462 GENERAL BIOLOGY 



is the chief control center of the voluntary activities of the 

 body, and the organ of the mind. 



The outer cellular layer of the hemisphere is known as the 

 cortex. The chief divisions of its topography as marked out 

 by the convolutions of its surface are called lobes, some of 

 which are named in figure 261. The distribution of the 



FIG. 261. Diagram of the chief tracts of projection fibers of the brain. Roman 

 numerals designate cranial nerve roots (as in fig. 262) Capital letters 

 designate fiber tracts, as mentioned in text, p, pons (Prom Howell, after Starr). 



fibers from the cells that compose the cortex is enormously 

 complicated, and is not fully known, but some of the major 

 features of that distribution are indicated in the figure. 

 This shows some of the main lines of communication between 

 the principal centers within the nervous system. There are 

 three principal sorts of fibres passing out from the cells of 

 the cortex of the hemispheres, i) commissural fibers 



