xvi THE NERVOUS SYSTEM 231 



heart beat, the tone of muscles, the size of blood vessels, 

 etc. Because so many important centers are located in 

 the bulb, it has been called the vital knot. 



The pom lies just in front of the bulb and serves 

 chiefly as a bridge to connect the cerebrum with the 

 other parts of the brain. 



159. The Cerebellum. The cerebellum or little brain 

 has the gray matter on the outside and the white within. 

 The white matter radiates from the center to all parts 

 of the surface, giving it a treelike appearance when cut 

 open, and for this reason it is called the tree of life. It 

 forms about one eighth of the entire brain, and consists 

 of two closely connected halves, each furrowed by nu- 

 merous shallow convolutions. 



The uses of the cerebellum are not fully known at 

 present, but we know that it has to do with balancing 

 the body in standing, walking, and running, and in mak- 

 ing the muscles work together in these movements. 

 When the cerebellum of any animal is diseased, injured, 

 or removed, it is unable to guide and control its muscles 

 of locomotion. 



160. The Cerebrum. The cerebrum is nearly divided 

 into two hemispheres by a deep longitudinal cleft or 

 fissure. Each hemisphere is marked by several deep 

 fissures and many small ones, which divide it into four 

 main parts called lobes, each having numerous convolu- 

 tions. These lobes are known as the frontal, parietal, 

 temporal, and occipital. See figures 123 and 126. The 

 gray matter, which varies in thickness from one twelftJi 

 to one sixth of an inch, forms the entire outer surface 



