124 



HUMAN BIOLOGY 



shallow or slow until carbon dioxid accumulates again. 

 The Four Kinds of Nerve Action and the Centers that con- 

 trol them. The cord controls chiefly reflex action ; the 

 medulla controls chiefly automatic action ; the cerebellum 



controls chiefly coordinate, or 

 harmonizing, action ; the cere- 

 brum controls the purely vol- 

 untary acts, for it is the seat 

 of consciousness and thought. 

 The medulla, like the cord, 

 has the gray matter on the 

 inside (Fig. 109). 



Structure of the Cere- 



FlG. 112. SENSORY AND MOTOR 



FIBERS. (Jegi.) 



bellum. - The cerebellum, 

 like the cerebrum, has the 

 gray matter or cells on the outside. The gray matter is 

 folded into furrows that are not nearly so winding as the 

 folds in the cerebrum (see Fig. 115). The fibers going 

 to the surface 

 cells have a 

 branched arrange- 

 ment called the 

 arbor vita, or tree 

 of life, which is 

 shown where the 

 cerebellum is cut. 



FIG. 113. BRAIN OF A MONKEY. Numerals 

 show location of motor centers. (See Fig. 115.) 



The cerebellum, 

 like the cere- 

 brum, is deeply 

 cleft and thus divided into halves, called hemispheres, 

 connected by a band of white matter. 



The work of the cerebellum is to aid the cerebrum in 

 controlling the muscles. It coordinates the muscular move- 



