616 



THE NERVOUS SYSTEM 



of the base of the cell a second set, whose processes are distributed in 

 a plane nearly corresponding to that in which their cell bodies lie. The 

 <-- ^ axon is usually given off from the basal surface 



of the cell, and passes from this point directly 

 inward to the white matter of the cerebral 

 medulla. 



The FOURTH LAYER, that of the LARGE 

 PYRAMIDAL CELLS, is also a thick layer. Its 

 cells are of the same shape, and distribute their 

 processes after the same manner as those of 

 the small pyramidal cell layer. The motor 

 area is specially characterized by the large size 

 of the cells of this layer. The largest of these, 

 the Cells of Betz, are about six times as large 

 as the small pyramidal cells, which have 

 a fairly uniform size of about ten mi- 

 crons. 



The FIFTH, or INNER POLYMORPHOUS CELL, 

 LAYER, is thicker than the preceding. Its cells 

 are of very varied form pyramidal, stellate, 

 fusiform, and granule cells and are less 

 densely packed than is the case in the more 

 superficial layers. They are intermediate in 

 size, between the cells of the second and the 

 third layers. The axons of the inner polymor- 

 phic cells, in large part, pass to the white 

 matter of the medulla, though some of them are 

 distributed laterally to neighboring convolu- 

 tions. Their dendrons are partially distrib- 

 uted within the layer in which they arise, but 

 by far the larger portion pass to the more su- 

 perficial pyramidal cell layers. Many of 

 the nerve cells of this layer, e.g., the granule 

 cells, are very small, their nucleus being 

 covered with an extremely narrow shell of 

 cytoplasm. 



It is noticeable that, as a rule, the dendronic processes from the 

 cells of all five layers are distributed either in the same plane as their 

 cell bodies, or they pass toward the surface, where many of them enter 

 the superficial molecular layer. The axons, on the other hand, are di- 



FIG. 532. HUMAN COR- 

 TEX CEREBRI, MOTOR 

 AREA. 



a, tangential fiber kyer; 

 b, outer polymorphous 

 cells; c, small pyramidal 

 cells; d, large pyramidal 

 cells; e, inner polymor- 

 phous cells. Nissl's stain. 

 Moderately magnified. 

 (After Schlapp.) 



