. 59). In the uncus it is very dis- 

 tinct (Fig. 61) and is so thick and dense in the gyrus hippocampi 

 (the subiculum) as to be visible to the naked eye (Fig. 62). The 

 zonal layer of fibers is faint in the visuo-sensory and audito- 

 sensory cortex. The stratum zonalc appears to grow richer 

 with the education of the individual. The function of the plex- 

 iform layer is commonly thought to be association. 



2. The layer of small pyramids (Figs. 58 and 59), as well 

 as the third layer, is situated in the supraradiary zone. It is 

 composed chiefly of small closely packed cell-bodies, pyramidal 

 in shape. They measure eight or ten microns in diameter. Their 

 apices point toward the surface. From the apices, surfaces and 

 lateral angles, dendrites are given off which ramify in the stratum 

 zonale of the first layer. The axone issues from the base of the 

 pyramid and runs down through the subjacent layers. Among 

 the small pyramids are a few ploymorphous cells. 



3. The layer of medium-sized pyramids (Fig. 58) is a nearly 

 pure layer; and, like the overlying layer, is nearly uniform through- 

 out the cerebral cortex. In arrangement of cell-bodies and proc- 

 esses it is like the second layer. The pyramids get farther apart 

 and become larger in size as the layer is descended. They measure 

 ten to fifteen microns in their vertical diameter. In the anterior 

 central gyrus these cells are motor. Layers "two" and "three" 

 might be combined in one as w r as formerly the custom. 



4. External Layer of Large Pyramids (Figs. 58 and 59). This 

 layer coincides in position with the line of Baillarger. The 

 pyramids are larger and farther apart than in the above layer, 

 and show a considerable accession of Nissl bodies as compared 

 with the smaller pyramids. They measure i$fi to 20;* by 25^ 

 to 3o/z, and form "one of the most important criteria in dividing 

 the brain surface into different histological territories" (Campbell). 

 As these cells, in the anterior central gyrus, degenerate in amyo- 

 trophic lateral sclerosis, they are considered motor in function. 

 The apical processes appear to reach the first layer and ramify 

 in the stratum zonale; the lateral and basal dendrites arborize 



