126 HUMAN BIOLOGY 



membraneous net, or mesh work, 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. no). 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 cere- 

 brum is white matter. This consists largely of associa- 

 tion -A fibers (Fig. in) ivhich 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 con- 

 nects 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 mes- 

 sages to all the voluntary muscles, and more indirectly 

 to the involuntary muscles. The cerebral fibers are of 

 three kinds : sensory, associational (connecting cells in cere- 

 brum), and motor (Figs. 111,112). It is estimated that the 

 cerebrum alone contains 9,200,000,000 cells. 



