206 



THE NERVOUS SYSTEM 



eighteen months old these bones have grown together, forming a 

 comparatively firm, hard structure. The spinal cord is protected 



When the position of the growing plant is changed, gravity again determines the direction of 

 the growth of the root and of the stem. 



by the spinal or vertebral column and moist lining membranes 

 similar to those in the skull. The vertebral column in an adult, 

 made up of twenty-six movable bones called vertebrae, permits 

 flexibility, and gives protection to the delicate spinal cord inclosed. 

 From both the brain and spinal cord, nerves extend to all parts of 

 the body. These nerves are embedded in other tissues which 

 afford them protection. 



The structure of the brain. The cerebrum. The larger part 

 of the brain is called the cerebrum. It is divided into two hemi- 

 spheres. The outer surface, the cortex, is composed of gray matter 



(cell bodies and syn- 

 netfoftive 

 geotropism. 



"positive^ 



apses), and shows 

 many irregular convo- 

 lutions. Underlying 

 the cortex are found 

 nerve fibers. These 

 are axons of the neu- 

 rons. They make up 

 the white matter. 

 Certain definite areas of the cerebrum are concerned with definite 

 functions. These include motor areas which control movements, 



This plant responds to gravity : stems, negatively ; roots, 

 positively. 



