Like many other organs of the body, its functional activity depends upon the presence of cells, the bodies of which are exclusively 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 another, connecting together the various cell bodies.
Fig. 4 shows a cell and its processes which 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.