648 



STRUCTURE OF THE CEREBRUM. 



[CH. XLVI. 



is remarkable in being broken up by fibres arranged in groups 

 passing to the outer layers. 



It is a noticeable fact that the different layers do not bear the 

 same relation to one another in thickness in different regions. In 

 the area about the fissure of Rolando, which we shall presently see 

 is called the sensori-motor area, the large pyramidal cells of the 



H 



Fig. 481. Principal types of cells in the cerebral cortex. 



A, medium-sized pyramidal cell of the second layer. 



B, large pyramidal cell of third layer. 



C, polymorphous cell of fourth layer. 



D, cell of which the axis-cylinder process is ascending. 



E, neuroglia cell. 



F, cell of the first, or molecular, layer, forming an intermediate cell-station between 



sensory fibres and motor cells. Notice the tangential direction of the nerve-fibres. 



G, sensory fibre from the white matter. 

 H, white matter. 



I, collateral of the white matter. (Ramon y Cajal.) 



third layer are conspicuous in size and number, and numerous 

 large cells are found in the fourth layer. These latter attain 

 their greatest development in the pre-central and post-central 

 convolutions. The granular layer is very marked in the occipital 

 region, forming a distinct and broad division of the fourth layer. 

 The large cells are scarce. In the frontal region, the pyramidal 

 and fourth layers are well marked but the cells are less 



