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 membraneous net, or meshwork, of blood vessels covers the cerebrum and plentifully supplies it with blood.

Structure of the Cerebrum.—The gray matter, or cell mass of the cerebrum, forms a surface layer, called the cortex ("bark"), about one eighth of an inch thick. This gray layer is deeply folded, the folds, or convolutions, being separated by deep furrows, some of them an inch deep (see Fig. 110). Thus the area of the surface layer is increased to several times what it would be if smooth. Intelligence increases with increase in the number and depth of the convolutions. The greater part of the cerebrum is white matter. This consists largely of associational fibers (Fig. 111) which connect the cells in the gray matter with each other and with important interior ganglia at the base of the cerebrum (Fig. 112). These basal ganglia are the largest parts of the brains of the lower vertebrates (Animal Biology, Figs. 222, 259). Why do these animals not need large cerebrums? The human cerebrum comprises nearly seven eighths of the weight of the brain. A deep fissure divides it into the right and left cerebral hemispheres. A band of white matter connects the hemispheres.

Functions of the Cerebrum.—The cerebrum is the seat of consciousness and thought, and of all activity controlled by the will. It also directs the work of the lower nerve centers in the spinal cord, medulla, and cerebellum.

It receives sensory messages from all parts of the skin and through the special senses. It sends out motor messages to all the voluntary muscles, and more indirectly to the involuntary muscles. The cerebral fibers are of three kinds: sensory, associational (connecting cells in cerebrum), and motor (Figs. 111, 112). It is estimated that the cerebrum alone contains 9,200,000,000 cells.