398 THE NERVES AND THE SENSE ORGANS 



of large groups of neurons; there are smaller groups in 

 different parts of the body. 



400. The Brain and Its Parts. The brain is the 

 largest part of the nervous system (Fig. 294). It weighs 

 about 3 pounds; in exceptional cases, 4 pounds. In other 



mammals there is 

 a distinct front 

 portion, or fore- 

 brain, called the 

 cerebrum. In 

 man this is the 

 upper portion. 

 There is also a 

 midbrain and a 

 hindbrain. The 

 hindbrain con- 

 sists of the cere- 

 bellum, the pons, 

 and the bulb . The bulb is also called the medulla oblongata. 

 The cerebrum is gray because of the cell bodies on its 

 surface (cf. 399). Its interior is white, and consists of 

 bundles of nerve fibers ("nerves")- The surface of the 

 cerebrum of an adult person has a great many folds, or 

 convolutions; these greatly increase its area. The in- 

 creased area of the cerebrum is needed as its cells increase 

 in number. A deep dent from front to back divides the 

 cerebrum into a right and a left hemisphere. All the 

 parts of the cerebrum are connected by nerve fibers, and 

 a multitude of fibers connect the cerebrum with the mid- 

 brain, hindbrain, and spinal cord, as well as with the 



Fissure 



of Sylvius PonsVarolu 



Cerebellum 



Fig. 294. 

 The Parts of the Brain. 



