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THE POPULAR SCIENCE MONTHLY. 



Like many other organs of the body, its functional activity 

 depends npon the presence of cells, the bodies of which are exclu- 

 sively found upon its surface, extending to a depth varying from 

 one eighth to one fourth of an inch, and constituting the so-called 



gray matter of the brain. The convoluted arrangement of the 

 surface, as can readily be understood, more than doubles its area. 

 Beneath the gray matter, or cortex, is found the white matter, 

 which consists of fine fibrous processes extending from the bodies 

 of the cells in the gray matter, and connecting those in one part 

 of the cortex with those in another part. 



Fig. 3 shows the course of the fibrous processes of the cells of 

 the cortex of the brain as they pass from one convolution to an- 

 other, connecting together the various cell bodies. 



Fig. 4 shows a cell and its processes wliich properly constitute 

 the essential anatomical and physiological unit of the brain, and 

 indeed, speaking more generally, any nervous system. 



Fig. 5 shows how these cells in the cortex, or gray matter of 

 the brain, besides sending out processes as already described, also 

 send processes to cells distributed the whole length of the spinal 

 cord. These cells in the spinal cord in their turn send similar 

 processes out along the nerves, to terminate in the skin, muscles. 



